Linux 4.19.133

[linux/fpc-iii.git] / drivers / hwtracing / stm / core.c

// SPDX-License-Identifier: GPL-2.0
/*
 * System Trace Module (STM) infrastructure
 * Copyright (c) 2014, Intel Corporation.
 *
 * STM class implements generic infrastructure for  System Trace Module devices
 * as defined in MIPI STPv2 specification.
 */

#include <linux/pm_runtime.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/device.h>
#include <linux/compat.h>
#include <linux/kdev_t.h>
#include <linux/srcu.h>
#include <linux/slab.h>
#include <linux/stm.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include "stm.h"

#include <uapi/linux/stm.h>

static unsigned int stm_core_up;

/*
 * The SRCU here makes sure that STM device doesn't disappear from under a
 * stm_source_write() caller, which may want to have as little overhead as
 * possible.
 */
static struct srcu_struct stm_source_srcu;

static ssize_t masters_show(struct device *dev,
                            struct device_attribute *attr,
                            char *buf)
{
        struct stm_device *stm = to_stm_device(dev);
        int ret;

        ret = sprintf(buf, "%u %u\n", stm->data->sw_start, stm->data->sw_end);

        return ret;
}

static DEVICE_ATTR_RO(masters);

static ssize_t channels_show(struct device *dev,
                             struct device_attribute *attr,
                             char *buf)
{
        struct stm_device *stm = to_stm_device(dev);
        int ret;

        ret = sprintf(buf, "%u\n", stm->data->sw_nchannels);

        return ret;
}

static DEVICE_ATTR_RO(channels);

static ssize_t hw_override_show(struct device *dev,
                                struct device_attribute *attr,
                                char *buf)
{
        struct stm_device *stm = to_stm_device(dev);
        int ret;

        ret = sprintf(buf, "%u\n", stm->data->hw_override);

        return ret;
}

static DEVICE_ATTR_RO(hw_override);

static struct attribute *stm_attrs[] = {
        &dev_attr_masters.attr,
        &dev_attr_channels.attr,
        &dev_attr_hw_override.attr,
        NULL,
};

ATTRIBUTE_GROUPS(stm);

static struct class stm_class = {
        .name           = "stm",
        .dev_groups     = stm_groups,
};

static int stm_dev_match(struct device *dev, const void *data)
{
        const char *name = data;

        return sysfs_streq(name, dev_name(dev));
}

/**
 * stm_find_device() - find stm device by name
 * @buf:        character buffer containing the name
 *
 * This is called when either policy gets assigned to an stm device or an
 * stm_source device gets linked to an stm device.
 *
 * This grabs device's reference (get_device()) and module reference, both
 * of which the calling path needs to make sure to drop with stm_put_device().
 *
 * Return:      stm device pointer or null if lookup failed.
 */
struct stm_device *stm_find_device(const char *buf)
{
        struct stm_device *stm;
        struct device *dev;

        if (!stm_core_up)
                return NULL;

        dev = class_find_device(&stm_class, NULL, buf, stm_dev_match);
        if (!dev)
                return NULL;

        stm = to_stm_device(dev);
        if (!try_module_get(stm->owner)) {
                /* matches class_find_device() above */
                put_device(dev);
                return NULL;
        }

        return stm;
}

/**
 * stm_put_device() - drop references on the stm device
 * @stm:        stm device, previously acquired by stm_find_device()
 *
 * This drops the module reference and device reference taken by
 * stm_find_device() or stm_char_open().
 */
void stm_put_device(struct stm_device *stm)
{
        module_put(stm->owner);
        put_device(&stm->dev);
}

/*
 * Internally we only care about software-writable masters here, that is the
 * ones in the range [stm_data->sw_start..stm_data..sw_end], however we need
 * original master numbers to be visible externally, since they are the ones
 * that will appear in the STP stream. Thus, the internal bookkeeping uses
 * $master - stm_data->sw_start to reference master descriptors and such.
 */

#define __stm_master(_s, _m)                            \
        ((_s)->masters[(_m) - (_s)->data->sw_start])

static inline struct stp_master *
stm_master(struct stm_device *stm, unsigned int idx)
{
        if (idx < stm->data->sw_start || idx > stm->data->sw_end)
                return NULL;

        return __stm_master(stm, idx);
}

static int stp_master_alloc(struct stm_device *stm, unsigned int idx)
{
        struct stp_master *master;

        master = kzalloc(struct_size(master, chan_map,
                                     BITS_TO_LONGS(stm->data->sw_nchannels)),
                         GFP_ATOMIC);
        if (!master)
                return -ENOMEM;

        master->nr_free = stm->data->sw_nchannels;
        __stm_master(stm, idx) = master;

        return 0;
}

static void stp_master_free(struct stm_device *stm, unsigned int idx)
{
        struct stp_master *master = stm_master(stm, idx);

        if (!master)
                return;

        __stm_master(stm, idx) = NULL;
        kfree(master);
}

static void stm_output_claim(struct stm_device *stm, struct stm_output *output)
{
        struct stp_master *master = stm_master(stm, output->master);

        lockdep_assert_held(&stm->mc_lock);
        lockdep_assert_held(&output->lock);

        if (WARN_ON_ONCE(master->nr_free < output->nr_chans))
                return;

        bitmap_allocate_region(&master->chan_map[0], output->channel,
                               ilog2(output->nr_chans));

        master->nr_free -= output->nr_chans;
}

static void
stm_output_disclaim(struct stm_device *stm, struct stm_output *output)
{
        struct stp_master *master = stm_master(stm, output->master);

        lockdep_assert_held(&stm->mc_lock);
        lockdep_assert_held(&output->lock);

        bitmap_release_region(&master->chan_map[0], output->channel,
                              ilog2(output->nr_chans));

        master->nr_free += output->nr_chans;
        output->nr_chans = 0;
}

/*
 * This is like bitmap_find_free_region(), except it can ignore @start bits
 * at the beginning.
 */
static int find_free_channels(unsigned long *bitmap, unsigned int start,
                              unsigned int end, unsigned int width)
{
        unsigned int pos;
        int i;

        for (pos = start; pos < end + 1; pos = ALIGN(pos, width)) {
                pos = find_next_zero_bit(bitmap, end + 1, pos);
                if (pos + width > end + 1)
                        break;

                if (pos & (width - 1))
                        continue;

                for (i = 1; i < width && !test_bit(pos + i, bitmap); i++)
                        ;
                if (i == width)
                        return pos;

                /* step over [pos..pos+i) to continue search */
                pos += i;
        }

        return -1;
}

static int
stm_find_master_chan(struct stm_device *stm, unsigned int width,
                     unsigned int *mstart, unsigned int mend,
                     unsigned int *cstart, unsigned int cend)
{
        struct stp_master *master;
        unsigned int midx;
        int pos, err;

        for (midx = *mstart; midx <= mend; midx++) {
                if (!stm_master(stm, midx)) {
                        err = stp_master_alloc(stm, midx);
                        if (err)
                                return err;
                }

                master = stm_master(stm, midx);

                if (!master->nr_free)
                        continue;

                pos = find_free_channels(master->chan_map, *cstart, cend,
                                         width);
                if (pos < 0)
                        continue;

                *mstart = midx;
                *cstart = pos;
                return 0;
        }

        return -ENOSPC;
}

static int stm_output_assign(struct stm_device *stm, unsigned int width,
                             struct stp_policy_node *policy_node,
                             struct stm_output *output)
{
        unsigned int midx, cidx, mend, cend;
        int ret = -EINVAL;

        if (width > stm->data->sw_nchannels)
                return -EINVAL;

        if (policy_node) {
                stp_policy_node_get_ranges(policy_node,
                                           &midx, &mend, &cidx, &cend);
        } else {
                midx = stm->data->sw_start;
                cidx = 0;
                mend = stm->data->sw_end;
                cend = stm->data->sw_nchannels - 1;
        }

        spin_lock(&stm->mc_lock);
        spin_lock(&output->lock);
        /* output is already assigned -- shouldn't happen */
        if (WARN_ON_ONCE(output->nr_chans))
                goto unlock;

        ret = stm_find_master_chan(stm, width, &midx, mend, &cidx, cend);
        if (ret < 0)
                goto unlock;

        output->master = midx;
        output->channel = cidx;
        output->nr_chans = width;
        stm_output_claim(stm, output);
        dev_dbg(&stm->dev, "assigned %u:%u (+%u)\n", midx, cidx, width);

        ret = 0;
unlock:
        spin_unlock(&output->lock);
        spin_unlock(&stm->mc_lock);

        return ret;
}

static void stm_output_free(struct stm_device *stm, struct stm_output *output)
{
        spin_lock(&stm->mc_lock);
        spin_lock(&output->lock);
        if (output->nr_chans)
                stm_output_disclaim(stm, output);
        spin_unlock(&output->lock);
        spin_unlock(&stm->mc_lock);
}

static void stm_output_init(struct stm_output *output)
{
        spin_lock_init(&output->lock);
}

static int major_match(struct device *dev, const void *data)
{
        unsigned int major = *(unsigned int *)data;

        return MAJOR(dev->devt) == major;
}

static int stm_char_open(struct inode *inode, struct file *file)
{
        struct stm_file *stmf;
        struct device *dev;
        unsigned int major = imajor(inode);
        int err = -ENOMEM;

        dev = class_find_device(&stm_class, NULL, &major, major_match);
        if (!dev)
                return -ENODEV;

        stmf = kzalloc(sizeof(*stmf), GFP_KERNEL);
        if (!stmf)
                goto err_put_device;

        err = -ENODEV;
        stm_output_init(&stmf->output);
        stmf->stm = to_stm_device(dev);

        if (!try_module_get(stmf->stm->owner))
                goto err_free;

        file->private_data = stmf;

        return nonseekable_open(inode, file);

err_free:
        kfree(stmf);
err_put_device:
        /* matches class_find_device() above */
        put_device(dev);

        return err;
}

static int stm_char_release(struct inode *inode, struct file *file)
{
        struct stm_file *stmf = file->private_data;
        struct stm_device *stm = stmf->stm;

        if (stm->data->unlink)
                stm->data->unlink(stm->data, stmf->output.master,
                                  stmf->output.channel);

        stm_output_free(stm, &stmf->output);

        /*
         * matches the stm_char_open()'s
         * class_find_device() + try_module_get()
         */
        stm_put_device(stm);
        kfree(stmf);

        return 0;
}

static int stm_file_assign(struct stm_file *stmf, char *id, unsigned int width)
{
        struct stm_device *stm = stmf->stm;
        int ret;

        stmf->policy_node = stp_policy_node_lookup(stm, id);

        ret = stm_output_assign(stm, width, stmf->policy_node, &stmf->output);

        if (stmf->policy_node)
                stp_policy_node_put(stmf->policy_node);

        return ret;
}

static ssize_t notrace stm_write(struct stm_data *data, unsigned int master,
                          unsigned int channel, const char *buf, size_t count)
{
        unsigned int flags = STP_PACKET_TIMESTAMPED;
        const unsigned char *p = buf, nil = 0;
        size_t pos;
        ssize_t sz;

        for (pos = 0, p = buf; count > pos; pos += sz, p += sz) {
                sz = min_t(unsigned int, count - pos, 8);
                sz = data->packet(data, master, channel, STP_PACKET_DATA, flags,
                                  sz, p);
                flags = 0;

                if (sz < 0)
                        break;
        }

        data->packet(data, master, channel, STP_PACKET_FLAG, 0, 0, &nil);

        return pos;
}

static ssize_t stm_char_write(struct file *file, const char __user *buf,
                              size_t count, loff_t *ppos)
{
        struct stm_file *stmf = file->private_data;
        struct stm_device *stm = stmf->stm;
        char *kbuf;
        int err;

        if (count + 1 > PAGE_SIZE)
                count = PAGE_SIZE - 1;

        /*
         * if no m/c have been assigned to this writer up to this
         * point, use "default" policy entry
         */
        if (!stmf->output.nr_chans) {
                err = stm_file_assign(stmf, "default", 1);
                /*
                 * EBUSY means that somebody else just assigned this
                 * output, which is just fine for write()
                 */
                if (err && err != -EBUSY)
                        return err;
        }

        kbuf = kmalloc(count + 1, GFP_KERNEL);
        if (!kbuf)
                return -ENOMEM;

        err = copy_from_user(kbuf, buf, count);
        if (err) {
                kfree(kbuf);
                return -EFAULT;
        }

        pm_runtime_get_sync(&stm->dev);

        count = stm_write(stm->data, stmf->output.master, stmf->output.channel,
                          kbuf, count);

        pm_runtime_mark_last_busy(&stm->dev);
        pm_runtime_put_autosuspend(&stm->dev);
        kfree(kbuf);

        return count;
}

static void stm_mmap_open(struct vm_area_struct *vma)
{
        struct stm_file *stmf = vma->vm_file->private_data;
        struct stm_device *stm = stmf->stm;

        pm_runtime_get(&stm->dev);
}

static void stm_mmap_close(struct vm_area_struct *vma)
{
        struct stm_file *stmf = vma->vm_file->private_data;
        struct stm_device *stm = stmf->stm;

        pm_runtime_mark_last_busy(&stm->dev);
        pm_runtime_put_autosuspend(&stm->dev);
}

static const struct vm_operations_struct stm_mmap_vmops = {
        .open   = stm_mmap_open,
        .close  = stm_mmap_close,
};

static int stm_char_mmap(struct file *file, struct vm_area_struct *vma)
{
        struct stm_file *stmf = file->private_data;
        struct stm_device *stm = stmf->stm;
        unsigned long size, phys;

        if (!stm->data->mmio_addr)
                return -EOPNOTSUPP;

        if (vma->vm_pgoff)
                return -EINVAL;

        size = vma->vm_end - vma->vm_start;

        if (stmf->output.nr_chans * stm->data->sw_mmiosz != size)
                return -EINVAL;

        phys = stm->data->mmio_addr(stm->data, stmf->output.master,
                                    stmf->output.channel,
                                    stmf->output.nr_chans);

        if (!phys)
                return -EINVAL;

        pm_runtime_get_sync(&stm->dev);

        vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
        vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
        vma->vm_ops = &stm_mmap_vmops;
        vm_iomap_memory(vma, phys, size);

        return 0;
}

static int stm_char_policy_set_ioctl(struct stm_file *stmf, void __user *arg)
{
        struct stm_device *stm = stmf->stm;
        struct stp_policy_id *id;
        int ret = -EINVAL, wlimit = 1;
        u32 size;

        if (stmf->output.nr_chans)
                return -EBUSY;

        if (copy_from_user(&size, arg, sizeof(size)))
                return -EFAULT;

        if (size < sizeof(*id) || size >= PATH_MAX + sizeof(*id))
                return -EINVAL;

        /*
         * size + 1 to make sure the .id string at the bottom is terminated,
         * which is also why memdup_user() is not useful here
         */
        id = kzalloc(size + 1, GFP_KERNEL);
        if (!id)
                return -ENOMEM;

        if (copy_from_user(id, arg, size)) {
                ret = -EFAULT;
                goto err_free;
        }

        if (id->__reserved_0 || id->__reserved_1)
                goto err_free;

        if (stm->data->sw_mmiosz)
                wlimit = PAGE_SIZE / stm->data->sw_mmiosz;

        if (id->width < 1 || id->width > wlimit)
                goto err_free;

        ret = stm_file_assign(stmf, id->id, id->width);
        if (ret)
                goto err_free;

        if (stm->data->link)
                ret = stm->data->link(stm->data, stmf->output.master,
                                      stmf->output.channel);

        if (ret)
                stm_output_free(stmf->stm, &stmf->output);

err_free:
        kfree(id);

        return ret;
}

static int stm_char_policy_get_ioctl(struct stm_file *stmf, void __user *arg)
{
        struct stp_policy_id id = {
                .size           = sizeof(id),
                .master         = stmf->output.master,
                .channel        = stmf->output.channel,
                .width          = stmf->output.nr_chans,
                .__reserved_0   = 0,
                .__reserved_1   = 0,
        };

        return copy_to_user(arg, &id, id.size) ? -EFAULT : 0;
}

static long
stm_char_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
        struct stm_file *stmf = file->private_data;
        struct stm_data *stm_data = stmf->stm->data;
        int err = -ENOTTY;
        u64 options;

        switch (cmd) {
        case STP_POLICY_ID_SET:
                err = stm_char_policy_set_ioctl(stmf, (void __user *)arg);
                if (err)
                        return err;

                return stm_char_policy_get_ioctl(stmf, (void __user *)arg);

        case STP_POLICY_ID_GET:
                return stm_char_policy_get_ioctl(stmf, (void __user *)arg);

        case STP_SET_OPTIONS:
                if (copy_from_user(&options, (u64 __user *)arg, sizeof(u64)))
                        return -EFAULT;

                if (stm_data->set_options)
                        err = stm_data->set_options(stm_data,
                                                    stmf->output.master,
                                                    stmf->output.channel,
                                                    stmf->output.nr_chans,
                                                    options);

                break;
        default:
                break;
        }

        return err;
}

#ifdef CONFIG_COMPAT
static long
stm_char_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
        return stm_char_ioctl(file, cmd, (unsigned long)compat_ptr(arg));
}
#else
#define stm_char_compat_ioctl   NULL
#endif

static const struct file_operations stm_fops = {
        .open           = stm_char_open,
        .release        = stm_char_release,
        .write          = stm_char_write,
        .mmap           = stm_char_mmap,
        .unlocked_ioctl = stm_char_ioctl,
        .compat_ioctl   = stm_char_compat_ioctl,
        .llseek         = no_llseek,
};

static void stm_device_release(struct device *dev)
{
        struct stm_device *stm = to_stm_device(dev);

        vfree(stm);
}

int stm_register_device(struct device *parent, struct stm_data *stm_data,
                        struct module *owner)
{
        struct stm_device *stm;
        unsigned int nmasters;
        int err = -ENOMEM;

        if (!stm_core_up)
                return -EPROBE_DEFER;

        if (!stm_data->packet || !stm_data->sw_nchannels)
                return -EINVAL;

        nmasters = stm_data->sw_end - stm_data->sw_start + 1;
        stm = vzalloc(sizeof(*stm) + nmasters * sizeof(void *));
        if (!stm)
                return -ENOMEM;

        stm->major = register_chrdev(0, stm_data->name, &stm_fops);
        if (stm->major < 0)
                goto err_free;

        device_initialize(&stm->dev);
        stm->dev.devt = MKDEV(stm->major, 0);
        stm->dev.class = &stm_class;
        stm->dev.parent = parent;
        stm->dev.release = stm_device_release;

        mutex_init(&stm->link_mutex);
        spin_lock_init(&stm->link_lock);
        INIT_LIST_HEAD(&stm->link_list);

        /* initialize the object before it is accessible via sysfs */
        spin_lock_init(&stm->mc_lock);
        mutex_init(&stm->policy_mutex);
        stm->sw_nmasters = nmasters;
        stm->owner = owner;
        stm->data = stm_data;
        stm_data->stm = stm;

        err = kobject_set_name(&stm->dev.kobj, "%s", stm_data->name);
        if (err)
                goto err_device;

        err = device_add(&stm->dev);
        if (err)
                goto err_device;

        /*
         * Use delayed autosuspend to avoid bouncing back and forth
         * on recurring character device writes, with the initial
         * delay time of 2 seconds.
         */
        pm_runtime_no_callbacks(&stm->dev);
        pm_runtime_use_autosuspend(&stm->dev);
        pm_runtime_set_autosuspend_delay(&stm->dev, 2000);
        pm_runtime_set_suspended(&stm->dev);
        pm_runtime_enable(&stm->dev);

        return 0;

err_device:
        unregister_chrdev(stm->major, stm_data->name);

        /* matches device_initialize() above */
        put_device(&stm->dev);
err_free:
        vfree(stm);

        return err;
}
EXPORT_SYMBOL_GPL(stm_register_device);

static int __stm_source_link_drop(struct stm_source_device *src,
                                  struct stm_device *stm);

void stm_unregister_device(struct stm_data *stm_data)
{
        struct stm_device *stm = stm_data->stm;
        struct stm_source_device *src, *iter;
        int i, ret;

        pm_runtime_dont_use_autosuspend(&stm->dev);
        pm_runtime_disable(&stm->dev);

        mutex_lock(&stm->link_mutex);
        list_for_each_entry_safe(src, iter, &stm->link_list, link_entry) {
                ret = __stm_source_link_drop(src, stm);
                /*
                 * src <-> stm link must not change under the same
                 * stm::link_mutex, so complain loudly if it has;
                 * also in this situation ret!=0 means this src is
                 * not connected to this stm and it should be otherwise
                 * safe to proceed with the tear-down of stm.
                 */
                WARN_ON_ONCE(ret);
        }
        mutex_unlock(&stm->link_mutex);

        synchronize_srcu(&stm_source_srcu);

        unregister_chrdev(stm->major, stm_data->name);

        mutex_lock(&stm->policy_mutex);
        if (stm->policy)
                stp_policy_unbind(stm->policy);
        mutex_unlock(&stm->policy_mutex);

        for (i = stm->data->sw_start; i <= stm->data->sw_end; i++)
                stp_master_free(stm, i);

        device_unregister(&stm->dev);
        stm_data->stm = NULL;
}
EXPORT_SYMBOL_GPL(stm_unregister_device);

/*
 * stm::link_list access serialization uses a spinlock and a mutex; holding
 * either of them guarantees that the list is stable; modification requires
 * holding both of them.
 *
 * Lock ordering is as follows:
 *   stm::link_mutex
 *     stm::link_lock
 *       src::link_lock
 */

/**
 * stm_source_link_add() - connect an stm_source device to an stm device
 * @src:        stm_source device
 * @stm:        stm device
 *
 * This function establishes a link from stm_source to an stm device so that
 * the former can send out trace data to the latter.
 *
 * Return:      0 on success, -errno otherwise.
 */
static int stm_source_link_add(struct stm_source_device *src,
                               struct stm_device *stm)
{
        char *id;
        int err;

        mutex_lock(&stm->link_mutex);
        spin_lock(&stm->link_lock);
        spin_lock(&src->link_lock);

        /* src->link is dereferenced under stm_source_srcu but not the list */
        rcu_assign_pointer(src->link, stm);
        list_add_tail(&src->link_entry, &stm->link_list);

        spin_unlock(&src->link_lock);
        spin_unlock(&stm->link_lock);
        mutex_unlock(&stm->link_mutex);

        id = kstrdup(src->data->name, GFP_KERNEL);
        if (id) {
                src->policy_node =
                        stp_policy_node_lookup(stm, id);

                kfree(id);
        }

        err = stm_output_assign(stm, src->data->nr_chans,
                                src->policy_node, &src->output);

        if (src->policy_node)
                stp_policy_node_put(src->policy_node);

        if (err)
                goto fail_detach;

        /* this is to notify the STM device that a new link has been made */
        if (stm->data->link)
                err = stm->data->link(stm->data, src->output.master,
                                      src->output.channel);

        if (err)
                goto fail_free_output;

        /* this is to let the source carry out all necessary preparations */
        if (src->data->link)
                src->data->link(src->data);

        return 0;

fail_free_output:
        stm_output_free(stm, &src->output);

fail_detach:
        mutex_lock(&stm->link_mutex);
        spin_lock(&stm->link_lock);
        spin_lock(&src->link_lock);

        rcu_assign_pointer(src->link, NULL);
        list_del_init(&src->link_entry);

        spin_unlock(&src->link_lock);
        spin_unlock(&stm->link_lock);
        mutex_unlock(&stm->link_mutex);

        return err;
}

/**
 * __stm_source_link_drop() - detach stm_source from an stm device
 * @src:        stm_source device
 * @stm:        stm device
 *
 * If @stm is @src::link, disconnect them from one another and put the
 * reference on the @stm device.
 *
 * Caller must hold stm::link_mutex.
 */
static int __stm_source_link_drop(struct stm_source_device *src,
                                  struct stm_device *stm)
{
        struct stm_device *link;
        int ret = 0;

        lockdep_assert_held(&stm->link_mutex);

        /* for stm::link_list modification, we hold both mutex and spinlock */
        spin_lock(&stm->link_lock);
        spin_lock(&src->link_lock);
        link = srcu_dereference_check(src->link, &stm_source_srcu, 1);

        /*
         * The linked device may have changed since we last looked, because
         * we weren't holding the src::link_lock back then; if this is the
         * case, tell the caller to retry.
         */
        if (link != stm) {
                ret = -EAGAIN;
                goto unlock;
        }

        stm_output_free(link, &src->output);
        list_del_init(&src->link_entry);
        pm_runtime_mark_last_busy(&link->dev);
        pm_runtime_put_autosuspend(&link->dev);
        /* matches stm_find_device() from stm_source_link_store() */
        stm_put_device(link);
        rcu_assign_pointer(src->link, NULL);

unlock:
        spin_unlock(&src->link_lock);
        spin_unlock(&stm->link_lock);

        /*
         * Call the unlink callbacks for both source and stm, when we know
         * that we have actually performed the unlinking.
         */
        if (!ret) {
                if (src->data->unlink)
                        src->data->unlink(src->data);

                if (stm->data->unlink)
                        stm->data->unlink(stm->data, src->output.master,
                                          src->output.channel);
        }

        return ret;
}

/**
 * stm_source_link_drop() - detach stm_source from its stm device
 * @src:        stm_source device
 *
 * Unlinking means disconnecting from source's STM device; after this
 * writes will be unsuccessful until it is linked to a new STM device.
 *
 * This will happen on "stm_source_link" sysfs attribute write to undo
 * the existing link (if any), or on linked STM device's de-registration.
 */
static void stm_source_link_drop(struct stm_source_device *src)
{
        struct stm_device *stm;
        int idx, ret;

retry:
        idx = srcu_read_lock(&stm_source_srcu);
        /*
         * The stm device will be valid for the duration of this
         * read section, but the link may change before we grab
         * the src::link_lock in __stm_source_link_drop().
         */
        stm = srcu_dereference(src->link, &stm_source_srcu);

        ret = 0;
        if (stm) {
                mutex_lock(&stm->link_mutex);
                ret = __stm_source_link_drop(src, stm);
                mutex_unlock(&stm->link_mutex);
        }

        srcu_read_unlock(&stm_source_srcu, idx);

        /* if it did change, retry */
        if (ret == -EAGAIN)
                goto retry;
}

static ssize_t stm_source_link_show(struct device *dev,
                                    struct device_attribute *attr,
                                    char *buf)
{
        struct stm_source_device *src = to_stm_source_device(dev);
        struct stm_device *stm;
        int idx, ret;

        idx = srcu_read_lock(&stm_source_srcu);
        stm = srcu_dereference(src->link, &stm_source_srcu);
        ret = sprintf(buf, "%s\n",
                      stm ? dev_name(&stm->dev) : "<none>");
        srcu_read_unlock(&stm_source_srcu, idx);

        return ret;
}

static ssize_t stm_source_link_store(struct device *dev,
                                     struct device_attribute *attr,
                                     const char *buf, size_t count)
{
        struct stm_source_device *src = to_stm_source_device(dev);
        struct stm_device *link;
        int err;

        stm_source_link_drop(src);

        link = stm_find_device(buf);
        if (!link)
                return -EINVAL;

        pm_runtime_get(&link->dev);

        err = stm_source_link_add(src, link);
        if (err) {
                pm_runtime_put_autosuspend(&link->dev);
                /* matches the stm_find_device() above */
                stm_put_device(link);
        }

        return err ? : count;
}

static DEVICE_ATTR_RW(stm_source_link);

static struct attribute *stm_source_attrs[] = {
        &dev_attr_stm_source_link.attr,
        NULL,
};

ATTRIBUTE_GROUPS(stm_source);

static struct class stm_source_class = {
        .name           = "stm_source",
        .dev_groups     = stm_source_groups,
};

static void stm_source_device_release(struct device *dev)
{
        struct stm_source_device *src = to_stm_source_device(dev);

        kfree(src);
}

/**
 * stm_source_register_device() - register an stm_source device
 * @parent:     parent device
 * @data:       device description structure
 *
 * This will create a device of stm_source class that can write
 * data to an stm device once linked.
 *
 * Return:      0 on success, -errno otherwise.
 */
int stm_source_register_device(struct device *parent,
                               struct stm_source_data *data)
{
        struct stm_source_device *src;
        int err;

        if (!stm_core_up)
                return -EPROBE_DEFER;

        src = kzalloc(sizeof(*src), GFP_KERNEL);
        if (!src)
                return -ENOMEM;

        device_initialize(&src->dev);
        src->dev.class = &stm_source_class;
        src->dev.parent = parent;
        src->dev.release = stm_source_device_release;

        err = kobject_set_name(&src->dev.kobj, "%s", data->name);
        if (err)
                goto err;

        pm_runtime_no_callbacks(&src->dev);
        pm_runtime_forbid(&src->dev);

        err = device_add(&src->dev);
        if (err)
                goto err;

        stm_output_init(&src->output);
        spin_lock_init(&src->link_lock);
        INIT_LIST_HEAD(&src->link_entry);
        src->data = data;
        data->src = src;

        return 0;

err:
        put_device(&src->dev);

        return err;
}
EXPORT_SYMBOL_GPL(stm_source_register_device);

/**
 * stm_source_unregister_device() - unregister an stm_source device
 * @data:       device description that was used to register the device
 *
 * This will remove a previously created stm_source device from the system.
 */
void stm_source_unregister_device(struct stm_source_data *data)
{
        struct stm_source_device *src = data->src;

        stm_source_link_drop(src);

        device_unregister(&src->dev);
}
EXPORT_SYMBOL_GPL(stm_source_unregister_device);

int notrace stm_source_write(struct stm_source_data *data,
                             unsigned int chan,
                             const char *buf, size_t count)
{
        struct stm_source_device *src = data->src;
        struct stm_device *stm;
        int idx;

        if (!src->output.nr_chans)
                return -ENODEV;

        if (chan >= src->output.nr_chans)
                return -EINVAL;

        idx = srcu_read_lock(&stm_source_srcu);

        stm = srcu_dereference(src->link, &stm_source_srcu);
        if (stm)
                count = stm_write(stm->data, src->output.master,
                                  src->output.channel + chan,
                                  buf, count);
        else
                count = -ENODEV;

        srcu_read_unlock(&stm_source_srcu, idx);

        return count;
}
EXPORT_SYMBOL_GPL(stm_source_write);

static int __init stm_core_init(void)
{
        int err;

        err = class_register(&stm_class);
        if (err)
                return err;

        err = class_register(&stm_source_class);
        if (err)
                goto err_stm;

        err = stp_configfs_init();
        if (err)
                goto err_src;

        init_srcu_struct(&stm_source_srcu);

        stm_core_up++;

        return 0;

err_src:
        class_unregister(&stm_source_class);
err_stm:
        class_unregister(&stm_class);

        return err;
}

module_init(stm_core_init);

static void __exit stm_core_exit(void)
{
        cleanup_srcu_struct(&stm_source_srcu);
        class_unregister(&stm_source_class);
        class_unregister(&stm_class);
        stp_configfs_exit();
}

module_exit(stm_core_exit);

MODULE_LICENSE("GPL v2");
MODULE_DESCRIPTION("System Trace Module device class");
MODULE_AUTHOR("Alexander Shishkin <alexander.shishkin@linux.intel.com>");