Merge remote-tracking branch 's5p/for-next'

[linux-2.6/next.git] / drivers / scsi / scsi_lib.c

/*
 *  scsi_lib.c Copyright (C) 1999 Eric Youngdale
 *
 *  SCSI queueing library.
 *      Initial versions: Eric Youngdale (eric@andante.org).
 *                        Based upon conversations with large numbers
 *                        of people at Linux Expo.
 */

#include <linux/bio.h>
#include <linux/bitops.h>
#include <linux/blkdev.h>
#include <linux/completion.h>
#include <linux/kernel.h>
#include <linux/mempool.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/delay.h>
#include <linux/hardirq.h>
#include <linux/scatterlist.h>

#include <scsi/scsi.h>
#include <scsi/scsi_cmnd.h>
#include <scsi/scsi_dbg.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi_driver.h>
#include <scsi/scsi_eh.h>
#include <scsi/scsi_host.h>

#include "scsi_priv.h"
#include "scsi_logging.h"


#define SG_MEMPOOL_NR           ARRAY_SIZE(scsi_sg_pools)
#define SG_MEMPOOL_SIZE         2

struct scsi_host_sg_pool {
        size_t          size;
        char            *name;
        struct kmem_cache       *slab;
        mempool_t       *pool;
};

#define SP(x) { x, "sgpool-" __stringify(x) }
#if (SCSI_MAX_SG_SEGMENTS < 32)
#error SCSI_MAX_SG_SEGMENTS is too small (must be 32 or greater)
#endif
static struct scsi_host_sg_pool scsi_sg_pools[] = {
        SP(8),
        SP(16),
#if (SCSI_MAX_SG_SEGMENTS > 32)
        SP(32),
#if (SCSI_MAX_SG_SEGMENTS > 64)
        SP(64),
#if (SCSI_MAX_SG_SEGMENTS > 128)
        SP(128),
#if (SCSI_MAX_SG_SEGMENTS > 256)
#error SCSI_MAX_SG_SEGMENTS is too large (256 MAX)
#endif
#endif
#endif
#endif
        SP(SCSI_MAX_SG_SEGMENTS)
};
#undef SP

struct kmem_cache *scsi_sdb_cache;

/*
 * When to reinvoke queueing after a resource shortage. It's 3 msecs to
 * not change behaviour from the previous unplug mechanism, experimentation
 * may prove this needs changing.
 */
#define SCSI_QUEUE_DELAY        3

/*
 * Function:    scsi_unprep_request()
 *
 * Purpose:     Remove all preparation done for a request, including its
 *              associated scsi_cmnd, so that it can be requeued.
 *
 * Arguments:   req     - request to unprepare
 *
 * Lock status: Assumed that no locks are held upon entry.
 *
 * Returns:     Nothing.
 */
static void scsi_unprep_request(struct request *req)
{
        struct scsi_cmnd *cmd = req->special;

        blk_unprep_request(req);
        req->special = NULL;

        scsi_put_command(cmd);
}

/**
 * __scsi_queue_insert - private queue insertion
 * @cmd: The SCSI command being requeued
 * @reason:  The reason for the requeue
 * @unbusy: Whether the queue should be unbusied
 *
 * This is a private queue insertion.  The public interface
 * scsi_queue_insert() always assumes the queue should be unbusied
 * because it's always called before the completion.  This function is
 * for a requeue after completion, which should only occur in this
 * file.
 */
static int __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, int unbusy)
{
        struct Scsi_Host *host = cmd->device->host;
        struct scsi_device *device = cmd->device;
        struct scsi_target *starget = scsi_target(device);
        struct request_queue *q = device->request_queue;
        unsigned long flags;

        SCSI_LOG_MLQUEUE(1,
                 printk("Inserting command %p into mlqueue\n", cmd));

        /*
         * Set the appropriate busy bit for the device/host.
         *
         * If the host/device isn't busy, assume that something actually
         * completed, and that we should be able to queue a command now.
         *
         * Note that the prior mid-layer assumption that any host could
         * always queue at least one command is now broken.  The mid-layer
         * will implement a user specifiable stall (see
         * scsi_host.max_host_blocked and scsi_device.max_device_blocked)
         * if a command is requeued with no other commands outstanding
         * either for the device or for the host.
         */
        switch (reason) {
        case SCSI_MLQUEUE_HOST_BUSY:
                host->host_blocked = host->max_host_blocked;
                break;
        case SCSI_MLQUEUE_DEVICE_BUSY:
        case SCSI_MLQUEUE_EH_RETRY:
                device->device_blocked = device->max_device_blocked;
                break;
        case SCSI_MLQUEUE_TARGET_BUSY:
                starget->target_blocked = starget->max_target_blocked;
                break;
        }

        /*
         * Decrement the counters, since these commands are no longer
         * active on the host/device.
         */
        if (unbusy)
                scsi_device_unbusy(device);

        /*
         * Requeue this command.  It will go before all other commands
         * that are already in the queue.
         */
        spin_lock_irqsave(q->queue_lock, flags);
        blk_requeue_request(q, cmd->request);
        spin_unlock_irqrestore(q->queue_lock, flags);

        kblockd_schedule_work(q, &device->requeue_work);

        return 0;
}

/*
 * Function:    scsi_queue_insert()
 *
 * Purpose:     Insert a command in the midlevel queue.
 *
 * Arguments:   cmd    - command that we are adding to queue.
 *              reason - why we are inserting command to queue.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       We do this for one of two cases.  Either the host is busy
 *              and it cannot accept any more commands for the time being,
 *              or the device returned QUEUE_FULL and can accept no more
 *              commands.
 * Notes:       This could be called either from an interrupt context or a
 *              normal process context.
 */
int scsi_queue_insert(struct scsi_cmnd *cmd, int reason)
{
        return __scsi_queue_insert(cmd, reason, 1);
}
/**
 * scsi_execute - insert request and wait for the result
 * @sdev:       scsi device
 * @cmd:        scsi command
 * @data_direction: data direction
 * @buffer:     data buffer
 * @bufflen:    len of buffer
 * @sense:      optional sense buffer
 * @timeout:    request timeout in seconds
 * @retries:    number of times to retry request
 * @flags:      or into request flags;
 * @resid:      optional residual length
 *
 * returns the req->errors value which is the scsi_cmnd result
 * field.
 */
int scsi_execute(struct scsi_device *sdev, const unsigned char *cmd,
                 int data_direction, void *buffer, unsigned bufflen,
                 unsigned char *sense, int timeout, int retries, int flags,
                 int *resid)
{
        struct request *req;
        int write = (data_direction == DMA_TO_DEVICE);
        int ret = DRIVER_ERROR << 24;

        req = blk_get_request(sdev->request_queue, write, __GFP_WAIT);
        if (!req)
                return ret;

        if (bufflen &&  blk_rq_map_kern(sdev->request_queue, req,
                                        buffer, bufflen, __GFP_WAIT))
                goto out;

        req->cmd_len = COMMAND_SIZE(cmd[0]);
        memcpy(req->cmd, cmd, req->cmd_len);
        req->sense = sense;
        req->sense_len = 0;
        req->retries = retries;
        req->timeout = timeout;
        req->cmd_type = REQ_TYPE_BLOCK_PC;
        req->cmd_flags |= flags | REQ_QUIET | REQ_PREEMPT;

        /*
         * head injection *required* here otherwise quiesce won't work
         */
        blk_execute_rq(req->q, NULL, req, 1);

        /*
         * Some devices (USB mass-storage in particular) may transfer
         * garbage data together with a residue indicating that the data
         * is invalid.  Prevent the garbage from being misinterpreted
         * and prevent security leaks by zeroing out the excess data.
         */
        if (unlikely(req->resid_len > 0 && req->resid_len <= bufflen))
                memset(buffer + (bufflen - req->resid_len), 0, req->resid_len);

        if (resid)
                *resid = req->resid_len;
        ret = req->errors;
 out:
        blk_put_request(req);

        return ret;
}
EXPORT_SYMBOL(scsi_execute);


int scsi_execute_req(struct scsi_device *sdev, const unsigned char *cmd,
                     int data_direction, void *buffer, unsigned bufflen,
                     struct scsi_sense_hdr *sshdr, int timeout, int retries,
                     int *resid)
{
        char *sense = NULL;
        int result;
        
        if (sshdr) {
                sense = kzalloc(SCSI_SENSE_BUFFERSIZE, GFP_NOIO);
                if (!sense)
                        return DRIVER_ERROR << 24;
        }
        result = scsi_execute(sdev, cmd, data_direction, buffer, bufflen,
                              sense, timeout, retries, 0, resid);
        if (sshdr)
                scsi_normalize_sense(sense, SCSI_SENSE_BUFFERSIZE, sshdr);

        kfree(sense);
        return result;
}
EXPORT_SYMBOL(scsi_execute_req);

/*
 * Function:    scsi_init_cmd_errh()
 *
 * Purpose:     Initialize cmd fields related to error handling.
 *
 * Arguments:   cmd     - command that is ready to be queued.
 *
 * Notes:       This function has the job of initializing a number of
 *              fields related to error handling.   Typically this will
 *              be called once for each command, as required.
 */
static void scsi_init_cmd_errh(struct scsi_cmnd *cmd)
{
        cmd->serial_number = 0;
        scsi_set_resid(cmd, 0);
        memset(cmd->sense_buffer, 0, SCSI_SENSE_BUFFERSIZE);
        if (cmd->cmd_len == 0)
                cmd->cmd_len = scsi_command_size(cmd->cmnd);
}

void scsi_device_unbusy(struct scsi_device *sdev)
{
        struct Scsi_Host *shost = sdev->host;
        struct scsi_target *starget = scsi_target(sdev);
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        shost->host_busy--;
        starget->target_busy--;
        if (unlikely(scsi_host_in_recovery(shost) &&
                     (shost->host_failed || shost->host_eh_scheduled)))
                scsi_eh_wakeup(shost);
        spin_unlock(shost->host_lock);
        spin_lock(sdev->request_queue->queue_lock);
        sdev->device_busy--;
        spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
}

/*
 * Called for single_lun devices on IO completion. Clear starget_sdev_user,
 * and call blk_run_queue for all the scsi_devices on the target -
 * including current_sdev first.
 *
 * Called with *no* scsi locks held.
 */
static void scsi_single_lun_run(struct scsi_device *current_sdev)
{
        struct Scsi_Host *shost = current_sdev->host;
        struct scsi_device *sdev, *tmp;
        struct scsi_target *starget = scsi_target(current_sdev);
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        starget->starget_sdev_user = NULL;
        spin_unlock_irqrestore(shost->host_lock, flags);

        /*
         * Call blk_run_queue for all LUNs on the target, starting with
         * current_sdev. We race with others (to set starget_sdev_user),
         * but in most cases, we will be first. Ideally, each LU on the
         * target would get some limited time or requests on the target.
         */
        blk_run_queue(current_sdev->request_queue);

        spin_lock_irqsave(shost->host_lock, flags);
        if (starget->starget_sdev_user)
                goto out;
        list_for_each_entry_safe(sdev, tmp, &starget->devices,
                        same_target_siblings) {
                if (sdev == current_sdev)
                        continue;
                if (scsi_device_get(sdev))
                        continue;

                spin_unlock_irqrestore(shost->host_lock, flags);
                blk_run_queue(sdev->request_queue);
                spin_lock_irqsave(shost->host_lock, flags);
        
                scsi_device_put(sdev);
        }
 out:
        spin_unlock_irqrestore(shost->host_lock, flags);
}

static inline int scsi_device_is_busy(struct scsi_device *sdev)
{
        if (sdev->device_busy >= sdev->queue_depth || sdev->device_blocked)
                return 1;

        return 0;
}

static inline int scsi_target_is_busy(struct scsi_target *starget)
{
        return ((starget->can_queue > 0 &&
                 starget->target_busy >= starget->can_queue) ||
                 starget->target_blocked);
}

static inline int scsi_host_is_busy(struct Scsi_Host *shost)
{
        if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) ||
            shost->host_blocked || shost->host_self_blocked)
                return 1;

        return 0;
}

/*
 * Function:    scsi_run_queue()
 *
 * Purpose:     Select a proper request queue to serve next
 *
 * Arguments:   q       - last request's queue
 *
 * Returns:     Nothing
 *
 * Notes:       The previous command was completely finished, start
 *              a new one if possible.
 */
static void scsi_run_queue(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost;
        LIST_HEAD(starved_list);
        unsigned long flags;

        /* if the device is dead, sdev will be NULL, so no queue to run */
        if (!sdev)
                return;

        shost = sdev->host;
        if (scsi_target(sdev)->single_lun)
                scsi_single_lun_run(sdev);

        spin_lock_irqsave(shost->host_lock, flags);
        list_splice_init(&shost->starved_list, &starved_list);

        while (!list_empty(&starved_list)) {
                /*
                 * As long as shost is accepting commands and we have
                 * starved queues, call blk_run_queue. scsi_request_fn
                 * drops the queue_lock and can add us back to the
                 * starved_list.
                 *
                 * host_lock protects the starved_list and starved_entry.
                 * scsi_request_fn must get the host_lock before checking
                 * or modifying starved_list or starved_entry.
                 */
                if (scsi_host_is_busy(shost))
                        break;

                sdev = list_entry(starved_list.next,
                                  struct scsi_device, starved_entry);
                list_del_init(&sdev->starved_entry);
                if (scsi_target_is_busy(scsi_target(sdev))) {
                        list_move_tail(&sdev->starved_entry,
                                       &shost->starved_list);
                        continue;
                }

                spin_unlock(shost->host_lock);
                spin_lock(sdev->request_queue->queue_lock);
                __blk_run_queue(sdev->request_queue);
                spin_unlock(sdev->request_queue->queue_lock);
                spin_lock(shost->host_lock);
        }
        /* put any unprocessed entries back */
        list_splice(&starved_list, &shost->starved_list);
        spin_unlock_irqrestore(shost->host_lock, flags);

        blk_run_queue(q);
}

void scsi_requeue_run_queue(struct work_struct *work)
{
        struct scsi_device *sdev;
        struct request_queue *q;

        sdev = container_of(work, struct scsi_device, requeue_work);
        q = sdev->request_queue;
        scsi_run_queue(q);
}

/*
 * Function:    scsi_requeue_command()
 *
 * Purpose:     Handle post-processing of completed commands.
 *
 * Arguments:   q       - queue to operate on
 *              cmd     - command that may need to be requeued.
 *
 * Returns:     Nothing
 *
 * Notes:       After command completion, there may be blocks left
 *              over which weren't finished by the previous command
 *              this can be for a number of reasons - the main one is
 *              I/O errors in the middle of the request, in which case
 *              we need to request the blocks that come after the bad
 *              sector.
 * Notes:       Upon return, cmd is a stale pointer.
 */
static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;
        unsigned long flags;

        spin_lock_irqsave(q->queue_lock, flags);
        scsi_unprep_request(req);
        blk_requeue_request(q, req);
        spin_unlock_irqrestore(q->queue_lock, flags);

        scsi_run_queue(q);
}

void scsi_next_command(struct scsi_cmnd *cmd)
{
        struct scsi_device *sdev = cmd->device;
        struct request_queue *q = sdev->request_queue;

        /* need to hold a reference on the device before we let go of the cmd */
        get_device(&sdev->sdev_gendev);

        scsi_put_command(cmd);
        scsi_run_queue(q);

        /* ok to remove device now */
        put_device(&sdev->sdev_gendev);
}

void scsi_run_host_queues(struct Scsi_Host *shost)
{
        struct scsi_device *sdev;

        shost_for_each_device(sdev, shost)
                scsi_run_queue(sdev->request_queue);
}

static void __scsi_release_buffers(struct scsi_cmnd *, int);

/*
 * Function:    scsi_end_request()
 *
 * Purpose:     Post-processing of completed commands (usually invoked at end
 *              of upper level post-processing and scsi_io_completion).
 *
 * Arguments:   cmd      - command that is complete.
 *              error    - 0 if I/O indicates success, < 0 for I/O error.
 *              bytes    - number of bytes of completed I/O
 *              requeue  - indicates whether we should requeue leftovers.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     cmd if requeue required, NULL otherwise.
 *
 * Notes:       This is called for block device requests in order to
 *              mark some number of sectors as complete.
 * 
 *              We are guaranteeing that the request queue will be goosed
 *              at some point during this call.
 * Notes:       If cmd was requeued, upon return it will be a stale pointer.
 */
static struct scsi_cmnd *scsi_end_request(struct scsi_cmnd *cmd, int error,
                                          int bytes, int requeue)
{
        struct request_queue *q = cmd->device->request_queue;
        struct request *req = cmd->request;

        /*
         * If there are blocks left over at the end, set up the command
         * to queue the remainder of them.
         */
        if (blk_end_request(req, error, bytes)) {
                /* kill remainder if no retrys */
                if (error && scsi_noretry_cmd(cmd))
                        blk_end_request_all(req, error);
                else {
                        if (requeue) {
                                /*
                                 * Bleah.  Leftovers again.  Stick the
                                 * leftovers in the front of the
                                 * queue, and goose the queue again.
                                 */
                                scsi_release_buffers(cmd);
                                scsi_requeue_command(q, cmd);
                                cmd = NULL;
                        }
                        return cmd;
                }
        }

        /*
         * This will goose the queue request function at the end, so we don't
         * need to worry about launching another command.
         */
        __scsi_release_buffers(cmd, 0);
        scsi_next_command(cmd);
        return NULL;
}

static inline unsigned int scsi_sgtable_index(unsigned short nents)
{
        unsigned int index;

        BUG_ON(nents > SCSI_MAX_SG_SEGMENTS);

        if (nents <= 8)
                index = 0;
        else
                index = get_count_order(nents) - 3;

        return index;
}

static void scsi_sg_free(struct scatterlist *sgl, unsigned int nents)
{
        struct scsi_host_sg_pool *sgp;

        sgp = scsi_sg_pools + scsi_sgtable_index(nents);
        mempool_free(sgl, sgp->pool);
}

static struct scatterlist *scsi_sg_alloc(unsigned int nents, gfp_t gfp_mask)
{
        struct scsi_host_sg_pool *sgp;

        sgp = scsi_sg_pools + scsi_sgtable_index(nents);
        return mempool_alloc(sgp->pool, gfp_mask);
}

static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents,
                              gfp_t gfp_mask)
{
        int ret;

        BUG_ON(!nents);

        ret = __sg_alloc_table(&sdb->table, nents, SCSI_MAX_SG_SEGMENTS,
                               gfp_mask, scsi_sg_alloc);
        if (unlikely(ret))
                __sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS,
                                scsi_sg_free);

        return ret;
}

static void scsi_free_sgtable(struct scsi_data_buffer *sdb)
{
        __sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS, scsi_sg_free);
}

static void __scsi_release_buffers(struct scsi_cmnd *cmd, int do_bidi_check)
{

        if (cmd->sdb.table.nents)
                scsi_free_sgtable(&cmd->sdb);

        memset(&cmd->sdb, 0, sizeof(cmd->sdb));

        if (do_bidi_check && scsi_bidi_cmnd(cmd)) {
                struct scsi_data_buffer *bidi_sdb =
                        cmd->request->next_rq->special;
                scsi_free_sgtable(bidi_sdb);
                kmem_cache_free(scsi_sdb_cache, bidi_sdb);
                cmd->request->next_rq->special = NULL;
        }

        if (scsi_prot_sg_count(cmd))
                scsi_free_sgtable(cmd->prot_sdb);
}

/*
 * Function:    scsi_release_buffers()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd     - command that we are bailing.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       In the event that an upper level driver rejects a
 *              command, we must release resources allocated during
 *              the __init_io() function.  Primarily this would involve
 *              the scatter-gather table, and potentially any bounce
 *              buffers.
 */
void scsi_release_buffers(struct scsi_cmnd *cmd)
{
        __scsi_release_buffers(cmd, 1);
}
EXPORT_SYMBOL(scsi_release_buffers);

static int __scsi_error_from_host_byte(struct scsi_cmnd *cmd, int result)
{
        int error = 0;

        switch(host_byte(result)) {
        case DID_TRANSPORT_FAILFAST:
                error = -ENOLINK;
                break;
        case DID_TARGET_FAILURE:
                cmd->result |= (DID_OK << 16);
                error = -EREMOTEIO;
                break;
        case DID_NEXUS_FAILURE:
                cmd->result |= (DID_OK << 16);
                error = -EBADE;
                break;
        default:
                error = -EIO;
                break;
        }

        return error;
}

/*
 * Function:    scsi_io_completion()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd   - command that is finished.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This function is matched in terms of capabilities to
 *              the function that created the scatter-gather list.
 *              In other words, if there are no bounce buffers
 *              (the normal case for most drivers), we don't need
 *              the logic to deal with cleaning up afterwards.
 *
 *              We must call scsi_end_request().  This will finish off
 *              the specified number of sectors.  If we are done, the
 *              command block will be released and the queue function
 *              will be goosed.  If we are not done then we have to
 *              figure out what to do next:
 *
 *              a) We can call scsi_requeue_command().  The request
 *                 will be unprepared and put back on the queue.  Then
 *                 a new command will be created for it.  This should
 *                 be used if we made forward progress, or if we want
 *                 to switch from READ(10) to READ(6) for example.
 *
 *              b) We can call scsi_queue_insert().  The request will
 *                 be put back on the queue and retried using the same
 *                 command as before, possibly after a delay.
 *
 *              c) We can call blk_end_request() with -EIO to fail
 *                 the remainder of the request.
 */
void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
{
        int result = cmd->result;
        struct request_queue *q = cmd->device->request_queue;
        struct request *req = cmd->request;
        int error = 0;
        struct scsi_sense_hdr sshdr;
        int sense_valid = 0;
        int sense_deferred = 0;
        enum {ACTION_FAIL, ACTION_REPREP, ACTION_RETRY,
              ACTION_DELAYED_RETRY} action;
        char *description = NULL;

        if (result) {
                sense_valid = scsi_command_normalize_sense(cmd, &sshdr);
                if (sense_valid)
                        sense_deferred = scsi_sense_is_deferred(&sshdr);
        }

        if (req->cmd_type == REQ_TYPE_BLOCK_PC) { /* SG_IO ioctl from block level */
                req->errors = result;
                if (result) {
                        if (sense_valid && req->sense) {
                                /*
                                 * SG_IO wants current and deferred errors
                                 */
                                int len = 8 + cmd->sense_buffer[7];

                                if (len > SCSI_SENSE_BUFFERSIZE)
                                        len = SCSI_SENSE_BUFFERSIZE;
                                memcpy(req->sense, cmd->sense_buffer,  len);
                                req->sense_len = len;
                        }
                        if (!sense_deferred)
                                error = __scsi_error_from_host_byte(cmd, result);
                }

                req->resid_len = scsi_get_resid(cmd);

                if (scsi_bidi_cmnd(cmd)) {
                        /*
                         * Bidi commands Must be complete as a whole,
                         * both sides at once.
                         */
                        req->next_rq->resid_len = scsi_in(cmd)->resid;

                        scsi_release_buffers(cmd);
                        blk_end_request_all(req, 0);

                        scsi_next_command(cmd);
                        return;
                }
        }

        /* no bidi support for !REQ_TYPE_BLOCK_PC yet */
        BUG_ON(blk_bidi_rq(req));

        /*
         * Next deal with any sectors which we were able to correctly
         * handle.
         */
        SCSI_LOG_HLCOMPLETE(1, printk("%u sectors total, "
                                      "%d bytes done.\n",
                                      blk_rq_sectors(req), good_bytes));

        /*
         * Recovered errors need reporting, but they're always treated
         * as success, so fiddle the result code here.  For BLOCK_PC
         * we already took a copy of the original into rq->errors which
         * is what gets returned to the user
         */
        if (sense_valid && (sshdr.sense_key == RECOVERED_ERROR)) {
                /* if ATA PASS-THROUGH INFORMATION AVAILABLE skip
                 * print since caller wants ATA registers. Only occurs on
                 * SCSI ATA PASS_THROUGH commands when CK_COND=1
                 */
                if ((sshdr.asc == 0x0) && (sshdr.ascq == 0x1d))
                        ;
                else if (!(req->cmd_flags & REQ_QUIET))
                        scsi_print_sense("", cmd);
                result = 0;
                /* BLOCK_PC may have set error */
                error = 0;
        }

        /*
         * A number of bytes were successfully read.  If there
         * are leftovers and there is some kind of error
         * (result != 0), retry the rest.
         */
        if (scsi_end_request(cmd, error, good_bytes, result == 0) == NULL)
                return;

        error = __scsi_error_from_host_byte(cmd, result);

        if (host_byte(result) == DID_RESET) {
                /* Third party bus reset or reset for error recovery
                 * reasons.  Just retry the command and see what
                 * happens.
                 */
                action = ACTION_RETRY;
        } else if (sense_valid && !sense_deferred) {
                switch (sshdr.sense_key) {
                case UNIT_ATTENTION:
                        if (cmd->device->removable) {
                                /* Detected disc change.  Set a bit
                                 * and quietly refuse further access.
                                 */
                                cmd->device->changed = 1;
                                description = "Media Changed";
                                action = ACTION_FAIL;
                        } else {
                                /* Must have been a power glitch, or a
                                 * bus reset.  Could not have been a
                                 * media change, so we just retry the
                                 * command and see what happens.
                                 */
                                action = ACTION_RETRY;
                        }
                        break;
                case ILLEGAL_REQUEST:
                        /* If we had an ILLEGAL REQUEST returned, then
                         * we may have performed an unsupported
                         * command.  The only thing this should be
                         * would be a ten byte read where only a six
                         * byte read was supported.  Also, on a system
                         * where READ CAPACITY failed, we may have
                         * read past the end of the disk.
                         */
                        if ((cmd->device->use_10_for_rw &&
                            sshdr.asc == 0x20 && sshdr.ascq == 0x00) &&
                            (cmd->cmnd[0] == READ_10 ||
                             cmd->cmnd[0] == WRITE_10)) {
                                /* This will issue a new 6-byte command. */
                                cmd->device->use_10_for_rw = 0;
                                action = ACTION_REPREP;
                        } else if (sshdr.asc == 0x10) /* DIX */ {
                                description = "Host Data Integrity Failure";
                                action = ACTION_FAIL;
                                error = -EILSEQ;
                        /* INVALID COMMAND OPCODE or INVALID FIELD IN CDB */
                        } else if ((sshdr.asc == 0x20 || sshdr.asc == 0x24) &&
                                   (cmd->cmnd[0] == UNMAP ||
                                    cmd->cmnd[0] == WRITE_SAME_16 ||
                                    cmd->cmnd[0] == WRITE_SAME)) {
                                description = "Discard failure";
                                action = ACTION_FAIL;
                        } else
                                action = ACTION_FAIL;
                        break;
                case ABORTED_COMMAND:
                        action = ACTION_FAIL;
                        if (sshdr.asc == 0x10) { /* DIF */
                                description = "Target Data Integrity Failure";
                                error = -EILSEQ;
                        }
                        break;
                case NOT_READY:
                        /* If the device is in the process of becoming
                         * ready, or has a temporary blockage, retry.
                         */
                        if (sshdr.asc == 0x04) {
                                switch (sshdr.ascq) {
                                case 0x01: /* becoming ready */
                                case 0x04: /* format in progress */
                                case 0x05: /* rebuild in progress */
                                case 0x06: /* recalculation in progress */
                                case 0x07: /* operation in progress */
                                case 0x08: /* Long write in progress */
                                case 0x09: /* self test in progress */
                                case 0x14: /* space allocation in progress */
                                        action = ACTION_DELAYED_RETRY;
                                        break;
                                default:
                                        description = "Device not ready";
                                        action = ACTION_FAIL;
                                        break;
                                }
                        } else {
                                description = "Device not ready";
                                action = ACTION_FAIL;
                        }
                        break;
                case VOLUME_OVERFLOW:
                        /* See SSC3rXX or current. */
                        action = ACTION_FAIL;
                        break;
                default:
                        description = "Unhandled sense code";
                        action = ACTION_FAIL;
                        break;
                }
        } else {
                description = "Unhandled error code";
                action = ACTION_FAIL;
        }

        switch (action) {
        case ACTION_FAIL:
                /* Give up and fail the remainder of the request */
                scsi_release_buffers(cmd);
                if (!(req->cmd_flags & REQ_QUIET)) {
                        if (description)
                                scmd_printk(KERN_INFO, cmd, "%s\n",
                                            description);
                        scsi_print_result(cmd);
                        if (driver_byte(result) & DRIVER_SENSE)
                                scsi_print_sense("", cmd);
                        scsi_print_command(cmd);
                }
                if (blk_end_request_err(req, error))
                        scsi_requeue_command(q, cmd);
                else
                        scsi_next_command(cmd);
                break;
        case ACTION_REPREP:
                /* Unprep the request and put it back at the head of the queue.
                 * A new command will be prepared and issued.
                 */
                scsi_release_buffers(cmd);
                scsi_requeue_command(q, cmd);
                break;
        case ACTION_RETRY:
                /* Retry the same command immediately */
                __scsi_queue_insert(cmd, SCSI_MLQUEUE_EH_RETRY, 0);
                break;
        case ACTION_DELAYED_RETRY:
                /* Retry the same command after a delay */
                __scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY, 0);
                break;
        }
}

static int scsi_init_sgtable(struct request *req, struct scsi_data_buffer *sdb,
                             gfp_t gfp_mask)
{
        int count;

        /*
         * If sg table allocation fails, requeue request later.
         */
        if (unlikely(scsi_alloc_sgtable(sdb, req->nr_phys_segments,
                                        gfp_mask))) {
                return BLKPREP_DEFER;
        }

        req->buffer = NULL;

        /* 
         * Next, walk the list, and fill in the addresses and sizes of
         * each segment.
         */
        count = blk_rq_map_sg(req->q, req, sdb->table.sgl);
        BUG_ON(count > sdb->table.nents);
        sdb->table.nents = count;
        sdb->length = blk_rq_bytes(req);
        return BLKPREP_OK;
}

/*
 * Function:    scsi_init_io()
 *
 * Purpose:     SCSI I/O initialize function.
 *
 * Arguments:   cmd   - Command descriptor we wish to initialize
 *
 * Returns:     0 on success
 *              BLKPREP_DEFER if the failure is retryable
 *              BLKPREP_KILL if the failure is fatal
 */
int scsi_init_io(struct scsi_cmnd *cmd, gfp_t gfp_mask)
{
        struct request *rq = cmd->request;

        int error = scsi_init_sgtable(rq, &cmd->sdb, gfp_mask);
        if (error)
                goto err_exit;

        if (blk_bidi_rq(rq)) {
                struct scsi_data_buffer *bidi_sdb = kmem_cache_zalloc(
                        scsi_sdb_cache, GFP_ATOMIC);
                if (!bidi_sdb) {
                        error = BLKPREP_DEFER;
                        goto err_exit;
                }

                rq->next_rq->special = bidi_sdb;
                error = scsi_init_sgtable(rq->next_rq, bidi_sdb, GFP_ATOMIC);
                if (error)
                        goto err_exit;
        }

        if (blk_integrity_rq(rq)) {
                struct scsi_data_buffer *prot_sdb = cmd->prot_sdb;
                int ivecs, count;

                BUG_ON(prot_sdb == NULL);
                ivecs = blk_rq_count_integrity_sg(rq->q, rq->bio);

                if (scsi_alloc_sgtable(prot_sdb, ivecs, gfp_mask)) {
                        error = BLKPREP_DEFER;
                        goto err_exit;
                }

                count = blk_rq_map_integrity_sg(rq->q, rq->bio,
                                                prot_sdb->table.sgl);
                BUG_ON(unlikely(count > ivecs));
                BUG_ON(unlikely(count > queue_max_integrity_segments(rq->q)));

                cmd->prot_sdb = prot_sdb;
                cmd->prot_sdb->table.nents = count;
        }

        return BLKPREP_OK ;

err_exit:
        scsi_release_buffers(cmd);
        cmd->request->special = NULL;
        scsi_put_command(cmd);
        return error;
}
EXPORT_SYMBOL(scsi_init_io);

static struct scsi_cmnd *scsi_get_cmd_from_req(struct scsi_device *sdev,
                struct request *req)
{
        struct scsi_cmnd *cmd;

        if (!req->special) {
                cmd = scsi_get_command(sdev, GFP_ATOMIC);
                if (unlikely(!cmd))
                        return NULL;
                req->special = cmd;
        } else {
                cmd = req->special;
        }

        /* pull a tag out of the request if we have one */
        cmd->tag = req->tag;
        cmd->request = req;

        cmd->cmnd = req->cmd;
        cmd->prot_op = SCSI_PROT_NORMAL;

        return cmd;
}

int scsi_setup_blk_pc_cmnd(struct scsi_device *sdev, struct request *req)
{
        struct scsi_cmnd *cmd;
        int ret = scsi_prep_state_check(sdev, req);

        if (ret != BLKPREP_OK)
                return ret;

        cmd = scsi_get_cmd_from_req(sdev, req);
        if (unlikely(!cmd))
                return BLKPREP_DEFER;

        /*
         * BLOCK_PC requests may transfer data, in which case they must
         * a bio attached to them.  Or they might contain a SCSI command
         * that does not transfer data, in which case they may optionally
         * submit a request without an attached bio.
         */
        if (req->bio) {
                int ret;

                BUG_ON(!req->nr_phys_segments);

                ret = scsi_init_io(cmd, GFP_ATOMIC);
                if (unlikely(ret))
                        return ret;
        } else {
                BUG_ON(blk_rq_bytes(req));

                memset(&cmd->sdb, 0, sizeof(cmd->sdb));
                req->buffer = NULL;
        }

        cmd->cmd_len = req->cmd_len;
        if (!blk_rq_bytes(req))
                cmd->sc_data_direction = DMA_NONE;
        else if (rq_data_dir(req) == WRITE)
                cmd->sc_data_direction = DMA_TO_DEVICE;
        else
                cmd->sc_data_direction = DMA_FROM_DEVICE;
        
        cmd->transfersize = blk_rq_bytes(req);
        cmd->allowed = req->retries;
        return BLKPREP_OK;
}
EXPORT_SYMBOL(scsi_setup_blk_pc_cmnd);

/*
 * Setup a REQ_TYPE_FS command.  These are simple read/write request
 * from filesystems that still need to be translated to SCSI CDBs from
 * the ULD.
 */
int scsi_setup_fs_cmnd(struct scsi_device *sdev, struct request *req)
{
        struct scsi_cmnd *cmd;
        int ret = scsi_prep_state_check(sdev, req);

        if (ret != BLKPREP_OK)
                return ret;

        if (unlikely(sdev->scsi_dh_data && sdev->scsi_dh_data->scsi_dh
                         && sdev->scsi_dh_data->scsi_dh->prep_fn)) {
                ret = sdev->scsi_dh_data->scsi_dh->prep_fn(sdev, req);
                if (ret != BLKPREP_OK)
                        return ret;
        }

        /*
         * Filesystem requests must transfer data.
         */
        BUG_ON(!req->nr_phys_segments);

        cmd = scsi_get_cmd_from_req(sdev, req);
        if (unlikely(!cmd))
                return BLKPREP_DEFER;

        memset(cmd->cmnd, 0, BLK_MAX_CDB);
        return scsi_init_io(cmd, GFP_ATOMIC);
}
EXPORT_SYMBOL(scsi_setup_fs_cmnd);

int scsi_prep_state_check(struct scsi_device *sdev, struct request *req)
{
        int ret = BLKPREP_OK;

        /*
         * If the device is not in running state we will reject some
         * or all commands.
         */
        if (unlikely(sdev->sdev_state != SDEV_RUNNING)) {
                switch (sdev->sdev_state) {
                case SDEV_OFFLINE:
                        /*
                         * If the device is offline we refuse to process any
                         * commands.  The device must be brought online
                         * before trying any recovery commands.
                         */
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to offline device\n");
                        ret = BLKPREP_KILL;
                        break;
                case SDEV_DEL:
                        /*
                         * If the device is fully deleted, we refuse to
                         * process any commands as well.
                         */
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to dead device\n");
                        ret = BLKPREP_KILL;
                        break;
                case SDEV_QUIESCE:
                case SDEV_BLOCK:
                case SDEV_CREATED_BLOCK:
                        /*
                         * If the devices is blocked we defer normal commands.
                         */
                        if (!(req->cmd_flags & REQ_PREEMPT))
                                ret = BLKPREP_DEFER;
                        break;
                default:
                        /*
                         * For any other not fully online state we only allow
                         * special commands.  In particular any user initiated
                         * command is not allowed.
                         */
                        if (!(req->cmd_flags & REQ_PREEMPT))
                                ret = BLKPREP_KILL;
                        break;
                }
        }
        return ret;
}
EXPORT_SYMBOL(scsi_prep_state_check);

int scsi_prep_return(struct request_queue *q, struct request *req, int ret)
{
        struct scsi_device *sdev = q->queuedata;

        switch (ret) {
        case BLKPREP_KILL:
                req->errors = DID_NO_CONNECT << 16;
                /* release the command and kill it */
                if (req->special) {
                        struct scsi_cmnd *cmd = req->special;
                        scsi_release_buffers(cmd);
                        scsi_put_command(cmd);
                        req->special = NULL;
                }
                break;
        case BLKPREP_DEFER:
                /*
                 * If we defer, the blk_peek_request() returns NULL, but the
                 * queue must be restarted, so we schedule a callback to happen
                 * shortly.
                 */
                if (sdev->device_busy == 0)
                        blk_delay_queue(q, SCSI_QUEUE_DELAY);
                break;
        default:
                req->cmd_flags |= REQ_DONTPREP;
        }

        return ret;
}
EXPORT_SYMBOL(scsi_prep_return);

int scsi_prep_fn(struct request_queue *q, struct request *req)
{
        struct scsi_device *sdev = q->queuedata;
        int ret = BLKPREP_KILL;

        if (req->cmd_type == REQ_TYPE_BLOCK_PC)
                ret = scsi_setup_blk_pc_cmnd(sdev, req);
        return scsi_prep_return(q, req, ret);
}
EXPORT_SYMBOL(scsi_prep_fn);

/*
 * scsi_dev_queue_ready: if we can send requests to sdev, return 1 else
 * return 0.
 *
 * Called with the queue_lock held.
 */
static inline int scsi_dev_queue_ready(struct request_queue *q,
                                  struct scsi_device *sdev)
{
        if (sdev->device_busy == 0 && sdev->device_blocked) {
                /*
                 * unblock after device_blocked iterates to zero
                 */
                if (--sdev->device_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                   sdev_printk(KERN_INFO, sdev,
                                   "unblocking device at zero depth\n"));
                } else {
                        blk_delay_queue(q, SCSI_QUEUE_DELAY);
                        return 0;
                }
        }
        if (scsi_device_is_busy(sdev))
                return 0;

        return 1;
}


/*
 * scsi_target_queue_ready: checks if there we can send commands to target
 * @sdev: scsi device on starget to check.
 *
 * Called with the host lock held.
 */
static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
                                           struct scsi_device *sdev)
{
        struct scsi_target *starget = scsi_target(sdev);

        if (starget->single_lun) {
                if (starget->starget_sdev_user &&
                    starget->starget_sdev_user != sdev)
                        return 0;
                starget->starget_sdev_user = sdev;
        }

        if (starget->target_busy == 0 && starget->target_blocked) {
                /*
                 * unblock after target_blocked iterates to zero
                 */
                if (--starget->target_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
                                         "unblocking target at zero depth\n"));
                } else
                        return 0;
        }

        if (scsi_target_is_busy(starget)) {
                if (list_empty(&sdev->starved_entry))
                        list_add_tail(&sdev->starved_entry,
                                      &shost->starved_list);
                return 0;
        }

        /* We're OK to process the command, so we can't be starved */
        if (!list_empty(&sdev->starved_entry))
                list_del_init(&sdev->starved_entry);
        return 1;
}

/*
 * scsi_host_queue_ready: if we can send requests to shost, return 1 else
 * return 0. We must end up running the queue again whenever 0 is
 * returned, else IO can hang.
 *
 * Called with host_lock held.
 */
static inline int scsi_host_queue_ready(struct request_queue *q,
                                   struct Scsi_Host *shost,
                                   struct scsi_device *sdev)
{
        if (scsi_host_in_recovery(shost))
                return 0;
        if (shost->host_busy == 0 && shost->host_blocked) {
                /*
                 * unblock after host_blocked iterates to zero
                 */
                if (--shost->host_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                printk("scsi%d unblocking host at zero depth\n",
                                        shost->host_no));
                } else {
                        return 0;
                }
        }
        if (scsi_host_is_busy(shost)) {
                if (list_empty(&sdev->starved_entry))
                        list_add_tail(&sdev->starved_entry, &shost->starved_list);
                return 0;
        }

        /* We're OK to process the command, so we can't be starved */
        if (!list_empty(&sdev->starved_entry))
                list_del_init(&sdev->starved_entry);

        return 1;
}

/*
 * Busy state exporting function for request stacking drivers.
 *
 * For efficiency, no lock is taken to check the busy state of
 * shost/starget/sdev, since the returned value is not guaranteed and
 * may be changed after request stacking drivers call the function,
 * regardless of taking lock or not.
 *
 * When scsi can't dispatch I/Os anymore and needs to kill I/Os
 * (e.g. !sdev), scsi needs to return 'not busy'.
 * Otherwise, request stacking drivers may hold requests forever.
 */
static int scsi_lld_busy(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost;
        struct scsi_target *starget;

        if (!sdev)
                return 0;

        shost = sdev->host;
        starget = scsi_target(sdev);

        if (scsi_host_in_recovery(shost) || scsi_host_is_busy(shost) ||
            scsi_target_is_busy(starget) || scsi_device_is_busy(sdev))
                return 1;

        return 0;
}

/*
 * Kill a request for a dead device
 */
static void scsi_kill_request(struct request *req, struct request_queue *q)
{
        struct scsi_cmnd *cmd = req->special;
        struct scsi_device *sdev;
        struct scsi_target *starget;
        struct Scsi_Host *shost;

        blk_start_request(req);

        sdev = cmd->device;
        starget = scsi_target(sdev);
        shost = sdev->host;
        scsi_init_cmd_errh(cmd);
        cmd->result = DID_NO_CONNECT << 16;
        atomic_inc(&cmd->device->iorequest_cnt);

        /*
         * SCSI request completion path will do scsi_device_unbusy(),
         * bump busy counts.  To bump the counters, we need to dance
         * with the locks as normal issue path does.
         */
        sdev->device_busy++;
        spin_unlock(sdev->request_queue->queue_lock);
        spin_lock(shost->host_lock);
        shost->host_busy++;
        starget->target_busy++;
        spin_unlock(shost->host_lock);
        spin_lock(sdev->request_queue->queue_lock);

        blk_complete_request(req);
}

static void scsi_softirq_done(struct request *rq)
{
        struct scsi_cmnd *cmd = rq->special;
        unsigned long wait_for = (cmd->allowed + 1) * rq->timeout;
        int disposition;

        INIT_LIST_HEAD(&cmd->eh_entry);

        atomic_inc(&cmd->device->iodone_cnt);
        if (cmd->result)
                atomic_inc(&cmd->device->ioerr_cnt);

        disposition = scsi_decide_disposition(cmd);
        if (disposition != SUCCESS &&
            time_before(cmd->jiffies_at_alloc + wait_for, jiffies)) {
                sdev_printk(KERN_ERR, cmd->device,
                            "timing out command, waited %lus\n",
                            wait_for/HZ);
                disposition = SUCCESS;
        }
                        
        scsi_log_completion(cmd, disposition);

        switch (disposition) {
                case SUCCESS:
                        scsi_finish_command(cmd);
                        break;
                case NEEDS_RETRY:
                        scsi_queue_insert(cmd, SCSI_MLQUEUE_EH_RETRY);
                        break;
                case ADD_TO_MLQUEUE:
                        scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY);
                        break;
                default:
                        if (!scsi_eh_scmd_add(cmd, 0))
                                scsi_finish_command(cmd);
        }
}

/*
 * Function:    scsi_request_fn()
 *
 * Purpose:     Main strategy routine for SCSI.
 *
 * Arguments:   q       - Pointer to actual queue.
 *
 * Returns:     Nothing
 *
 * Lock status: IO request lock assumed to be held when called.
 */
static void scsi_request_fn(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost;
        struct scsi_cmnd *cmd;
        struct request *req;

        if (!sdev) {
                printk("scsi: killing requests for dead queue\n");
                while ((req = blk_peek_request(q)) != NULL)
                        scsi_kill_request(req, q);
                return;
        }

        if(!get_device(&sdev->sdev_gendev))
                /* We must be tearing the block queue down already */
                return;

        /*
         * To start with, we keep looping until the queue is empty, or until
         * the host is no longer able to accept any more requests.
         */
        shost = sdev->host;
        for (;;) {
                int rtn;
                /*
                 * get next queueable request.  We do this early to make sure
                 * that the request is fully prepared even if we cannot 
                 * accept it.
                 */
                req = blk_peek_request(q);
                if (!req || !scsi_dev_queue_ready(q, sdev))
                        break;

                if (unlikely(!scsi_device_online(sdev))) {
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to offline device\n");
                        scsi_kill_request(req, q);
                        continue;
                }


                /*
                 * Remove the request from the request list.
                 */
                if (!(blk_queue_tagged(q) && !blk_queue_start_tag(q, req)))
                        blk_start_request(req);
                sdev->device_busy++;

                spin_unlock(q->queue_lock);
                cmd = req->special;
                if (unlikely(cmd == NULL)) {
                        printk(KERN_CRIT "impossible request in %s.\n"
                                         "please mail a stack trace to "
                                         "linux-scsi@vger.kernel.org\n",
                                         __func__);
                        blk_dump_rq_flags(req, "foo");
                        BUG();
                }
                spin_lock(shost->host_lock);

                /*
                 * We hit this when the driver is using a host wide
                 * tag map. For device level tag maps the queue_depth check
                 * in the device ready fn would prevent us from trying
                 * to allocate a tag. Since the map is a shared host resource
                 * we add the dev to the starved list so it eventually gets
                 * a run when a tag is freed.
                 */
                if (blk_queue_tagged(q) && !blk_rq_tagged(req)) {
                        if (list_empty(&sdev->starved_entry))
                                list_add_tail(&sdev->starved_entry,
                                              &shost->starved_list);
                        goto not_ready;
                }

                if (!scsi_target_queue_ready(shost, sdev))
                        goto not_ready;

                if (!scsi_host_queue_ready(q, shost, sdev))
                        goto not_ready;

                scsi_target(sdev)->target_busy++;
                shost->host_busy++;

                /*
                 * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
                 *              take the lock again.
                 */
                spin_unlock_irq(shost->host_lock);

                /*
                 * Finally, initialize any error handling parameters, and set up
                 * the timers for timeouts.
                 */
                scsi_init_cmd_errh(cmd);

                /*
                 * Dispatch the command to the low-level driver.
                 */
                rtn = scsi_dispatch_cmd(cmd);
                spin_lock_irq(q->queue_lock);
                if (rtn)
                        goto out_delay;
        }

        goto out;

 not_ready:
        spin_unlock_irq(shost->host_lock);

        /*
         * lock q, handle tag, requeue req, and decrement device_busy. We
         * must return with queue_lock held.
         *
         * Decrementing device_busy without checking it is OK, as all such
         * cases (host limits or settings) should run the queue at some
         * later time.
         */
        spin_lock_irq(q->queue_lock);
        blk_requeue_request(q, req);
        sdev->device_busy--;
out_delay:
        if (sdev->device_busy == 0)
                blk_delay_queue(q, SCSI_QUEUE_DELAY);
out:
        /* must be careful here...if we trigger the ->remove() function
         * we cannot be holding the q lock */
        spin_unlock_irq(q->queue_lock);
        put_device(&sdev->sdev_gendev);
        spin_lock_irq(q->queue_lock);
}

u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
{
        struct device *host_dev;
        u64 bounce_limit = 0xffffffff;

        if (shost->unchecked_isa_dma)
                return BLK_BOUNCE_ISA;
        /*
         * Platforms with virtual-DMA translation
         * hardware have no practical limit.
         */
        if (!PCI_DMA_BUS_IS_PHYS)
                return BLK_BOUNCE_ANY;

        host_dev = scsi_get_device(shost);
        if (host_dev && host_dev->dma_mask)
                bounce_limit = *host_dev->dma_mask;

        return bounce_limit;
}
EXPORT_SYMBOL(scsi_calculate_bounce_limit);

struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
                                         request_fn_proc *request_fn)
{
        struct request_queue *q;
        struct device *dev = shost->shost_gendev.parent;

        q = blk_init_queue(request_fn, NULL);
        if (!q)
                return NULL;

        /*
         * this limit is imposed by hardware restrictions
         */
        blk_queue_max_segments(q, min_t(unsigned short, shost->sg_tablesize,
                                        SCSI_MAX_SG_CHAIN_SEGMENTS));

        if (scsi_host_prot_dma(shost)) {
                shost->sg_prot_tablesize =
                        min_not_zero(shost->sg_prot_tablesize,
                                     (unsigned short)SCSI_MAX_PROT_SG_SEGMENTS);
                BUG_ON(shost->sg_prot_tablesize < shost->sg_tablesize);
                blk_queue_max_integrity_segments(q, shost->sg_prot_tablesize);
        }

        blk_queue_max_hw_sectors(q, shost->max_sectors);
        blk_queue_bounce_limit(q, scsi_calculate_bounce_limit(shost));
        blk_queue_segment_boundary(q, shost->dma_boundary);
        dma_set_seg_boundary(dev, shost->dma_boundary);

        blk_queue_max_segment_size(q, dma_get_max_seg_size(dev));

        if (!shost->use_clustering)
                q->limits.cluster = 0;

        /*
         * set a reasonable default alignment on word boundaries: the
         * host and device may alter it using
         * blk_queue_update_dma_alignment() later.
         */
        blk_queue_dma_alignment(q, 0x03);

        return q;
}
EXPORT_SYMBOL(__scsi_alloc_queue);

struct request_queue *scsi_alloc_queue(struct scsi_device *sdev)
{
        struct request_queue *q;

        q = __scsi_alloc_queue(sdev->host, scsi_request_fn);
        if (!q)
                return NULL;

        blk_queue_prep_rq(q, scsi_prep_fn);
        blk_queue_softirq_done(q, scsi_softirq_done);
        blk_queue_rq_timed_out(q, scsi_times_out);
        blk_queue_lld_busy(q, scsi_lld_busy);
        return q;
}

void scsi_free_queue(struct request_queue *q)
{
        blk_cleanup_queue(q);
}

/*
 * Function:    scsi_block_requests()
 *
 * Purpose:     Utility function used by low-level drivers to prevent further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 */
void scsi_block_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 1;
}
EXPORT_SYMBOL(scsi_block_requests);

/*
 * Function:    scsi_unblock_requests()
 *
 * Purpose:     Utility function used by low-level drivers to allow further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 *
 *              This is done as an API function so that changes to the
 *              internals of the scsi mid-layer won't require wholesale
 *              changes to drivers that use this feature.
 */
void scsi_unblock_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 0;
        scsi_run_host_queues(shost);
}
EXPORT_SYMBOL(scsi_unblock_requests);

int __init scsi_init_queue(void)
{
        int i;

        scsi_sdb_cache = kmem_cache_create("scsi_data_buffer",
                                           sizeof(struct scsi_data_buffer),
                                           0, 0, NULL);
        if (!scsi_sdb_cache) {
                printk(KERN_ERR "SCSI: can't init scsi sdb cache\n");
                return -ENOMEM;
        }

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                int size = sgp->size * sizeof(struct scatterlist);

                sgp->slab = kmem_cache_create(sgp->name, size, 0,
                                SLAB_HWCACHE_ALIGN, NULL);
                if (!sgp->slab) {
                        printk(KERN_ERR "SCSI: can't init sg slab %s\n",
                                        sgp->name);
                        goto cleanup_sdb;
                }

                sgp->pool = mempool_create_slab_pool(SG_MEMPOOL_SIZE,
                                                     sgp->slab);
                if (!sgp->pool) {
                        printk(KERN_ERR "SCSI: can't init sg mempool %s\n",
                                        sgp->name);
                        goto cleanup_sdb;
                }
        }

        return 0;

cleanup_sdb:
        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                if (sgp->pool)
                        mempool_destroy(sgp->pool);
                if (sgp->slab)
                        kmem_cache_destroy(sgp->slab);
        }
        kmem_cache_destroy(scsi_sdb_cache);

        return -ENOMEM;
}

void scsi_exit_queue(void)
{
        int i;

        kmem_cache_destroy(scsi_sdb_cache);

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                mempool_destroy(sgp->pool);
                kmem_cache_destroy(sgp->slab);
        }
}

/**
 *      scsi_mode_select - issue a mode select
 *      @sdev:  SCSI device to be queried
 *      @pf:    Page format bit (1 == standard, 0 == vendor specific)
 *      @sp:    Save page bit (0 == don't save, 1 == save)
 *      @modepage: mode page being requested
 *      @buffer: request buffer (may not be smaller than eight bytes)
 *      @len:   length of request buffer.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *      @data: returns a structure abstracting the mode header data
 *      @sshdr: place to put sense data (or NULL if no sense to be collected).
 *              must be SCSI_SENSE_BUFFERSIZE big.
 *
 *      Returns zero if successful; negative error number or scsi
 *      status on error
 *
 */
int
scsi_mode_select(struct scsi_device *sdev, int pf, int sp, int modepage,
                 unsigned char *buffer, int len, int timeout, int retries,
                 struct scsi_mode_data *data, struct scsi_sense_hdr *sshdr)
{
        unsigned char cmd[10];
        unsigned char *real_buffer;
        int ret;

        memset(cmd, 0, sizeof(cmd));
        cmd[1] = (pf ? 0x10 : 0) | (sp ? 0x01 : 0);

        if (sdev->use_10_for_ms) {
                if (len > 65535)
                        return -EINVAL;
                real_buffer = kmalloc(8 + len, GFP_KERNEL);
                if (!real_buffer)
                        return -ENOMEM;
                memcpy(real_buffer + 8, buffer, len);
                len += 8;
                real_buffer[0] = 0;
                real_buffer[1] = 0;
                real_buffer[2] = data->medium_type;
                real_buffer[3] = data->device_specific;
                real_buffer[4] = data->longlba ? 0x01 : 0;
                real_buffer[5] = 0;
                real_buffer[6] = data->block_descriptor_length >> 8;
                real_buffer[7] = data->block_descriptor_length;

                cmd[0] = MODE_SELECT_10;
                cmd[7] = len >> 8;
                cmd[8] = len;
        } else {
                if (len > 255 || data->block_descriptor_length > 255 ||
                    data->longlba)
                        return -EINVAL;

                real_buffer = kmalloc(4 + len, GFP_KERNEL);
                if (!real_buffer)
                        return -ENOMEM;
                memcpy(real_buffer + 4, buffer, len);
                len += 4;
                real_buffer[0] = 0;
                real_buffer[1] = data->medium_type;
                real_buffer[2] = data->device_specific;
                real_buffer[3] = data->block_descriptor_length;
                

                cmd[0] = MODE_SELECT;
                cmd[4] = len;
        }

        ret = scsi_execute_req(sdev, cmd, DMA_TO_DEVICE, real_buffer, len,
                               sshdr, timeout, retries, NULL);
        kfree(real_buffer);
        return ret;
}
EXPORT_SYMBOL_GPL(scsi_mode_select);

/**
 *      scsi_mode_sense - issue a mode sense, falling back from 10 to six bytes if necessary.
 *      @sdev:  SCSI device to be queried
 *      @dbd:   set if mode sense will allow block descriptors to be returned
 *      @modepage: mode page being requested
 *      @buffer: request buffer (may not be smaller than eight bytes)
 *      @len:   length of request buffer.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *      @data: returns a structure abstracting the mode header data
 *      @sshdr: place to put sense data (or NULL if no sense to be collected).
 *              must be SCSI_SENSE_BUFFERSIZE big.
 *
 *      Returns zero if unsuccessful, or the header offset (either 4
 *      or 8 depending on whether a six or ten byte command was
 *      issued) if successful.
 */
int
scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
                  unsigned char *buffer, int len, int timeout, int retries,
                  struct scsi_mode_data *data, struct scsi_sense_hdr *sshdr)
{
        unsigned char cmd[12];
        int use_10_for_ms;
        int header_length;
        int result;
        struct scsi_sense_hdr my_sshdr;

        memset(data, 0, sizeof(*data));
        memset(&cmd[0], 0, 12);
        cmd[1] = dbd & 0x18;    /* allows DBD and LLBA bits */
        cmd[2] = modepage;

        /* caller might not be interested in sense, but we need it */
        if (!sshdr)
                sshdr = &my_sshdr;

 retry:
        use_10_for_ms = sdev->use_10_for_ms;

        if (use_10_for_ms) {
                if (len < 8)
                        len = 8;

                cmd[0] = MODE_SENSE_10;
                cmd[8] = len;
                header_length = 8;
        } else {
                if (len < 4)
                        len = 4;

                cmd[0] = MODE_SENSE;
                cmd[4] = len;
                header_length = 4;
        }

        memset(buffer, 0, len);

        result = scsi_execute_req(sdev, cmd, DMA_FROM_DEVICE, buffer, len,
                                  sshdr, timeout, retries, NULL);

        /* This code looks awful: what it's doing is making sure an
         * ILLEGAL REQUEST sense return identifies the actual command
         * byte as the problem.  MODE_SENSE commands can return
         * ILLEGAL REQUEST if the code page isn't supported */

        if (use_10_for_ms && !scsi_status_is_good(result) &&
            (driver_byte(result) & DRIVER_SENSE)) {
                if (scsi_sense_valid(sshdr)) {
                        if ((sshdr->sense_key == ILLEGAL_REQUEST) &&
                            (sshdr->asc == 0x20) && (sshdr->ascq == 0)) {
                                /* 
                                 * Invalid command operation code
                                 */
                                sdev->use_10_for_ms = 0;
                                goto retry;
                        }
                }
        }

        if(scsi_status_is_good(result)) {
                if (unlikely(buffer[0] == 0x86 && buffer[1] == 0x0b &&
                             (modepage == 6 || modepage == 8))) {
                        /* Initio breakage? */
                        header_length = 0;
                        data->length = 13;
                        data->medium_type = 0;
                        data->device_specific = 0;
                        data->longlba = 0;
                        data->block_descriptor_length = 0;
                } else if(use_10_for_ms) {
                        data->length = buffer[0]*256 + buffer[1] + 2;
                        data->medium_type = buffer[2];
                        data->device_specific = buffer[3];
                        data->longlba = buffer[4] & 0x01;
                        data->block_descriptor_length = buffer[6]*256
                                + buffer[7];
                } else {
                        data->length = buffer[0] + 1;
                        data->medium_type = buffer[1];
                        data->device_specific = buffer[2];
                        data->block_descriptor_length = buffer[3];
                }
                data->header_length = header_length;
        }

        return result;
}
EXPORT_SYMBOL(scsi_mode_sense);

/**
 *      scsi_test_unit_ready - test if unit is ready
 *      @sdev:  scsi device to change the state of.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *      @sshdr_external: Optional pointer to struct scsi_sense_hdr for
 *              returning sense. Make sure that this is cleared before passing
 *              in.
 *
 *      Returns zero if unsuccessful or an error if TUR failed.  For
 *      removable media, UNIT_ATTENTION sets ->changed flag.
 **/
int
scsi_test_unit_ready(struct scsi_device *sdev, int timeout, int retries,
                     struct scsi_sense_hdr *sshdr_external)
{
        char cmd[] = {
                TEST_UNIT_READY, 0, 0, 0, 0, 0,
        };
        struct scsi_sense_hdr *sshdr;
        int result;

        if (!sshdr_external)
                sshdr = kzalloc(sizeof(*sshdr), GFP_KERNEL);
        else
                sshdr = sshdr_external;

        /* try to eat the UNIT_ATTENTION if there are enough retries */
        do {
                result = scsi_execute_req(sdev, cmd, DMA_NONE, NULL, 0, sshdr,
                                          timeout, retries, NULL);
                if (sdev->removable && scsi_sense_valid(sshdr) &&
                    sshdr->sense_key == UNIT_ATTENTION)
                        sdev->changed = 1;
        } while (scsi_sense_valid(sshdr) &&
                 sshdr->sense_key == UNIT_ATTENTION && --retries);

        if (!sshdr_external)
                kfree(sshdr);
        return result;
}
EXPORT_SYMBOL(scsi_test_unit_ready);

/**
 *      scsi_device_set_state - Take the given device through the device state model.
 *      @sdev:  scsi device to change the state of.
 *      @state: state to change to.
 *
 *      Returns zero if unsuccessful or an error if the requested 
 *      transition is illegal.
 */
int
scsi_device_set_state(struct scsi_device *sdev, enum scsi_device_state state)
{
        enum scsi_device_state oldstate = sdev->sdev_state;

        if (state == oldstate)
                return 0;

        switch (state) {
        case SDEV_CREATED:
                switch (oldstate) {
                case SDEV_CREATED_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;
                        
        case SDEV_RUNNING:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_OFFLINE:
                case SDEV_QUIESCE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_QUIESCE:
                switch (oldstate) {
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_OFFLINE:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_QUIESCE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_BLOCK:
                switch (oldstate) {
                case SDEV_RUNNING:
                case SDEV_CREATED_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_CREATED_BLOCK:
                switch (oldstate) {
                case SDEV_CREATED:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_CANCEL:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_QUIESCE:
                case SDEV_OFFLINE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_DEL:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                case SDEV_CANCEL:
                        break;
                default:
                        goto illegal;
                }
                break;

        }
        sdev->sdev_state = state;
        return 0;

 illegal:
        SCSI_LOG_ERROR_RECOVERY(1, 
                                sdev_printk(KERN_ERR, sdev,
                                            "Illegal state transition %s->%s\n",
                                            scsi_device_state_name(oldstate),
                                            scsi_device_state_name(state))
                                );
        return -EINVAL;
}
EXPORT_SYMBOL(scsi_device_set_state);

/**
 *      sdev_evt_emit - emit a single SCSI device uevent
 *      @sdev: associated SCSI device
 *      @evt: event to emit
 *
 *      Send a single uevent (scsi_event) to the associated scsi_device.
 */
static void scsi_evt_emit(struct scsi_device *sdev, struct scsi_event *evt)
{
        int idx = 0;
        char *envp[3];

        switch (evt->evt_type) {
        case SDEV_EVT_MEDIA_CHANGE:
                envp[idx++] = "SDEV_MEDIA_CHANGE=1";
                break;

        default:
                /* do nothing */
                break;
        }

        envp[idx++] = NULL;

        kobject_uevent_env(&sdev->sdev_gendev.kobj, KOBJ_CHANGE, envp);
}

/**
 *      sdev_evt_thread - send a uevent for each scsi event
 *      @work: work struct for scsi_device
 *
 *      Dispatch queued events to their associated scsi_device kobjects
 *      as uevents.
 */
void scsi_evt_thread(struct work_struct *work)
{
        struct scsi_device *sdev;
        LIST_HEAD(event_list);

        sdev = container_of(work, struct scsi_device, event_work);

        while (1) {
                struct scsi_event *evt;
                struct list_head *this, *tmp;
                unsigned long flags;

                spin_lock_irqsave(&sdev->list_lock, flags);
                list_splice_init(&sdev->event_list, &event_list);
                spin_unlock_irqrestore(&sdev->list_lock, flags);

                if (list_empty(&event_list))
                        break;

                list_for_each_safe(this, tmp, &event_list) {
                        evt = list_entry(this, struct scsi_event, node);
                        list_del(&evt->node);
                        scsi_evt_emit(sdev, evt);
                        kfree(evt);
                }
        }
}

/**
 *      sdev_evt_send - send asserted event to uevent thread
 *      @sdev: scsi_device event occurred on
 *      @evt: event to send
 *
 *      Assert scsi device event asynchronously.
 */
void sdev_evt_send(struct scsi_device *sdev, struct scsi_event *evt)
{
        unsigned long flags;

#if 0
        /* FIXME: currently this check eliminates all media change events
         * for polled devices.  Need to update to discriminate between AN
         * and polled events */
        if (!test_bit(evt->evt_type, sdev->supported_events)) {
                kfree(evt);
                return;
        }
#endif

        spin_lock_irqsave(&sdev->list_lock, flags);
        list_add_tail(&evt->node, &sdev->event_list);
        schedule_work(&sdev->event_work);
        spin_unlock_irqrestore(&sdev->list_lock, flags);
}
EXPORT_SYMBOL_GPL(sdev_evt_send);

/**
 *      sdev_evt_alloc - allocate a new scsi event
 *      @evt_type: type of event to allocate
 *      @gfpflags: GFP flags for allocation
 *
 *      Allocates and returns a new scsi_event.
 */
struct scsi_event *sdev_evt_alloc(enum scsi_device_event evt_type,
                                  gfp_t gfpflags)
{
        struct scsi_event *evt = kzalloc(sizeof(struct scsi_event), gfpflags);
        if (!evt)
                return NULL;

        evt->evt_type = evt_type;
        INIT_LIST_HEAD(&evt->node);

        /* evt_type-specific initialization, if any */
        switch (evt_type) {
        case SDEV_EVT_MEDIA_CHANGE:
        default:
                /* do nothing */
                break;
        }

        return evt;
}
EXPORT_SYMBOL_GPL(sdev_evt_alloc);

/**
 *      sdev_evt_send_simple - send asserted event to uevent thread
 *      @sdev: scsi_device event occurred on
 *      @evt_type: type of event to send
 *      @gfpflags: GFP flags for allocation
 *
 *      Assert scsi device event asynchronously, given an event type.
 */
void sdev_evt_send_simple(struct scsi_device *sdev,
                          enum scsi_device_event evt_type, gfp_t gfpflags)
{
        struct scsi_event *evt = sdev_evt_alloc(evt_type, gfpflags);
        if (!evt) {
                sdev_printk(KERN_ERR, sdev, "event %d eaten due to OOM\n",
                            evt_type);
                return;
        }

        sdev_evt_send(sdev, evt);
}
EXPORT_SYMBOL_GPL(sdev_evt_send_simple);

/**
 *      scsi_device_quiesce - Block user issued commands.
 *      @sdev:  scsi device to quiesce.
 *
 *      This works by trying to transition to the SDEV_QUIESCE state
 *      (which must be a legal transition).  When the device is in this
 *      state, only special requests will be accepted, all others will
 *      be deferred.  Since special requests may also be requeued requests,
 *      a successful return doesn't guarantee the device will be 
 *      totally quiescent.
 *
 *      Must be called with user context, may sleep.
 *
 *      Returns zero if unsuccessful or an error if not.
 */
int
scsi_device_quiesce(struct scsi_device *sdev)
{
        int err = scsi_device_set_state(sdev, SDEV_QUIESCE);
        if (err)
                return err;

        scsi_run_queue(sdev->request_queue);
        while (sdev->device_busy) {
                msleep_interruptible(200);
                scsi_run_queue(sdev->request_queue);
        }
        return 0;
}
EXPORT_SYMBOL(scsi_device_quiesce);

/**
 *      scsi_device_resume - Restart user issued commands to a quiesced device.
 *      @sdev:  scsi device to resume.
 *
 *      Moves the device from quiesced back to running and restarts the
 *      queues.
 *
 *      Must be called with user context, may sleep.
 */
void
scsi_device_resume(struct scsi_device *sdev)
{
        if(scsi_device_set_state(sdev, SDEV_RUNNING))
                return;
        scsi_run_queue(sdev->request_queue);
}
EXPORT_SYMBOL(scsi_device_resume);

static void
device_quiesce_fn(struct scsi_device *sdev, void *data)
{
        scsi_device_quiesce(sdev);
}

void
scsi_target_quiesce(struct scsi_target *starget)
{
        starget_for_each_device(starget, NULL, device_quiesce_fn);
}
EXPORT_SYMBOL(scsi_target_quiesce);

static void
device_resume_fn(struct scsi_device *sdev, void *data)
{
        scsi_device_resume(sdev);
}

void
scsi_target_resume(struct scsi_target *starget)
{
        starget_for_each_device(starget, NULL, device_resume_fn);
}
EXPORT_SYMBOL(scsi_target_resume);

/**
 * scsi_internal_device_block - internal function to put a device temporarily into the SDEV_BLOCK state
 * @sdev:       device to block
 *
 * Block request made by scsi lld's to temporarily stop all
 * scsi commands on the specified device.  Called from interrupt
 * or normal process context.
 *
 * Returns zero if successful or error if not
 *
 * Notes:       
 *      This routine transitions the device to the SDEV_BLOCK state
 *      (which must be a legal transition).  When the device is in this
 *      state, all commands are deferred until the scsi lld reenables
 *      the device with scsi_device_unblock or device_block_tmo fires.
 *      This routine assumes the host_lock is held on entry.
 */
int
scsi_internal_device_block(struct scsi_device *sdev)
{
        struct request_queue *q = sdev->request_queue;
        unsigned long flags;
        int err = 0;

        err = scsi_device_set_state(sdev, SDEV_BLOCK);
        if (err) {
                err = scsi_device_set_state(sdev, SDEV_CREATED_BLOCK);

                if (err)
                        return err;
        }

        /* 
         * The device has transitioned to SDEV_BLOCK.  Stop the
         * block layer from calling the midlayer with this device's
         * request queue. 
         */
        spin_lock_irqsave(q->queue_lock, flags);
        blk_stop_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return 0;
}
EXPORT_SYMBOL_GPL(scsi_internal_device_block);
 
/**
 * scsi_internal_device_unblock - resume a device after a block request
 * @sdev:       device to resume
 *
 * Called by scsi lld's or the midlayer to restart the device queue
 * for the previously suspended scsi device.  Called from interrupt or
 * normal process context.
 *
 * Returns zero if successful or error if not.
 *
 * Notes:       
 *      This routine transitions the device to the SDEV_RUNNING state
 *      (which must be a legal transition) allowing the midlayer to
 *      goose the queue for this device.  This routine assumes the 
 *      host_lock is held upon entry.
 */
int
scsi_internal_device_unblock(struct scsi_device *sdev)
{
        struct request_queue *q = sdev->request_queue; 
        unsigned long flags;
        
        /* 
         * Try to transition the scsi device to SDEV_RUNNING
         * and goose the device queue if successful.  
         */
        if (sdev->sdev_state == SDEV_BLOCK)
                sdev->sdev_state = SDEV_RUNNING;
        else if (sdev->sdev_state == SDEV_CREATED_BLOCK)
                sdev->sdev_state = SDEV_CREATED;
        else if (sdev->sdev_state != SDEV_CANCEL &&
                 sdev->sdev_state != SDEV_OFFLINE)
                return -EINVAL;

        spin_lock_irqsave(q->queue_lock, flags);
        blk_start_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return 0;
}
EXPORT_SYMBOL_GPL(scsi_internal_device_unblock);

static void
device_block(struct scsi_device *sdev, void *data)
{
        scsi_internal_device_block(sdev);
}

static int
target_block(struct device *dev, void *data)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_block);
        return 0;
}

void
scsi_target_block(struct device *dev)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_block);
        else
                device_for_each_child(dev, NULL, target_block);
}
EXPORT_SYMBOL_GPL(scsi_target_block);

static void
device_unblock(struct scsi_device *sdev, void *data)
{
        scsi_internal_device_unblock(sdev);
}

static int
target_unblock(struct device *dev, void *data)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_unblock);
        return 0;
}

void
scsi_target_unblock(struct device *dev)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_unblock);
        else
                device_for_each_child(dev, NULL, target_unblock);
}
EXPORT_SYMBOL_GPL(scsi_target_unblock);

/**
 * scsi_kmap_atomic_sg - find and atomically map an sg-elemnt
 * @sgl:        scatter-gather list
 * @sg_count:   number of segments in sg
 * @offset:     offset in bytes into sg, on return offset into the mapped area
 * @len:        bytes to map, on return number of bytes mapped
 *
 * Returns virtual address of the start of the mapped page
 */
void *scsi_kmap_atomic_sg(struct scatterlist *sgl, int sg_count,
                          size_t *offset, size_t *len)
{
        int i;
        size_t sg_len = 0, len_complete = 0;
        struct scatterlist *sg;
        struct page *page;

        WARN_ON(!irqs_disabled());

        for_each_sg(sgl, sg, sg_count, i) {
                len_complete = sg_len; /* Complete sg-entries */
                sg_len += sg->length;
                if (sg_len > *offset)
                        break;
        }

        if (unlikely(i == sg_count)) {
                printk(KERN_ERR "%s: Bytes in sg: %zu, requested offset %zu, "
                        "elements %d\n",
                       __func__, sg_len, *offset, sg_count);
                WARN_ON(1);
                return NULL;
        }

        /* Offset starting from the beginning of first page in this sg-entry */
        *offset = *offset - len_complete + sg->offset;

        /* Assumption: contiguous pages can be accessed as "page + i" */
        page = nth_page(sg_page(sg), (*offset >> PAGE_SHIFT));
        *offset &= ~PAGE_MASK;

        /* Bytes in this sg-entry from *offset to the end of the page */
        sg_len = PAGE_SIZE - *offset;
        if (*len > sg_len)
                *len = sg_len;

        return kmap_atomic(page, KM_BIO_SRC_IRQ);
}
EXPORT_SYMBOL(scsi_kmap_atomic_sg);

/**
 * scsi_kunmap_atomic_sg - atomically unmap a virtual address, previously mapped with scsi_kmap_atomic_sg
 * @virt:       virtual address to be unmapped
 */
void scsi_kunmap_atomic_sg(void *virt)
{
        kunmap_atomic(virt, KM_BIO_SRC_IRQ);
}
EXPORT_SYMBOL(scsi_kunmap_atomic_sg);