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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	                          Linux Kernel 2.6 series
2	                 SCSI mid_level - lower_level driver interface
3	                 =============================================
4	
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.
16	
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).
23	
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).]
34	
35	The SCSI mid level isolates an LLD from other layers such as the SCSI
36	upper layer drivers and the block layer.
37	
38	This version of the document roughly matches linux kernel version 2.6.8 .
39	
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).
52	
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.
57	
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.
68	
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.
72	
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.
85	
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
94	
95	Those functions in group a) are listed in a section entitled "Mid level
96	supplied functions" below.
97	
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.
106	
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.
110	
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 */ }"
115	
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.
120	
121	
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.
131	
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.
138	
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:
143	
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	------------------------------------------------------------
160	
161	If the LLD wants to adjust the default queue settings, it can invoke
162	scsi_adjust_queue_depth() in its slave_configure() routine.
163	
164	*** For scsi devices that the mid level tries to scan but do not
165	    respond, a slave_alloc(), slave_destroy() pair is called.
166	
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:
172	
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	------------------------------------------------------------
182	                     
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.
187	
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.
192	
193	
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:
199	
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	------------------------------------------------------------
208	
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:
216	
217	                  SCSI DEVICE hot unplug
218	LLD                      mid level                 LLD
219	===----------------------=========-----------------===------
220	scsi_remove_device() -------+
221	                            |
222	                     slave_destroy()
223	------------------------------------------------------------
224	
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().
229	
230	
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"
241	
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).
249	
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).
253	
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	------------------------------------------------------------
273	
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.
277	
278	*** For scsi devices that the mid level tries to scan but do not
279	    respond, a slave_alloc(), slave_destroy() pair is called.
280	
281	Here is an LLD shutdown sequence:
282	
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	------------------------------------------------------------
293	
294	An LLD need not define slave_destroy() (i.e. it is optional). 
295	
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.
302	
303	
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.
312	
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
321	
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).
331	
332	^^ struct Scsi_Host actually has 2 reference counts which are manipulated
333	in parallel by these functions.
334	
335	
336	Conventions
337	===========
338	First, Linus Torvalds's thoughts on C coding style can be found in the
339	Documentation/CodingStyle file. 
340	
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;"
346	
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.
352	
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.
357	
358	
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_".
367	
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()]
388	
389	
390	Details:
391	
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)
408	
409	
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)
435	
436	
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)
456	
457	
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)
483	
484	
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)
498	
499	
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)
515	
516	
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)
529	
530	
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)
554	
555	
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)
569	
570	
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)
588	
589	
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)
608	
609	
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)
629	
630	
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)
648	
649	
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)
665	
666	
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)
685	
686	
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)
698	
699	
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)
721	
722	
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)
735	
736	
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)
753	
754	
755	
756	
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.
767	
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()].
772	
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.
776	
777	The interface functions are listed below in alphabetical order.
778	
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
795	
796	
797	Details:
798	
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])
825	
826	
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)
847	
848	
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)
873	
874	
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: Invoked from scsi_eh thread. No other commands will be
886	 *      queued on current host during eh.
887	 *
888	 *      Optionally defined in: LLD
889	 **/
890	     int eh_abort_handler(struct scsi_cmnd * scp)
891	
892	
893	/**
894	 *      eh_bus_reset_handler - issue SCSI bus reset
895	 *      @scp: SCSI bus that contains this device should be reset
896	 *
897	 *      Returns SUCCESS if command aborted else FAILED
898	 *
899	 *      Locks: None held
900	 *
901	 *      Calling context: kernel thread
902	 *
903	 *      Notes: Invoked from scsi_eh thread. No other commands will be
904	 *      queued on current host during eh.
905	 *
906	 *      Optionally defined in: LLD
907	 **/
908	     int eh_bus_reset_handler(struct scsi_cmnd * scp)
909	
910	
911	/**
912	 *      eh_device_reset_handler - issue SCSI device reset
913	 *      @scp: identifies SCSI device to be reset
914	 *
915	 *      Returns SUCCESS if command aborted else FAILED
916	 *
917	 *      Locks: None held
918	 *
919	 *      Calling context: kernel thread
920	 *
921	 *      Notes: Invoked from scsi_eh thread. No other commands will be
922	 *      queued on current host during eh.
923	 *
924	 *      Optionally defined in: LLD
925	 **/
926	     int eh_device_reset_handler(struct scsi_cmnd * scp)
927	
928	
929	/**
930	 *      eh_host_reset_handler - reset host (host bus adapter)
931	 *      @scp: SCSI host that contains this device should be reset
932	 *
933	 *      Returns SUCCESS if command aborted else FAILED
934	 *
935	 *      Locks: None held
936	 *
937	 *      Calling context: kernel thread
938	 *
939	 *      Notes: Invoked from scsi_eh thread. No other commands will be
940	 *      queued on current host during eh. 
941	 *      With the default eh_strategy in place, if none of the _abort_, 
942	 *      _device_reset_, _bus_reset_ or this eh handler function are 
943	 *      defined (or they all return FAILED) then the device in question 
944	 *      will be set offline whenever eh is invoked.
945	 *
946	 *      Optionally defined in: LLD
947	 **/
948	     int eh_host_reset_handler(struct scsi_cmnd * scp)
949	
950	
951	/**
952	 *      info - supply information about given host: driver name plus data
953	 *             to distinguish given host
954	 *      @shp: host to supply information about
955	 *
956	 *      Return ASCII null terminated string. [This driver is assumed to
957	 *      manage the memory pointed to and maintain it, typically for the
958	 *      lifetime of this host.]
959	 *
960	 *      Locks: none
961	 *
962	 *      Calling context: process
963	 *
964	 *      Notes: Often supplies PCI or ISA information such as IO addresses
965	 *      and interrupt numbers. If not supplied struct Scsi_Host::name used
966	 *      instead. It is assumed the returned information fits on one line 
967	 *      (i.e. does not included embedded newlines).
968	 *      The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
969	 *      function (or struct Scsi_Host::name if this function is not
970	 *      available).
971	 *      In a similar manner, init_this_scsi_driver() outputs to the console
972	 *      each host's "info" (or name) for the driver it is registering.
973	 *      Also if proc_info() is not supplied, the output of this function
974	 *      is used instead.
975	 *
976	 *      Optionally defined in: LLD
977	 **/
978	    const char * info(struct Scsi_Host * shp)
979	
980	
981	/**
982	 *      ioctl - driver can respond to ioctls
983	 *      @sdp: device that ioctl was issued for
984	 *      @cmd: ioctl number
985	 *      @arg: pointer to read or write data from. Since it points to
986	 *            user space, should use appropriate kernel functions
987	 *            (e.g. copy_from_user() ). In the Unix style this argument
988	 *            can also be viewed as an unsigned long.
989	 *
990	 *      Returns negative "errno" value when there is a problem. 0 or a
991	 *      positive value indicates success and is returned to the user space.
992	 *
993	 *      Locks: none
994	 *
995	 *      Calling context: process
996	 *
997	 *      Notes: The SCSI subsystem uses a "trickle down" ioctl model.
998	 *      The user issues an ioctl() against an upper level driver
999	 *      (e.g. /dev/sdc) and if the upper level driver doesn't recognize
1000	 *      the 'cmd' then it is passed to the SCSI mid level. If the SCSI
1001	 *      mid level does not recognize it, then the LLD that controls
1002	 *      the device receives the ioctl. According to recent Unix standards
1003	 *      unsupported ioctl() 'cmd' numbers should return -ENOTTY.
1004	 *
1005	 *      Optionally defined in: LLD
1006	 **/
1007	    int ioctl(struct scsi_device *sdp, int cmd, void *arg)
1008	
1009	
1010	/**
1011	 *      proc_info - supports /proc/scsi/{driver_name}/{host_no}
1012	 *      @buffer: anchor point to output to (0==writeto1_read0) or fetch from
1013	 *               (1==writeto1_read0).
1014	 *      @start: where "interesting" data is written to. Ignored when
1015	 *              1==writeto1_read0.
1016	 *      @offset: offset within buffer 0==writeto1_read0 is actually
1017	 *               interested in. Ignored when 1==writeto1_read0 .
1018	 *      @length: maximum (or actual) extent of buffer
1019	 *      @host_no: host number of interest (struct Scsi_Host::host_no)
1020	 *      @writeto1_read0: 1 -> data coming from user space towards driver
1021	 *                            (e.g. "echo some_string > /proc/scsi/xyz/2")
1022	 *                       0 -> user what data from this driver
1023	 *                            (e.g. "cat /proc/scsi/xyz/2")
1024	 *
1025	 *      Returns length when 1==writeto1_read0. Otherwise number of chars
1026	 *      output to buffer past offset.
1027	 *
1028	 *      Locks: none held
1029	 *
1030	 *      Calling context: process
1031	 *
1032	 *      Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
1033	 *      support can now be configured out of the scsi subsystem.
1034	 *
1035	 *      Optionally defined in: LLD
1036	 **/
1037	    int proc_info(char * buffer, char ** start, off_t offset, 
1038	                  int length, int host_no, int writeto1_read0)
1039	
1040	
1041	/**
1042	 *      queuecommand - queue scsi command, invoke scp->scsi_done on completion
1043	 *      @shost: pointer to the scsi host object
1044	 *      @scp: pointer to scsi command object
1045	 *
1046	 *      Returns 0 on success.
1047	 *
1048	 *      If there's a failure, return either:
1049	 *
1050	 *      SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
1051	 *      SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
1052	 *
1053	 *      On both of these returns, the mid-layer will requeue the I/O
1054	 *
1055	 *      - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
1056	 *      device will be paused, and it will be unpaused when a command to
1057	 *      the device returns (or after a brief delay if there are no more
1058	 *      outstanding commands to it).  Commands to other devices continue
1059	 *      to be processed normally.
1060	 *
1061	 *      - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
1062	 *      is paused and will be unpaused when any command returns from
1063	 *      the host (or after a brief delay if there are no outstanding
1064	 *      commands to the host).
1065	 *
1066	 *      For compatibility with earlier versions of queuecommand, any
1067	 *      other return value is treated the same as
1068	 *      SCSI_MLQUEUE_HOST_BUSY.
1069	 *
1070	 *      Other types of errors that are detected immediately may be
1071	 *      flagged by setting scp->result to an appropriate value,
1072	 *      invoking the scp->scsi_done callback, and then returning 0
1073	 *      from this function. If the command is not performed
1074	 *      immediately (and the LLD is starting (or will start) the given
1075	 *      command) then this function should place 0 in scp->result and
1076	 *      return 0.
1077	 *
1078	 *      Command ownership.  If the driver returns zero, it owns the
1079	 *      command and must take responsibility for ensuring the
1080	 *      scp->scsi_done callback is executed.  Note: the driver may
1081	 *      call scp->scsi_done before returning zero, but after it has
1082	 *      called scp->scsi_done, it may not return any value other than
1083	 *      zero.  If the driver makes a non-zero return, it must not
1084	 *      execute the command's scsi_done callback at any time.
1085	 *
1086	 *      Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
1087	 *             held on entry (with "irqsave") and is expected to be
1088	 *             held on return. From 2.6.37 onwards, queuecommand is
1089	 *             called without any locks held.
1090	 *
1091	 *      Calling context: in interrupt (soft irq) or process context
1092	 *
1093	 *      Notes: This function should be relatively fast. Normally it
1094	 *      will not wait for IO to complete. Hence the scp->scsi_done
1095	 *      callback is invoked (often directly from an interrupt service
1096	 *      routine) some time after this function has returned. In some
1097	 *      cases (e.g. pseudo adapter drivers that manufacture the
1098	 *      response to a SCSI INQUIRY) the scp->scsi_done callback may be
1099	 *      invoked before this function returns.  If the scp->scsi_done
1100	 *      callback is not invoked within a certain period the SCSI mid
1101	 *      level will commence error processing.  If a status of CHECK
1102	 *      CONDITION is placed in "result" when the scp->scsi_done
1103	 *      callback is invoked, then the LLD driver should perform
1104	 *      autosense and fill in the struct scsi_cmnd::sense_buffer
1105	 *      array. The scsi_cmnd::sense_buffer array is zeroed prior to
1106	 *      the mid level queuing a command to an LLD.
1107	 *
1108	 *      Defined in: LLD
1109	 **/
1110	    int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
1111	
1112	
1113	/**
1114	 *      release - release all resources associated with given host
1115	 *      @shp: host to be released.
1116	 *
1117	 *      Return value ignored (could soon be a function returning void).
1118	 *
1119	 *      Locks: none held
1120	 *
1121	 *      Calling context: process
1122	 *
1123	 *      Notes: Invoked from scsi_module.c's exit_this_scsi_driver().
1124	 *      LLD's implementation of this function should call 
1125	 *      scsi_unregister(shp) prior to returning.
1126	 *      Only needed for old-style host templates.
1127	 *
1128	 *      Defined in: LLD (required in "passive initialization model",
1129	 *                       should not be defined in hotplug model)
1130	 **/
1131	    int release(struct Scsi_Host * shp)
1132	
1133	
1134	/**
1135	 *      slave_alloc -   prior to any commands being sent to a new device 
1136	 *                      (i.e. just prior to scan) this call is made
1137	 *      @sdp: pointer to new device (about to be scanned)
1138	 *
1139	 *      Returns 0 if ok. Any other return is assumed to be an error and
1140	 *      the device is ignored.
1141	 *
1142	 *      Locks: none
1143	 *
1144	 *      Calling context: process
1145	 *
1146	 *      Notes: Allows the driver to allocate any resources for a device
1147	 *      prior to its initial scan. The corresponding scsi device may not
1148	 *      exist but the mid level is just about to scan for it (i.e. send
1149	 *      and INQUIRY command plus ...). If a device is found then
1150	 *      slave_configure() will be called while if a device is not found
1151	 *      slave_destroy() is called.
1152	 *      For more details see the include/scsi/scsi_host.h file.
1153	 *
1154	 *      Optionally defined in: LLD
1155	 **/
1156	    int slave_alloc(struct scsi_device *sdp)
1157	
1158	
1159	/**
1160	 *      slave_configure - driver fine tuning for given device just after it
1161	 *                     has been first scanned (i.e. it responded to an
1162	 *                     INQUIRY)
1163	 *      @sdp: device that has just been attached
1164	 *
1165	 *      Returns 0 if ok. Any other return is assumed to be an error and
1166	 *      the device is taken offline. [offline devices will _not_ have
1167	 *      slave_destroy() called on them so clean up resources.]
1168	 *
1169	 *      Locks: none
1170	 *
1171	 *      Calling context: process
1172	 *
1173	 *      Notes: Allows the driver to inspect the response to the initial
1174	 *      INQUIRY done by the scanning code and take appropriate action.
1175	 *      For more details see the include/scsi/scsi_host.h file.
1176	 *
1177	 *      Optionally defined in: LLD
1178	 **/
1179	    int slave_configure(struct scsi_device *sdp)
1180	
1181	
1182	/**
1183	 *      slave_destroy - given device is about to be shut down. All
1184	 *                      activity has ceased on this device.
1185	 *      @sdp: device that is about to be shut down
1186	 *
1187	 *      Returns nothing
1188	 *
1189	 *      Locks: none
1190	 *
1191	 *      Calling context: process
1192	 *
1193	 *      Notes: Mid level structures for given device are still in place
1194	 *      but are about to be torn down. Any per device resources allocated
1195	 *      by this driver for given device should be freed now. No further
1196	 *      commands will be sent for this sdp instance. [However the device
1197	 *      could be re-attached in the future in which case a new instance
1198	 *      of struct scsi_device would be supplied by future slave_alloc()
1199	 *      and slave_configure() calls.]
1200	 *
1201	 *      Optionally defined in: LLD
1202	 **/
1203	    void slave_destroy(struct scsi_device *sdp)
1204	
1205	
1206	
1207	Data Structures
1208	===============
1209	struct scsi_host_template
1210	-------------------------
1211	There is one "struct scsi_host_template" instance per LLD ***. It is
1212	typically initialized as a file scope static in a driver's header file. That
1213	way members that are not explicitly initialized will be set to 0 or NULL.
1214	Member of interest:
1215	    name         - name of driver (may contain spaces, please limit to
1216	                   less than 80 characters)
1217	    proc_name    - name used in "/proc/scsi/<proc_name>/<host_no>" and
1218	                   by sysfs in one of its "drivers" directories. Hence
1219	                   "proc_name" should only contain characters acceptable
1220	                   to a Unix file name.
1221	   (*queuecommand)() - primary callback that the mid level uses to inject
1222	                   SCSI commands into an LLD.
1223	The structure is defined and commented in include/scsi/scsi_host.h
1224	
1225	*** In extreme situations a single driver may have several instances
1226	    if it controls several different classes of hardware (e.g. an LLD
1227	    that handles both ISA and PCI cards and has a separate instance of
1228	    struct scsi_host_template for each class).
1229	
1230	struct Scsi_Host
1231	----------------
1232	There is one struct Scsi_Host instance per host (HBA) that an LLD
1233	controls. The struct Scsi_Host structure has many members in common
1234	with "struct scsi_host_template". When a new struct Scsi_Host instance
1235	is created (in scsi_host_alloc() in hosts.c) those common members are
1236	initialized from the driver's struct scsi_host_template instance. Members
1237	of interest:
1238	    host_no      - system wide unique number that is used for identifying
1239	                   this host. Issued in ascending order from 0.
1240	    can_queue    - must be greater than 0; do not send more than can_queue
1241	                   commands to the adapter.
1242	    this_id      - scsi id of host (scsi initiator) or -1 if not known
1243	    sg_tablesize - maximum scatter gather elements allowed by host.
1244	                   0 implies scatter gather not supported by host
1245	    max_sectors  - maximum number of sectors (usually 512 bytes) allowed
1246	                   in a single SCSI command. The default value of 0 leads
1247	                   to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
1248	                   scsi_host.h) which is currently set to 1024. So for a
1249	                   disk the maximum transfer size is 512 KB when max_sectors
1250	                   is not defined. Note that this size may not be sufficient
1251	                   for disk firmware uploads.
1252	    cmd_per_lun  - maximum number of commands that can be queued on devices
1253	                   controlled by the host. Overridden by LLD calls to
1254	                   scsi_adjust_queue_depth().
1255	    unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
1256	                   restriction), 0=>can use full 32 bit (or better) DMA
1257	                   address space
1258	    use_clustering - 1=>SCSI commands in mid level's queue can be merged,
1259	                     0=>disallow SCSI command merging
1260	    hostt        - pointer to driver's struct scsi_host_template from which
1261	                   this struct Scsi_Host instance was spawned
1262	    hostt->proc_name  - name of LLD. This is the driver name that sysfs uses
1263	    transportt   - pointer to driver's struct scsi_transport_template instance
1264	                   (if any). FC and SPI transports currently supported.
1265	    sh_list      - a double linked list of pointers to all struct Scsi_Host
1266	                   instances (currently ordered by ascending host_no)
1267	    my_devices   - a double linked list of pointers to struct scsi_device 
1268	                   instances that belong to this host.
1269	    hostdata[0]  - area reserved for LLD at end of struct Scsi_Host. Size
1270	                   is set by the second argument (named 'xtr_bytes') to
1271	                   scsi_host_alloc() or scsi_register().
1272	    vendor_id    - a unique value that identifies the vendor supplying
1273	                   the LLD for the Scsi_Host.  Used most often in validating
1274	                   vendor-specific message requests.  Value consists of an
1275	                   identifier type and a vendor-specific value.
1276	                   See scsi_netlink.h for a description of valid formats.
1277	
1278	The scsi_host structure is defined in include/scsi/scsi_host.h
1279	
1280	struct scsi_device
1281	------------------
1282	Generally, there is one instance of this structure for each SCSI logical unit
1283	on a host. Scsi devices connected to a host are uniquely identified by a
1284	channel number, target id and logical unit number (lun).
1285	The structure is defined in include/scsi/scsi_device.h
1286	
1287	struct scsi_cmnd
1288	----------------
1289	Instances of this structure convey SCSI commands to the LLD and responses
1290	back to the mid level. The SCSI mid level will ensure that no more SCSI
1291	commands become queued against the LLD than are indicated by
1292	scsi_adjust_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
1293	be at least one instance of struct scsi_cmnd available for each SCSI device.
1294	Members of interest:
1295	    cmnd         - array containing SCSI command
1296	    cmnd_len     - length (in bytes) of SCSI command
1297	    sc_data_direction - direction of data transfer in data phase. See
1298	                "enum dma_data_direction" in include/linux/dma-mapping.h
1299	    request_bufflen - number of data bytes to transfer (0 if no data phase)
1300	    use_sg       - ==0 -> no scatter gather list, hence transfer data
1301	                          to/from request_buffer
1302	                 - >0 ->  scatter gather list (actually an array) in
1303	                          request_buffer with use_sg elements
1304	    request_buffer - either contains data buffer or scatter gather list
1305	                     depending on the setting of use_sg. Scatter gather
1306	                     elements are defined by 'struct scatterlist' found
1307	                     in include/asm/scatterlist.h .
1308	    done         - function pointer that should be invoked by LLD when the
1309	                   SCSI command is completed (successfully or otherwise).
1310	                   Should only be called by an LLD if the LLD has accepted
1311	                   the command (i.e. queuecommand() returned or will return
1312	                   0). The LLD may invoke 'done'  prior to queuecommand()
1313	                   finishing.
1314	    result       - should be set by LLD prior to calling 'done'. A value
1315	                   of 0 implies a successfully completed command (and all
1316	                   data (if any) has been transferred to or from the SCSI
1317	                   target device). 'result' is a 32 bit unsigned integer that
1318	                   can be viewed as 4 related bytes. The SCSI status value is
1319	                   in the LSB. See include/scsi/scsi.h status_byte(),
1320	                   msg_byte(), host_byte() and driver_byte() macros and
1321	                   related constants.
1322	    sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
1323	                   should be written when the SCSI status (LSB of 'result')
1324	                   is set to CHECK_CONDITION (2). When CHECK_CONDITION is
1325	                   set, if the top nibble of sense_buffer[0] has the value 7
1326	                   then the mid level will assume the sense_buffer array
1327	                   contains a valid SCSI sense buffer; otherwise the mid
1328	                   level will issue a REQUEST_SENSE SCSI command to
1329	                   retrieve the sense buffer. The latter strategy is error
1330	                   prone in the presence of command queuing so the LLD should
1331	                   always "auto-sense".
1332	    device       - pointer to scsi_device object that this command is
1333	                   associated with.
1334	    resid        - an LLD should set this signed integer to the requested
1335	                   transfer length (i.e. 'request_bufflen') less the number
1336	                   of bytes that are actually transferred. 'resid' is
1337	                   preset to 0 so an LLD can ignore it if it cannot detect
1338	                   underruns (overruns should be rare). If possible an LLD
1339	                   should set 'resid' prior to invoking 'done'. The most
1340	                   interesting case is data transfers from a SCSI target
1341	                   device (e.g. READs) that underrun.
1342	    underflow    - LLD should place (DID_ERROR << 16) in 'result' if
1343	                   actual number of bytes transferred is less than this
1344	                   figure. Not many LLDs implement this check and some that
1345	                   do just output an error message to the log rather than
1346	                   report a DID_ERROR. Better for an LLD to implement
1347	                   'resid'.
1348	
1349	It is recommended that a LLD set 'resid' on data transfers from a SCSI
1350	target device (e.g. READs). It is especially important that 'resid' is set
1351	when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
1352	(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
1353	data has been received then the safest approach is to indicate no bytes have
1354	been received. For example: to indicate that no valid data has been received
1355	a LLD might use these helpers:
1356	    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
1357	where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
1358	bytes blocks has been received 'resid' could be set like this:
1359	    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
1360	
1361	The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
1362	
1363	
1364	Locks
1365	=====
1366	Each struct Scsi_Host instance has a spin_lock called struct 
1367	Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in 
1368	hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
1369	is initialized to point at default_lock.  Thereafter lock and unlock
1370	operations performed by the mid level use the struct Scsi_Host::host_lock
1371	pointer.  Previously drivers could override the host_lock pointer but
1372	this is not allowed anymore.
1373	
1374	
1375	Autosense
1376	=========
1377	Autosense (or auto-sense) is defined in the SAM-2 document as "the
1378	automatic return of sense data to the application client coincident
1379	with the completion of a SCSI command" when a status of CHECK CONDITION
1380	occurs. LLDs should perform autosense. This should be done when the LLD
1381	detects a CHECK CONDITION status by either: 
1382	    a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
1383	       to perform an extra data in phase on such responses
1384	    b) or, the LLD issuing a REQUEST SENSE command itself
1385	
1386	Either way, when a status of CHECK CONDITION is detected, the mid level
1387	decides whether the LLD has performed autosense by checking struct 
1388	scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
1389	then autosense is assumed to have taken place. If it has another value (and
1390	this byte is initialized to 0 before each command) then the mid level will
1391	issue a REQUEST SENSE command.
1392	
1393	In the presence of queued commands the "nexus" that maintains sense
1394	buffer data from the command that failed until a following REQUEST SENSE
1395	may get out of synchronization. This is why it is best for the LLD
1396	to perform autosense.
1397	
1398	
1399	Changes since lk 2.4 series
1400	===========================
1401	io_request_lock has been replaced by several finer grained locks. The lock 
1402	relevant to LLDs is struct Scsi_Host::host_lock and there is
1403	one per SCSI host.
1404	
1405	The older error handling mechanism has been removed. This means the
1406	LLD interface functions abort() and reset() have been removed.
1407	The struct scsi_host_template::use_new_eh_code flag has been removed.
1408	
1409	In the 2.4 series the SCSI subsystem configuration descriptions were 
1410	aggregated with the configuration descriptions from all other Linux 
1411	subsystems in the Documentation/Configure.help file. In the 2.6 series, 
1412	the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
1413	file that contains both configuration and help information.
1414	
1415	struct SHT has been renamed to struct scsi_host_template.
1416	
1417	Addition of the "hotplug initialization model" and many extra functions
1418	to support it.
1419	
1420	
1421	Credits
1422	=======
1423	The following people have contributed to this document:
1424	        Mike Anderson <andmike at us dot ibm dot com>
1425	        James Bottomley <James dot Bottomley at hansenpartnership dot com>
1426	        Patrick Mansfield <patmans at us dot ibm dot com> 
1427	        Christoph Hellwig <hch at infradead dot org>
1428	        Doug Ledford <dledford at redhat dot com>
1429	        Andries Brouwer <Andries dot Brouwer at cwi dot nl>
1430	        Randy Dunlap <rdunlap at xenotime dot net>
1431	        Alan Stern <stern at rowland dot harvard dot edu>
1432	
1433	
1434	Douglas Gilbert
1435	dgilbert at interlog dot com
1436	21st September 2004
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