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1 Linux Kernel 2.6 series
2 SCSI mid_level - lower_level driver interface
3 =============================================
5 Introduction
6 ============
7 This document outlines the interface between the Linux SCSI mid level and
8 SCSI lower level drivers. Lower level drivers (LLDs) are variously called
9 host bus adapter (HBA) drivers and host drivers (HD). A "host" in this
10 context is a bridge between a computer IO bus (e.g. PCI or ISA) and a
11 single SCSI initiator port on a SCSI transport. An "initiator" port
12 (SCSI terminology, see SAM-3 at http://www.t10.org) sends SCSI commands
13 to "target" SCSI ports (e.g. disks). There can be many LLDs in a running
14 system, but only one per hardware type. Most LLDs can control one or more
15 SCSI HBAs. Some HBAs contain multiple hosts.
17 In some cases the SCSI transport is an external bus that already has
18 its own subsystem in Linux (e.g. USB and ieee1394). In such cases the
19 SCSI subsystem LLD is a software bridge to the other driver subsystem.
20 Examples are the usb-storage driver (found in the drivers/usb/storage
21 directory) and the ieee1394/sbp2 driver (found in the drivers/ieee1394
22 directory).
24 For example, the aic7xxx LLD controls Adaptec SCSI parallel interface
25 (SPI) controllers based on that company's 7xxx chip series. The aic7xxx
26 LLD can be built into the kernel or loaded as a module. There can only be
27 one aic7xxx LLD running in a Linux system but it may be controlling many
28 HBAs. These HBAs might be either on PCI daughter-boards or built into
29 the motherboard (or both). Some aic7xxx based HBAs are dual controllers
30 and thus represent two hosts. Like most modern HBAs, each aic7xxx host
31 has its own PCI device address. [The one-to-one correspondence between
32 a SCSI host and a PCI device is common but not required (e.g. with
33 ISA adapters).]
35 The SCSI mid level isolates an LLD from other layers such as the SCSI
36 upper layer drivers and the block layer.
38 This version of the document roughly matches linux kernel version 2.6.8 .
40 Documentation
41 =============
42 There is a SCSI documentation directory within the kernel source tree,
43 typically Documentation/scsi . Most documents are in plain
44 (i.e. ASCII) text. This file is named scsi_mid_low_api.txt and can be
45 found in that directory. A more recent copy of this document may be found
46 at http://web.archive.org/web/20070107183357rn_1/sg.torque.net/scsi/.
47 Many LLDs are documented there (e.g. aic7xxx.txt). The SCSI mid-level is
48 briefly described in scsi.txt which contains a url to a document
49 describing the SCSI subsystem in the lk 2.4 series. Two upper level
50 drivers have documents in that directory: st.txt (SCSI tape driver) and
51 scsi-generic.txt (for the sg driver).
53 Some documentation (or urls) for LLDs may be found in the C source code
54 or in the same directory as the C source code. For example to find a url
55 about the USB mass storage driver see the
56 /usr/src/linux/drivers/usb/storage directory.
58 Driver structure
59 ================
60 Traditionally an LLD for the SCSI subsystem has been at least two files in
61 the drivers/scsi directory. For example, a driver called "xyz" has a header
62 file "xyz.h" and a source file "xyz.c". [Actually there is no good reason
63 why this couldn't all be in one file; the header file is superfluous.] Some
64 drivers that have been ported to several operating systems have more than
65 two files. For example the aic7xxx driver has separate files for generic
66 and OS-specific code (e.g. FreeBSD and Linux). Such drivers tend to have
67 their own directory under the drivers/scsi directory.
69 When a new LLD is being added to Linux, the following files (found in the
70 drivers/scsi directory) will need some attention: Makefile and Kconfig .
71 It is probably best to study how existing LLDs are organized.
73 As the 2.5 series development kernels evolve into the 2.6 series
74 production series, changes are being introduced into this interface. An
75 example of this is driver initialization code where there are now 2 models
76 available. The older one, similar to what was found in the lk 2.4 series,
77 is based on hosts that are detected at HBA driver load time. This will be
78 referred to the "passive" initialization model. The newer model allows HBAs
79 to be hot plugged (and unplugged) during the lifetime of the LLD and will
80 be referred to as the "hotplug" initialization model. The newer model is
81 preferred as it can handle both traditional SCSI equipment that is
82 permanently connected as well as modern "SCSI" devices (e.g. USB or
83 IEEE 1394 connected digital cameras) that are hotplugged. Both
84 initialization models are discussed in the following sections.
86 An LLD interfaces to the SCSI subsystem several ways:
87 a) directly invoking functions supplied by the mid level
88 b) passing a set of function pointers to a registration function
89 supplied by the mid level. The mid level will then invoke these
90 functions at some point in the future. The LLD will supply
91 implementations of these functions.
92 c) direct access to instances of well known data structures maintained
93 by the mid level
95 Those functions in group a) are listed in a section entitled "Mid level
96 supplied functions" below.
98 Those functions in group b) are listed in a section entitled "Interface
99 functions" below. Their function pointers are placed in the members of
100 "struct scsi_host_template", an instance of which is passed to
101 scsi_host_alloc() ** . Those interface functions that the LLD does not
102 wish to supply should have NULL placed in the corresponding member of
103 struct scsi_host_template. Defining an instance of struct
104 scsi_host_template at file scope will cause NULL to be placed in function
105 pointer members not explicitly initialized.
107 Those usages in group c) should be handled with care, especially in a
108 "hotplug" environment. LLDs should be aware of the lifetime of instances
109 that are shared with the mid level and other layers.
111 All functions defined within an LLD and all data defined at file scope
112 should be static. For example the slave_alloc() function in an LLD
113 called "xxx" could be defined as
114 "static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }"
116 ** the scsi_host_alloc() function is a replacement for the rather vaguely
117 named scsi_register() function in most situations. The scsi_register()
118 and scsi_unregister() functions remain to support legacy LLDs that use
119 the passive initialization model.
122 Hotplug initialization model
123 ============================
124 In this model an LLD controls when SCSI hosts are introduced and removed
125 from the SCSI subsystem. Hosts can be introduced as early as driver
126 initialization and removed as late as driver shutdown. Typically a driver
127 will respond to a sysfs probe() callback that indicates an HBA has been
128 detected. After confirming that the new device is one that the LLD wants
129 to control, the LLD will initialize the HBA and then register a new host
130 with the SCSI mid level.
132 During LLD initialization the driver should register itself with the
133 appropriate IO bus on which it expects to find HBA(s) (e.g. the PCI bus).
134 This can probably be done via sysfs. Any driver parameters (especially
135 those that are writable after the driver is loaded) could also be
136 registered with sysfs at this point. The SCSI mid level first becomes
137 aware of an LLD when that LLD registers its first HBA.
139 At some later time, the LLD becomes aware of an HBA and what follows
140 is a typical sequence of calls between the LLD and the mid level.
141 This example shows the mid level scanning the newly introduced HBA for 3
142 scsi devices of which only the first 2 respond:
144 HBA PROBE: assume 2 SCSI devices found in scan
145 LLD mid level LLD
146 ===-------------------=========--------------------===------
147 scsi_host_alloc() -->
148 scsi_add_host() ---->
149 scsi_scan_host() -------+
150 |
151 slave_alloc()
152 slave_configure() --> scsi_adjust_queue_depth()
153 |
154 slave_alloc()
155 slave_configure()
156 |
157 slave_alloc() ***
158 slave_destroy() ***
159 ------------------------------------------------------------
161 If the LLD wants to adjust the default queue settings, it can invoke
162 scsi_adjust_queue_depth() in its slave_configure() routine.
164 *** For scsi devices that the mid level tries to scan but do not
165 respond, a slave_alloc(), slave_destroy() pair is called.
167 When an HBA is being removed it could be as part of an orderly shutdown
168 associated with the LLD module being unloaded (e.g. with the "rmmod"
169 command) or in response to a "hot unplug" indicated by sysfs()'s
170 remove() callback being invoked. In either case, the sequence is the
171 same:
173 HBA REMOVE: assume 2 SCSI devices attached
174 LLD mid level LLD
175 ===----------------------=========-----------------===------
176 scsi_remove_host() ---------+
177 |
178 slave_destroy()
179 slave_destroy()
180 scsi_host_put()
181 ------------------------------------------------------------
183 It may be useful for a LLD to keep track of struct Scsi_Host instances
184 (a pointer is returned by scsi_host_alloc()). Such instances are "owned"
185 by the mid-level. struct Scsi_Host instances are freed from
186 scsi_host_put() when the reference count hits zero.
188 Hot unplugging an HBA that controls a disk which is processing SCSI
189 commands on a mounted file system is an interesting situation. Reference
190 counting logic is being introduced into the mid level to cope with many
191 of the issues involved. See the section on reference counting below.
194 The hotplug concept may be extended to SCSI devices. Currently, when an
195 HBA is added, the scsi_scan_host() function causes a scan for SCSI devices
196 attached to the HBA's SCSI transport. On newer SCSI transports the HBA
197 may become aware of a new SCSI device _after_ the scan has completed.
198 An LLD can use this sequence to make the mid level aware of a SCSI device:
200 SCSI DEVICE hotplug
201 LLD mid level LLD
202 ===-------------------=========--------------------===------
203 scsi_add_device() ------+
204 |
205 slave_alloc()
206 slave_configure() [--> scsi_adjust_queue_depth()]
207 ------------------------------------------------------------
209 In a similar fashion, an LLD may become aware that a SCSI device has been
210 removed (unplugged) or the connection to it has been interrupted. Some
211 existing SCSI transports (e.g. SPI) may not become aware that a SCSI
212 device has been removed until a subsequent SCSI command fails which will
213 probably cause that device to be set offline by the mid level. An LLD that
214 detects the removal of a SCSI device can instigate its removal from
215 upper layers with this sequence:
217 SCSI DEVICE hot unplug
218 LLD mid level LLD
219 ===----------------------=========-----------------===------
220 scsi_remove_device() -------+
221 |
222 slave_destroy()
223 ------------------------------------------------------------
225 It may be useful for an LLD to keep track of struct scsi_device instances
226 (a pointer is passed as the parameter to slave_alloc() and
227 slave_configure() callbacks). Such instances are "owned" by the mid-level.
228 struct scsi_device instances are freed after slave_destroy().
231 Passive initialization model
232 ============================
233 These older LLDs include a file called "scsi_module.c" [yes the ".c" is a
234 little surprising] in their source code. For that file to work an
235 instance of struct scsi_host_template with the name "driver_template"
236 needs to be defined. Here is a typical code sequence used in this model:
237 static struct scsi_host_template driver_template = {
238 ...
239 };
240 #include "scsi_module.c"
242 The scsi_module.c file contains two functions:
243 - init_this_scsi_driver() which is executed when the LLD is
244 initialized (i.e. boot time or module load time)
245 - exit_this_scsi_driver() which is executed when the LLD is shut
246 down (i.e. module unload time)
247 Note: since these functions are tagged with __init and __exit qualifiers
248 an LLD should not call them explicitly (since the kernel does that).
250 Here is an example of an initialization sequence when two hosts are
251 detected (so detect() returns 2) and the SCSI bus scan on each host
252 finds 1 SCSI device (and a second device does not respond).
254 LLD mid level LLD
255 ===----------------------=========-----------------===------
256 init_this_scsi_driver() ----+
257 |
258 detect() -----------------+
259 | |
260 | scsi_register()
261 | scsi_register()
262 |
263 slave_alloc()
264 slave_configure() --> scsi_adjust_queue_depth()
265 slave_alloc() ***
266 slave_destroy() ***
267 |
268 slave_alloc()
269 slave_configure()
270 slave_alloc() ***
271 slave_destroy() ***
272 ------------------------------------------------------------
274 The mid level invokes scsi_adjust_queue_depth() with tagged queuing off and
275 "cmd_per_lun" for that host as the queue length. These settings can be
276 overridden by a slave_configure() supplied by the LLD.
278 *** For scsi devices that the mid level tries to scan but do not
279 respond, a slave_alloc(), slave_destroy() pair is called.
281 Here is an LLD shutdown sequence:
283 LLD mid level LLD
284 ===----------------------=========-----------------===------
285 exit_this_scsi_driver() ----+
286 |
287 slave_destroy()
288 release() --> scsi_unregister()
289 |
290 slave_destroy()
291 release() --> scsi_unregister()
292 ------------------------------------------------------------
294 An LLD need not define slave_destroy() (i.e. it is optional).
296 The shortcoming of the "passive initialization model" is that host
297 registration and de-registration are (typically) tied to LLD initialization
298 and shutdown. Once the LLD is initialized then a new host that appears
299 (e.g. via hotplugging) cannot easily be added without a redundant
300 driver shutdown and re-initialization. It may be possible to write an LLD
301 that uses both initialization models.
304 Reference Counting
305 ==================
306 The Scsi_Host structure has had reference counting infrastructure added.
307 This effectively spreads the ownership of struct Scsi_Host instances
308 across the various SCSI layers which use them. Previously such instances
309 were exclusively owned by the mid level. LLDs would not usually need to
310 directly manipulate these reference counts but there may be some cases
311 where they do.
313 There are 3 reference counting functions of interest associated with
314 struct Scsi_Host:
315 - scsi_host_alloc(): returns a pointer to new instance of struct
316 Scsi_Host which has its reference count ^^ set to 1
317 - scsi_host_get(): adds 1 to the reference count of the given instance
318 - scsi_host_put(): decrements 1 from the reference count of the given
319 instance. If the reference count reaches 0 then the given instance
320 is freed
322 The Scsi_device structure has had reference counting infrastructure added.
323 This effectively spreads the ownership of struct Scsi_device instances
324 across the various SCSI layers which use them. Previously such instances
325 were exclusively owned by the mid level. See the access functions declared
326 towards the end of include/scsi/scsi_device.h . If an LLD wants to keep
327 a copy of a pointer to a Scsi_device instance it should use scsi_device_get()
328 to bump its reference count. When it is finished with the pointer it can
329 use scsi_device_put() to decrement its reference count (and potentially
330 delete it).
332 ^^ struct Scsi_Host actually has 2 reference counts which are manipulated
333 in parallel by these functions.
336 Conventions
337 ===========
338 First, Linus Torvalds's thoughts on C coding style can be found in the
339 Documentation/CodingStyle file.
341 Next, there is a movement to "outlaw" typedefs introducing synonyms for
342 struct tags. Both can be still found in the SCSI subsystem, but
343 the typedefs have been moved to a single file, scsi_typedefs.h to
344 make their future removal easier, for example:
345 "typedef struct scsi_cmnd Scsi_Cmnd;"
347 Also, most C99 enhancements are encouraged to the extent they are supported
348 by the relevant gcc compilers. So C99 style structure and array
349 initializers are encouraged where appropriate. Don't go too far,
350 VLAs are not properly supported yet. An exception to this is the use of
351 "//" style comments; /*...*/ comments are still preferred in Linux.
353 Well written, tested and documented code, need not be re-formatted to
354 comply with the above conventions. For example, the aic7xxx driver
355 comes to Linux from FreeBSD and Adaptec's own labs. No doubt FreeBSD
356 and Adaptec have their own coding conventions.
359 Mid level supplied functions
360 ============================
361 These functions are supplied by the SCSI mid level for use by LLDs.
362 The names (i.e. entry points) of these functions are exported
363 so an LLD that is a module can access them. The kernel will
364 arrange for the SCSI mid level to be loaded and initialized before any LLD
365 is initialized. The functions below are listed alphabetically and their
366 names all start with "scsi_".
368 Summary:
369 scsi_activate_tcq - turn on tag command queueing
370 scsi_add_device - creates new scsi device (lu) instance
371 scsi_add_host - perform sysfs registration and set up transport class
372 scsi_adjust_queue_depth - change the queue depth on a SCSI device
373 scsi_bios_ptable - return copy of block device's partition table
374 scsi_block_requests - prevent further commands being queued to given host
375 scsi_deactivate_tcq - turn off tag command queueing
376 scsi_host_alloc - return a new scsi_host instance whose refcount==1
377 scsi_host_get - increments Scsi_Host instance's refcount
378 scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
379 scsi_partsize - parse partition table into cylinders, heads + sectors
380 scsi_register - create and register a scsi host adapter instance.
381 scsi_remove_device - detach and remove a SCSI device
382 scsi_remove_host - detach and remove all SCSI devices owned by host
383 scsi_report_bus_reset - report scsi _bus_ reset observed
384 scsi_scan_host - scan SCSI bus
385 scsi_track_queue_full - track successive QUEUE_FULL events
386 scsi_unblock_requests - allow further commands to be queued to given host
387 scsi_unregister - [calls scsi_host_put()]
390 Details:
392 /**
393 * scsi_activate_tcq - turn on tag command queueing ("ordered" task attribute)
394 * @sdev: device to turn on TCQ for
395 * @depth: queue depth
396 *
397 * Returns nothing
398 *
399 * Might block: no
400 *
401 * Notes: Eventually, it is hoped depth would be the maximum depth
402 * the device could cope with and the real queue depth
403 * would be adjustable from 0 to depth.
404 *
405 * Defined (inline) in: include/scsi/scsi_tcq.h
406 **/
407 void scsi_activate_tcq(struct scsi_device *sdev, int depth)
410 /**
411 * scsi_add_device - creates new scsi device (lu) instance
412 * @shost: pointer to scsi host instance
413 * @channel: channel number (rarely other than 0)
414 * @id: target id number
415 * @lun: logical unit number
416 *
417 * Returns pointer to new struct scsi_device instance or
418 * ERR_PTR(-ENODEV) (or some other bent pointer) if something is
419 * wrong (e.g. no lu responds at given address)
420 *
421 * Might block: yes
422 *
423 * Notes: This call is usually performed internally during a scsi
424 * bus scan when an HBA is added (i.e. scsi_scan_host()). So it
425 * should only be called if the HBA becomes aware of a new scsi
426 * device (lu) after scsi_scan_host() has completed. If successful
427 * this call can lead to slave_alloc() and slave_configure() callbacks
428 * into the LLD.
429 *
430 * Defined in: drivers/scsi/scsi_scan.c
431 **/
432 struct scsi_device * scsi_add_device(struct Scsi_Host *shost,
433 unsigned int channel,
434 unsigned int id, unsigned int lun)
437 /**
438 * scsi_add_host - perform sysfs registration and set up transport class
439 * @shost: pointer to scsi host instance
440 * @dev: pointer to struct device of type scsi class
441 *
442 * Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
443 *
444 * Might block: no
445 *
446 * Notes: Only required in "hotplug initialization model" after a
447 * successful call to scsi_host_alloc(). This function does not
448 * scan the bus; this can be done by calling scsi_scan_host() or
449 * in some other transport-specific way. The LLD must set up
450 * the transport template before calling this function and may only
451 * access the transport class data after this function has been called.
452 *
453 * Defined in: drivers/scsi/hosts.c
454 **/
455 int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
458 /**
459 * scsi_adjust_queue_depth - allow LLD to change queue depth on a SCSI device
460 * @sdev: pointer to SCSI device to change queue depth on
461 * @tagged: 0 - no tagged queuing
462 * MSG_SIMPLE_TAG - simple tagged queuing
463 * MSG_ORDERED_TAG - ordered tagged queuing
464 * @tags Number of tags allowed if tagged queuing enabled,
465 * or number of commands the LLD can queue up
466 * in non-tagged mode (as per cmd_per_lun).
467 *
468 * Returns nothing
469 *
470 * Might block: no
471 *
472 * Notes: Can be invoked any time on a SCSI device controlled by this
473 * LLD. [Specifically during and after slave_configure() and prior to
474 * slave_destroy().] Can safely be invoked from interrupt code. Actual
475 * queue depth change may be delayed until the next command is being
476 * processed. See also scsi_activate_tcq() and scsi_deactivate_tcq().
477 *
478 * Defined in: drivers/scsi/scsi.c [see source code for more notes]
479 *
480 **/
481 void scsi_adjust_queue_depth(struct scsi_device * sdev, int tagged,
482 int tags)
485 /**
486 * scsi_bios_ptable - return copy of block device's partition table
487 * @dev: pointer to block device
488 *
489 * Returns pointer to partition table, or NULL for failure
490 *
491 * Might block: yes
492 *
493 * Notes: Caller owns memory returned (free with kfree() )
494 *
495 * Defined in: drivers/scsi/scsicam.c
496 **/
497 unsigned char *scsi_bios_ptable(struct block_device *dev)
500 /**
501 * scsi_block_requests - prevent further commands being queued to given host
502 *
503 * @shost: pointer to host to block commands on
504 *
505 * Returns nothing
506 *
507 * Might block: no
508 *
509 * Notes: There is no timer nor any other means by which the requests
510 * get unblocked other than the LLD calling scsi_unblock_requests().
511 *
512 * Defined in: drivers/scsi/scsi_lib.c
513 **/
514 void scsi_block_requests(struct Scsi_Host * shost)
517 /**
518 * scsi_deactivate_tcq - turn off tag command queueing
519 * @sdev: device to turn off TCQ for
520 * @depth: queue depth (stored in sdev)
521 *
522 * Returns nothing
523 *
524 * Might block: no
525 *
526 * Defined (inline) in: include/scsi/scsi_tcq.h
527 **/
528 void scsi_deactivate_tcq(struct scsi_device *sdev, int depth)
531 /**
532 * scsi_host_alloc - create a scsi host adapter instance and perform basic
533 * initialization.
534 * @sht: pointer to scsi host template
535 * @privsize: extra bytes to allocate in hostdata array (which is the
536 * last member of the returned Scsi_Host instance)
537 *
538 * Returns pointer to new Scsi_Host instance or NULL on failure
539 *
540 * Might block: yes
541 *
542 * Notes: When this call returns to the LLD, the SCSI bus scan on
543 * this host has _not_ yet been done.
544 * The hostdata array (by default zero length) is a per host scratch
545 * area for the LLD's exclusive use.
546 * Both associated refcounting objects have their refcount set to 1.
547 * Full registration (in sysfs) and a bus scan are performed later when
548 * scsi_add_host() and scsi_scan_host() are called.
549 *
550 * Defined in: drivers/scsi/hosts.c .
551 **/
552 struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
553 int privsize)
556 /**
557 * scsi_host_get - increment Scsi_Host instance refcount
558 * @shost: pointer to struct Scsi_Host instance
559 *
560 * Returns nothing
561 *
562 * Might block: currently may block but may be changed to not block
563 *
564 * Notes: Actually increments the counts in two sub-objects
565 *
566 * Defined in: drivers/scsi/hosts.c
567 **/
568 void scsi_host_get(struct Scsi_Host *shost)
571 /**
572 * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
573 * @shost: pointer to struct Scsi_Host instance
574 *
575 * Returns nothing
576 *
577 * Might block: currently may block but may be changed to not block
578 *
579 * Notes: Actually decrements the counts in two sub-objects. If the
580 * latter refcount reaches 0, the Scsi_Host instance is freed.
581 * The LLD need not worry exactly when the Scsi_Host instance is
582 * freed, it just shouldn't access the instance after it has balanced
583 * out its refcount usage.
584 *
585 * Defined in: drivers/scsi/hosts.c
586 **/
587 void scsi_host_put(struct Scsi_Host *shost)
590 /**
591 * scsi_partsize - parse partition table into cylinders, heads + sectors
592 * @buf: pointer to partition table
593 * @capacity: size of (total) disk in 512 byte sectors
594 * @cyls: outputs number of cylinders calculated via this pointer
595 * @hds: outputs number of heads calculated via this pointer
596 * @secs: outputs number of sectors calculated via this pointer
597 *
598 * Returns 0 on success, -1 on failure
599 *
600 * Might block: no
601 *
602 * Notes: Caller owns memory returned (free with kfree() )
603 *
604 * Defined in: drivers/scsi/scsicam.c
605 **/
606 int scsi_partsize(unsigned char *buf, unsigned long capacity,
607 unsigned int *cyls, unsigned int *hds, unsigned int *secs)
610 /**
611 * scsi_register - create and register a scsi host adapter instance.
612 * @sht: pointer to scsi host template
613 * @privsize: extra bytes to allocate in hostdata array (which is the
614 * last member of the returned Scsi_Host instance)
615 *
616 * Returns pointer to new Scsi_Host instance or NULL on failure
617 *
618 * Might block: yes
619 *
620 * Notes: When this call returns to the LLD, the SCSI bus scan on
621 * this host has _not_ yet been done.
622 * The hostdata array (by default zero length) is a per host scratch
623 * area for the LLD.
624 *
625 * Defined in: drivers/scsi/hosts.c .
626 **/
627 struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
628 int privsize)
631 /**
632 * scsi_remove_device - detach and remove a SCSI device
633 * @sdev: a pointer to a scsi device instance
634 *
635 * Returns value: 0 on success, -EINVAL if device not attached
636 *
637 * Might block: yes
638 *
639 * Notes: If an LLD becomes aware that a scsi device (lu) has
640 * been removed but its host is still present then it can request
641 * the removal of that scsi device. If successful this call will
642 * lead to the slave_destroy() callback being invoked. sdev is an
643 * invalid pointer after this call.
644 *
645 * Defined in: drivers/scsi/scsi_sysfs.c .
646 **/
647 int scsi_remove_device(struct scsi_device *sdev)
650 /**
651 * scsi_remove_host - detach and remove all SCSI devices owned by host
652 * @shost: a pointer to a scsi host instance
653 *
654 * Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
655 *
656 * Might block: yes
657 *
658 * Notes: Should only be invoked if the "hotplug initialization
659 * model" is being used. It should be called _prior_ to
660 * scsi_unregister().
661 *
662 * Defined in: drivers/scsi/hosts.c .
663 **/
664 int scsi_remove_host(struct Scsi_Host *shost)
667 /**
668 * scsi_report_bus_reset - report scsi _bus_ reset observed
669 * @shost: a pointer to a scsi host involved
670 * @channel: channel (within) host on which scsi bus reset occurred
671 *
672 * Returns nothing
673 *
674 * Might block: no
675 *
676 * Notes: This only needs to be called if the reset is one which
677 * originates from an unknown location. Resets originated by the
678 * mid level itself don't need to call this, but there should be
679 * no harm. The main purpose of this is to make sure that a
680 * CHECK_CONDITION is properly treated.
681 *
682 * Defined in: drivers/scsi/scsi_error.c .
683 **/
684 void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
687 /**
688 * scsi_scan_host - scan SCSI bus
689 * @shost: a pointer to a scsi host instance
690 *
691 * Might block: yes
692 *
693 * Notes: Should be called after scsi_add_host()
694 *
695 * Defined in: drivers/scsi/scsi_scan.c
696 **/
697 void scsi_scan_host(struct Scsi_Host *shost)
700 /**
701 * scsi_track_queue_full - track successive QUEUE_FULL events on given
702 * device to determine if and when there is a need
703 * to adjust the queue depth on the device.
704 * @sdev: pointer to SCSI device instance
705 * @depth: Current number of outstanding SCSI commands on this device,
706 * not counting the one returned as QUEUE_FULL.
707 *
708 * Returns 0 - no change needed
709 * >0 - adjust queue depth to this new depth
710 * -1 - drop back to untagged operation using host->cmd_per_lun
711 * as the untagged command depth
712 *
713 * Might block: no
714 *
715 * Notes: LLDs may call this at any time and we will do "The Right
716 * Thing"; interrupt context safe.
717 *
718 * Defined in: drivers/scsi/scsi.c .
719 **/
720 int scsi_track_queue_full(struct scsi_device *sdev, int depth)
723 /**
724 * scsi_unblock_requests - allow further commands to be queued to given host
725 *
726 * @shost: pointer to host to unblock commands on
727 *
728 * Returns nothing
729 *
730 * Might block: no
731 *
732 * Defined in: drivers/scsi/scsi_lib.c .
733 **/
734 void scsi_unblock_requests(struct Scsi_Host * shost)
737 /**
738 * scsi_unregister - unregister and free memory used by host instance
739 * @shp: pointer to scsi host instance to unregister.
740 *
741 * Returns nothing
742 *
743 * Might block: no
744 *
745 * Notes: Should not be invoked if the "hotplug initialization
746 * model" is being used. Called internally by exit_this_scsi_driver()
747 * in the "passive initialization model". Hence a LLD has no need to
748 * call this function directly.
749 *
750 * Defined in: drivers/scsi/hosts.c .
751 **/
752 void scsi_unregister(struct Scsi_Host * shp)
757 Interface Functions
758 ===================
759 Interface functions are supplied (defined) by LLDs and their function
760 pointers are placed in an instance of struct scsi_host_template which
761 is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
762 Some are mandatory. Interface functions should be declared static. The
763 accepted convention is that driver "xyz" will declare its slave_configure()
764 function as:
765 static int xyz_slave_configure(struct scsi_device * sdev);
766 and so forth for all interface functions listed below.
768 A pointer to this function should be placed in the 'slave_configure' member
769 of a "struct scsi_host_template" instance. A pointer to such an instance
770 should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
771 init_this_scsi_driver()].
773 The interface functions are also described in the include/scsi/scsi_host.h
774 file immediately above their definition point in "struct scsi_host_template".
775 In some cases more detail is given in scsi_host.h than below.
777 The interface functions are listed below in alphabetical order.
779 Summary:
780 bios_param - fetch head, sector, cylinder info for a disk
781 detect - detects HBAs this driver wants to control
782 eh_timed_out - notify the host that a command timer expired
783 eh_abort_handler - abort given command
784 eh_bus_reset_handler - issue SCSI bus reset
785 eh_device_reset_handler - issue SCSI device reset
786 eh_host_reset_handler - reset host (host bus adapter)
787 info - supply information about given host
788 ioctl - driver can respond to ioctls
789 proc_info - supports /proc/scsi/{driver_name}/{host_no}
790 queuecommand - queue scsi command, invoke 'done' on completion
791 release - release all resources associated with given host
792 slave_alloc - prior to any commands being sent to a new device
793 slave_configure - driver fine tuning for given device after attach
794 slave_destroy - given device is about to be shut down
797 Details:
799 /**
800 * bios_param - fetch head, sector, cylinder info for a disk
801 * @sdev: pointer to scsi device context (defined in
802 * include/scsi/scsi_device.h)
803 * @bdev: pointer to block device context (defined in fs.h)
804 * @capacity: device size (in 512 byte sectors)
805 * @params: three element array to place output:
806 * params[0] number of heads (max 255)
807 * params[1] number of sectors (max 63)
808 * params[2] number of cylinders
809 *
810 * Return value is ignored
811 *
812 * Locks: none
813 *
814 * Calling context: process (sd)
815 *
816 * Notes: an arbitrary geometry (based on READ CAPACITY) is used
817 * if this function is not provided. The params array is
818 * pre-initialized with made up values just in case this function
819 * doesn't output anything.
820 *
821 * Optionally defined in: LLD
822 **/
823 int bios_param(struct scsi_device * sdev, struct block_device *bdev,
824 sector_t capacity, int params[3])
827 /**
828 * detect - detects HBAs this driver wants to control
829 * @shtp: host template for this driver.
830 *
831 * Returns number of hosts this driver wants to control. 0 means no
832 * suitable hosts found.
833 *
834 * Locks: none held
835 *
836 * Calling context: process [invoked from init_this_scsi_driver()]
837 *
838 * Notes: First function called from the SCSI mid level on this
839 * driver. Upper level drivers (e.g. sd) may not (yet) be present.
840 * For each host found, this method should call scsi_register()
841 * [see hosts.c].
842 *
843 * Defined in: LLD (required if "passive initialization mode" is used,
844 * not invoked in "hotplug initialization mode")
845 **/
846 int detect(struct scsi_host_template * shtp)
849 /**
850 * eh_timed_out - The timer for the command has just fired
851 * @scp: identifies command timing out
852 *
853 * Returns:
854 *
855 * EH_HANDLED: I fixed the error, please complete the command
856 * EH_RESET_TIMER: I need more time, reset the timer and
857 * begin counting again
858 * EH_NOT_HANDLED Begin normal error recovery
859 *
860 *
861 * Locks: None held
862 *
863 * Calling context: interrupt
864 *
865 * Notes: This is to give the LLD an opportunity to do local recovery.
866 * This recovery is limited to determining if the outstanding command
867 * will ever complete. You may not abort and restart the command from
868 * this callback.
869 *
870 * Optionally defined in: LLD
871 **/
872 int eh_timed_out(struct scsi_cmnd * scp)
875 /**
876 * eh_abort_handler - abort command associated with scp
877 * @scp: identifies command to be aborted
878 *
879 * Returns SUCCESS if command aborted else FAILED
880 *
881 * Locks: None held
882 *
883 * Calling context: kernel thread
884 *
885 * Notes: If 'no_async_abort' is defined this callback
886 * will be invoked from scsi_eh thread. No other commands
887 * will then be queued on current host during eh.
888 * Otherwise it will be called whenever scsi_times_out()
889 * is called due to a command timeout.
890 *
891 * Optionally defined in: LLD
892 **/
893 int eh_abort_handler(struct scsi_cmnd * scp)
896 /**
897 * eh_bus_reset_handler - issue SCSI bus reset
898 * @scp: SCSI bus that contains this device should be reset
899 *
900 * Returns SUCCESS if command aborted else FAILED
901 *
902 * Locks: None held
903 *
904 * Calling context: kernel thread
905 *
906 * Notes: Invoked from scsi_eh thread. No other commands will be
907 * queued on current host during eh.
908 *
909 * Optionally defined in: LLD
910 **/
911 int eh_bus_reset_handler(struct scsi_cmnd * scp)
914 /**
915 * eh_device_reset_handler - issue SCSI device reset
916 * @scp: identifies SCSI device to be reset
917 *
918 * Returns SUCCESS if command aborted else FAILED
919 *
920 * Locks: None held
921 *
922 * Calling context: kernel thread
923 *
924 * Notes: Invoked from scsi_eh thread. No other commands will be
925 * queued on current host during eh.
926 *
927 * Optionally defined in: LLD
928 **/
929 int eh_device_reset_handler(struct scsi_cmnd * scp)
932 /**
933 * eh_host_reset_handler - reset host (host bus adapter)
934 * @scp: SCSI host that contains this device should be reset
935 *
936 * Returns SUCCESS if command aborted else FAILED
937 *
938 * Locks: None held
939 *
940 * Calling context: kernel thread
941 *
942 * Notes: Invoked from scsi_eh thread. No other commands will be
943 * queued on current host during eh.
944 * With the default eh_strategy in place, if none of the _abort_,
945 * _device_reset_, _bus_reset_ or this eh handler function are
946 * defined (or they all return FAILED) then the device in question
947 * will be set offline whenever eh is invoked.
948 *
949 * Optionally defined in: LLD
950 **/
951 int eh_host_reset_handler(struct scsi_cmnd * scp)
954 /**
955 * info - supply information about given host: driver name plus data
956 * to distinguish given host
957 * @shp: host to supply information about
958 *
959 * Return ASCII null terminated string. [This driver is assumed to
960 * manage the memory pointed to and maintain it, typically for the
961 * lifetime of this host.]
962 *
963 * Locks: none
964 *
965 * Calling context: process
966 *
967 * Notes: Often supplies PCI or ISA information such as IO addresses
968 * and interrupt numbers. If not supplied struct Scsi_Host::name used
969 * instead. It is assumed the returned information fits on one line
970 * (i.e. does not included embedded newlines).
971 * The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
972 * function (or struct Scsi_Host::name if this function is not
973 * available).
974 * In a similar manner, init_this_scsi_driver() outputs to the console
975 * each host's "info" (or name) for the driver it is registering.
976 * Also if proc_info() is not supplied, the output of this function
977 * is used instead.
978 *
979 * Optionally defined in: LLD
980 **/
981 const char * info(struct Scsi_Host * shp)
984 /**
985 * ioctl - driver can respond to ioctls
986 * @sdp: device that ioctl was issued for
987 * @cmd: ioctl number
988 * @arg: pointer to read or write data from. Since it points to
989 * user space, should use appropriate kernel functions
990 * (e.g. copy_from_user() ). In the Unix style this argument
991 * can also be viewed as an unsigned long.
992 *
993 * Returns negative "errno" value when there is a problem. 0 or a
994 * positive value indicates success and is returned to the user space.
995 *
996 * Locks: none
997 *
998 * Calling context: process
999 *
1000 * Notes: The SCSI subsystem uses a "trickle down" ioctl model.
1001 * The user issues an ioctl() against an upper level driver
1002 * (e.g. /dev/sdc) and if the upper level driver doesn't recognize
1003 * the 'cmd' then it is passed to the SCSI mid level. If the SCSI
1004 * mid level does not recognize it, then the LLD that controls
1005 * the device receives the ioctl. According to recent Unix standards
1006 * unsupported ioctl() 'cmd' numbers should return -ENOTTY.
1007 *
1008 * Optionally defined in: LLD
1009 **/
1010 int ioctl(struct scsi_device *sdp, int cmd, void *arg)
1013 /**
1014 * proc_info - supports /proc/scsi/{driver_name}/{host_no}
1015 * @buffer: anchor point to output to (0==writeto1_read0) or fetch from
1016 * (1==writeto1_read0).
1017 * @start: where "interesting" data is written to. Ignored when
1018 * 1==writeto1_read0.
1019 * @offset: offset within buffer 0==writeto1_read0 is actually
1020 * interested in. Ignored when 1==writeto1_read0 .
1021 * @length: maximum (or actual) extent of buffer
1022 * @host_no: host number of interest (struct Scsi_Host::host_no)
1023 * @writeto1_read0: 1 -> data coming from user space towards driver
1024 * (e.g. "echo some_string > /proc/scsi/xyz/2")
1025 * 0 -> user what data from this driver
1026 * (e.g. "cat /proc/scsi/xyz/2")
1027 *
1028 * Returns length when 1==writeto1_read0. Otherwise number of chars
1029 * output to buffer past offset.
1030 *
1031 * Locks: none held
1032 *
1033 * Calling context: process
1034 *
1035 * Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
1036 * support can now be configured out of the scsi subsystem.
1037 *
1038 * Optionally defined in: LLD
1039 **/
1040 int proc_info(char * buffer, char ** start, off_t offset,
1041 int length, int host_no, int writeto1_read0)
1044 /**
1045 * queuecommand - queue scsi command, invoke scp->scsi_done on completion
1046 * @shost: pointer to the scsi host object
1047 * @scp: pointer to scsi command object
1048 *
1049 * Returns 0 on success.
1050 *
1051 * If there's a failure, return either:
1052 *
1053 * SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
1054 * SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
1055 *
1056 * On both of these returns, the mid-layer will requeue the I/O
1057 *
1058 * - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
1059 * device will be paused, and it will be unpaused when a command to
1060 * the device returns (or after a brief delay if there are no more
1061 * outstanding commands to it). Commands to other devices continue
1062 * to be processed normally.
1063 *
1064 * - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
1065 * is paused and will be unpaused when any command returns from
1066 * the host (or after a brief delay if there are no outstanding
1067 * commands to the host).
1068 *
1069 * For compatibility with earlier versions of queuecommand, any
1070 * other return value is treated the same as
1071 * SCSI_MLQUEUE_HOST_BUSY.
1072 *
1073 * Other types of errors that are detected immediately may be
1074 * flagged by setting scp->result to an appropriate value,
1075 * invoking the scp->scsi_done callback, and then returning 0
1076 * from this function. If the command is not performed
1077 * immediately (and the LLD is starting (or will start) the given
1078 * command) then this function should place 0 in scp->result and
1079 * return 0.
1080 *
1081 * Command ownership. If the driver returns zero, it owns the
1082 * command and must take responsibility for ensuring the
1083 * scp->scsi_done callback is executed. Note: the driver may
1084 * call scp->scsi_done before returning zero, but after it has
1085 * called scp->scsi_done, it may not return any value other than
1086 * zero. If the driver makes a non-zero return, it must not
1087 * execute the command's scsi_done callback at any time.
1088 *
1089 * Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
1090 * held on entry (with "irqsave") and is expected to be
1091 * held on return. From 2.6.37 onwards, queuecommand is
1092 * called without any locks held.
1093 *
1094 * Calling context: in interrupt (soft irq) or process context
1095 *
1096 * Notes: This function should be relatively fast. Normally it
1097 * will not wait for IO to complete. Hence the scp->scsi_done
1098 * callback is invoked (often directly from an interrupt service
1099 * routine) some time after this function has returned. In some
1100 * cases (e.g. pseudo adapter drivers that manufacture the
1101 * response to a SCSI INQUIRY) the scp->scsi_done callback may be
1102 * invoked before this function returns. If the scp->scsi_done
1103 * callback is not invoked within a certain period the SCSI mid
1104 * level will commence error processing. If a status of CHECK
1105 * CONDITION is placed in "result" when the scp->scsi_done
1106 * callback is invoked, then the LLD driver should perform
1107 * autosense and fill in the struct scsi_cmnd::sense_buffer
1108 * array. The scsi_cmnd::sense_buffer array is zeroed prior to
1109 * the mid level queuing a command to an LLD.
1110 *
1111 * Defined in: LLD
1112 **/
1113 int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
1116 /**
1117 * release - release all resources associated with given host
1118 * @shp: host to be released.
1119 *
1120 * Return value ignored (could soon be a function returning void).
1121 *
1122 * Locks: none held
1123 *
1124 * Calling context: process
1125 *
1126 * Notes: Invoked from scsi_module.c's exit_this_scsi_driver().
1127 * LLD's implementation of this function should call
1128 * scsi_unregister(shp) prior to returning.
1129 * Only needed for old-style host templates.
1130 *
1131 * Defined in: LLD (required in "passive initialization model",
1132 * should not be defined in hotplug model)
1133 **/
1134 int release(struct Scsi_Host * shp)
1137 /**
1138 * slave_alloc - prior to any commands being sent to a new device
1139 * (i.e. just prior to scan) this call is made
1140 * @sdp: pointer to new device (about to be scanned)
1141 *
1142 * Returns 0 if ok. Any other return is assumed to be an error and
1143 * the device is ignored.
1144 *
1145 * Locks: none
1146 *
1147 * Calling context: process
1148 *
1149 * Notes: Allows the driver to allocate any resources for a device
1150 * prior to its initial scan. The corresponding scsi device may not
1151 * exist but the mid level is just about to scan for it (i.e. send
1152 * and INQUIRY command plus ...). If a device is found then
1153 * slave_configure() will be called while if a device is not found
1154 * slave_destroy() is called.
1155 * For more details see the include/scsi/scsi_host.h file.
1156 *
1157 * Optionally defined in: LLD
1158 **/
1159 int slave_alloc(struct scsi_device *sdp)
1162 /**
1163 * slave_configure - driver fine tuning for given device just after it
1164 * has been first scanned (i.e. it responded to an
1165 * INQUIRY)
1166 * @sdp: device that has just been attached
1167 *
1168 * Returns 0 if ok. Any other return is assumed to be an error and
1169 * the device is taken offline. [offline devices will _not_ have
1170 * slave_destroy() called on them so clean up resources.]
1171 *
1172 * Locks: none
1173 *
1174 * Calling context: process
1175 *
1176 * Notes: Allows the driver to inspect the response to the initial
1177 * INQUIRY done by the scanning code and take appropriate action.
1178 * For more details see the include/scsi/scsi_host.h file.
1179 *
1180 * Optionally defined in: LLD
1181 **/
1182 int slave_configure(struct scsi_device *sdp)
1185 /**
1186 * slave_destroy - given device is about to be shut down. All
1187 * activity has ceased on this device.
1188 * @sdp: device that is about to be shut down
1189 *
1190 * Returns nothing
1191 *
1192 * Locks: none
1193 *
1194 * Calling context: process
1195 *
1196 * Notes: Mid level structures for given device are still in place
1197 * but are about to be torn down. Any per device resources allocated
1198 * by this driver for given device should be freed now. No further
1199 * commands will be sent for this sdp instance. [However the device
1200 * could be re-attached in the future in which case a new instance
1201 * of struct scsi_device would be supplied by future slave_alloc()
1202 * and slave_configure() calls.]
1203 *
1204 * Optionally defined in: LLD
1205 **/
1206 void slave_destroy(struct scsi_device *sdp)
1210 Data Structures
1211 ===============
1212 struct scsi_host_template
1213 -------------------------
1214 There is one "struct scsi_host_template" instance per LLD ***. It is
1215 typically initialized as a file scope static in a driver's header file. That
1216 way members that are not explicitly initialized will be set to 0 or NULL.
1217 Member of interest:
1218 name - name of driver (may contain spaces, please limit to
1219 less than 80 characters)
1220 proc_name - name used in "/proc/scsi/<proc_name>/<host_no>" and
1221 by sysfs in one of its "drivers" directories. Hence
1222 "proc_name" should only contain characters acceptable
1223 to a Unix file name.
1224 (*queuecommand)() - primary callback that the mid level uses to inject
1225 SCSI commands into an LLD.
1226 The structure is defined and commented in include/scsi/scsi_host.h
1228 *** In extreme situations a single driver may have several instances
1229 if it controls several different classes of hardware (e.g. an LLD
1230 that handles both ISA and PCI cards and has a separate instance of
1231 struct scsi_host_template for each class).
1233 struct Scsi_Host
1234 ----------------
1235 There is one struct Scsi_Host instance per host (HBA) that an LLD
1236 controls. The struct Scsi_Host structure has many members in common
1237 with "struct scsi_host_template". When a new struct Scsi_Host instance
1238 is created (in scsi_host_alloc() in hosts.c) those common members are
1239 initialized from the driver's struct scsi_host_template instance. Members
1240 of interest:
1241 host_no - system wide unique number that is used for identifying
1242 this host. Issued in ascending order from 0.
1243 can_queue - must be greater than 0; do not send more than can_queue
1244 commands to the adapter.
1245 this_id - scsi id of host (scsi initiator) or -1 if not known
1246 sg_tablesize - maximum scatter gather elements allowed by host.
1247 0 implies scatter gather not supported by host
1248 max_sectors - maximum number of sectors (usually 512 bytes) allowed
1249 in a single SCSI command. The default value of 0 leads
1250 to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
1251 scsi_host.h) which is currently set to 1024. So for a
1252 disk the maximum transfer size is 512 KB when max_sectors
1253 is not defined. Note that this size may not be sufficient
1254 for disk firmware uploads.
1255 cmd_per_lun - maximum number of commands that can be queued on devices
1256 controlled by the host. Overridden by LLD calls to
1257 scsi_adjust_queue_depth().
1258 unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
1259 restriction), 0=>can use full 32 bit (or better) DMA
1260 address space
1261 use_clustering - 1=>SCSI commands in mid level's queue can be merged,
1262 0=>disallow SCSI command merging
1263 no_async_abort - 1=>Asynchronous aborts are not supported
1264 0=>Timed-out commands will be aborted asynchronously
1265 hostt - pointer to driver's struct scsi_host_template from which
1266 this struct Scsi_Host instance was spawned
1267 hostt->proc_name - name of LLD. This is the driver name that sysfs uses
1268 transportt - pointer to driver's struct scsi_transport_template instance
1269 (if any). FC and SPI transports currently supported.
1270 sh_list - a double linked list of pointers to all struct Scsi_Host
1271 instances (currently ordered by ascending host_no)
1272 my_devices - a double linked list of pointers to struct scsi_device
1273 instances that belong to this host.
1274 hostdata[0] - area reserved for LLD at end of struct Scsi_Host. Size
1275 is set by the second argument (named 'xtr_bytes') to
1276 scsi_host_alloc() or scsi_register().
1277 vendor_id - a unique value that identifies the vendor supplying
1278 the LLD for the Scsi_Host. Used most often in validating
1279 vendor-specific message requests. Value consists of an
1280 identifier type and a vendor-specific value.
1281 See scsi_netlink.h for a description of valid formats.
1283 The scsi_host structure is defined in include/scsi/scsi_host.h
1285 struct scsi_device
1286 ------------------
1287 Generally, there is one instance of this structure for each SCSI logical unit
1288 on a host. Scsi devices connected to a host are uniquely identified by a
1289 channel number, target id and logical unit number (lun).
1290 The structure is defined in include/scsi/scsi_device.h
1292 struct scsi_cmnd
1293 ----------------
1294 Instances of this structure convey SCSI commands to the LLD and responses
1295 back to the mid level. The SCSI mid level will ensure that no more SCSI
1296 commands become queued against the LLD than are indicated by
1297 scsi_adjust_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
1298 be at least one instance of struct scsi_cmnd available for each SCSI device.
1299 Members of interest:
1300 cmnd - array containing SCSI command
1301 cmnd_len - length (in bytes) of SCSI command
1302 sc_data_direction - direction of data transfer in data phase. See
1303 "enum dma_data_direction" in include/linux/dma-mapping.h
1304 request_bufflen - number of data bytes to transfer (0 if no data phase)
1305 use_sg - ==0 -> no scatter gather list, hence transfer data
1306 to/from request_buffer
1307 - >0 -> scatter gather list (actually an array) in
1308 request_buffer with use_sg elements
1309 request_buffer - either contains data buffer or scatter gather list
1310 depending on the setting of use_sg. Scatter gather
1311 elements are defined by 'struct scatterlist' found
1312 in include/asm/scatterlist.h .
1313 done - function pointer that should be invoked by LLD when the
1314 SCSI command is completed (successfully or otherwise).
1315 Should only be called by an LLD if the LLD has accepted
1316 the command (i.e. queuecommand() returned or will return
1317 0). The LLD may invoke 'done' prior to queuecommand()
1318 finishing.
1319 result - should be set by LLD prior to calling 'done'. A value
1320 of 0 implies a successfully completed command (and all
1321 data (if any) has been transferred to or from the SCSI
1322 target device). 'result' is a 32 bit unsigned integer that
1323 can be viewed as 4 related bytes. The SCSI status value is
1324 in the LSB. See include/scsi/scsi.h status_byte(),
1325 msg_byte(), host_byte() and driver_byte() macros and
1326 related constants.
1327 sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
1328 should be written when the SCSI status (LSB of 'result')
1329 is set to CHECK_CONDITION (2). When CHECK_CONDITION is
1330 set, if the top nibble of sense_buffer[0] has the value 7
1331 then the mid level will assume the sense_buffer array
1332 contains a valid SCSI sense buffer; otherwise the mid
1333 level will issue a REQUEST_SENSE SCSI command to
1334 retrieve the sense buffer. The latter strategy is error
1335 prone in the presence of command queuing so the LLD should
1336 always "auto-sense".
1337 device - pointer to scsi_device object that this command is
1338 associated with.
1339 resid - an LLD should set this signed integer to the requested
1340 transfer length (i.e. 'request_bufflen') less the number
1341 of bytes that are actually transferred. 'resid' is
1342 preset to 0 so an LLD can ignore it if it cannot detect
1343 underruns (overruns should be rare). If possible an LLD
1344 should set 'resid' prior to invoking 'done'. The most
1345 interesting case is data transfers from a SCSI target
1346 device (e.g. READs) that underrun.
1347 underflow - LLD should place (DID_ERROR << 16) in 'result' if
1348 actual number of bytes transferred is less than this
1349 figure. Not many LLDs implement this check and some that
1350 do just output an error message to the log rather than
1351 report a DID_ERROR. Better for an LLD to implement
1352 'resid'.
1354 It is recommended that a LLD set 'resid' on data transfers from a SCSI
1355 target device (e.g. READs). It is especially important that 'resid' is set
1356 when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
1357 (and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
1358 data has been received then the safest approach is to indicate no bytes have
1359 been received. For example: to indicate that no valid data has been received
1360 a LLD might use these helpers:
1361 scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
1362 where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
1363 bytes blocks has been received 'resid' could be set like this:
1364 scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
1366 The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
1369 Locks
1370 =====
1371 Each struct Scsi_Host instance has a spin_lock called struct
1372 Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in
1373 hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
1374 is initialized to point at default_lock. Thereafter lock and unlock
1375 operations performed by the mid level use the struct Scsi_Host::host_lock
1376 pointer. Previously drivers could override the host_lock pointer but
1377 this is not allowed anymore.
1380 Autosense
1381 =========
1382 Autosense (or auto-sense) is defined in the SAM-2 document as "the
1383 automatic return of sense data to the application client coincident
1384 with the completion of a SCSI command" when a status of CHECK CONDITION
1385 occurs. LLDs should perform autosense. This should be done when the LLD
1386 detects a CHECK CONDITION status by either:
1387 a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
1388 to perform an extra data in phase on such responses
1389 b) or, the LLD issuing a REQUEST SENSE command itself
1391 Either way, when a status of CHECK CONDITION is detected, the mid level
1392 decides whether the LLD has performed autosense by checking struct
1393 scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
1394 then autosense is assumed to have taken place. If it has another value (and
1395 this byte is initialized to 0 before each command) then the mid level will
1396 issue a REQUEST SENSE command.
1398 In the presence of queued commands the "nexus" that maintains sense
1399 buffer data from the command that failed until a following REQUEST SENSE
1400 may get out of synchronization. This is why it is best for the LLD
1401 to perform autosense.
1404 Changes since lk 2.4 series
1405 ===========================
1406 io_request_lock has been replaced by several finer grained locks. The lock
1407 relevant to LLDs is struct Scsi_Host::host_lock and there is
1408 one per SCSI host.
1410 The older error handling mechanism has been removed. This means the
1411 LLD interface functions abort() and reset() have been removed.
1412 The struct scsi_host_template::use_new_eh_code flag has been removed.
1414 In the 2.4 series the SCSI subsystem configuration descriptions were
1415 aggregated with the configuration descriptions from all other Linux
1416 subsystems in the Documentation/Configure.help file. In the 2.6 series,
1417 the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
1418 file that contains both configuration and help information.
1420 struct SHT has been renamed to struct scsi_host_template.
1422 Addition of the "hotplug initialization model" and many extra functions
1423 to support it.
1426 Credits
1427 =======
1428 The following people have contributed to this document:
1429 Mike Anderson <andmike at us dot ibm dot com>
1430 James Bottomley <James dot Bottomley at hansenpartnership dot com>
1431 Patrick Mansfield <patmans at us dot ibm dot com>
1432 Christoph Hellwig <hch at infradead dot org>
1433 Doug Ledford <dledford at redhat dot com>
1434 Andries Brouwer <Andries dot Brouwer at cwi dot nl>
1435 Randy Dunlap <rdunlap at xenotime dot net>
1436 Alan Stern <stern at rowland dot harvard dot edu>
1439 Douglas Gilbert
1440 dgilbert at interlog dot com
1441 21st September 2004