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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 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_change_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_change_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_change_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_change_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_change_queue_depth() with "cmd_per_lun" for that
275	host as the queue length. These settings can be overridden by a
276	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_add_device - creates new scsi device (lu) instance
370	   scsi_add_host - perform sysfs registration and set up transport class
371	   scsi_change_queue_depth - change the queue depth on a SCSI device
372	   scsi_bios_ptable - return copy of block device's partition table
373	   scsi_block_requests - prevent further commands being queued to given host
374	   scsi_host_alloc - return a new scsi_host instance whose refcount==1
375	   scsi_host_get - increments Scsi_Host instance's refcount
376	   scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
377	   scsi_partsize - parse partition table into cylinders, heads + sectors
378	   scsi_register - create and register a scsi host adapter instance.
379	   scsi_remove_device - detach and remove a SCSI device
380	   scsi_remove_host - detach and remove all SCSI devices owned by host
381	   scsi_report_bus_reset - report scsi _bus_ reset observed
382	   scsi_scan_host - scan SCSI bus
383	   scsi_track_queue_full - track successive QUEUE_FULL events 
384	   scsi_unblock_requests - allow further commands to be queued to given host
385	   scsi_unregister - [calls scsi_host_put()]
386	
387	
388	Details:
389	
390	/**
391	 * scsi_add_device - creates new scsi device (lu) instance
392	 * @shost:   pointer to scsi host instance
393	 * @channel: channel number (rarely other than 0)
394	 * @id:      target id number
395	 * @lun:     logical unit number
396	 *
397	 *      Returns pointer to new struct scsi_device instance or 
398	 *      ERR_PTR(-ENODEV) (or some other bent pointer) if something is
399	 *      wrong (e.g. no lu responds at given address)
400	 *
401	 *      Might block: yes
402	 *
403	 *      Notes: This call is usually performed internally during a scsi
404	 *      bus scan when an HBA is added (i.e. scsi_scan_host()). So it
405	 *      should only be called if the HBA becomes aware of a new scsi
406	 *      device (lu) after scsi_scan_host() has completed. If successful
407	 *      this call can lead to slave_alloc() and slave_configure() callbacks
408	 *      into the LLD.
409	 *
410	 *      Defined in: drivers/scsi/scsi_scan.c
411	 **/
412	struct scsi_device * scsi_add_device(struct Scsi_Host *shost, 
413	                                     unsigned int channel,
414	                                     unsigned int id, unsigned int lun)
415	
416	
417	/**
418	 * scsi_add_host - perform sysfs registration and set up transport class
419	 * @shost:   pointer to scsi host instance
420	 * @dev:     pointer to struct device of type scsi class
421	 *
422	 *      Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
423	 *
424	 *      Might block: no
425	 *
426	 *      Notes: Only required in "hotplug initialization model" after a
427	 *      successful call to scsi_host_alloc().  This function does not
428	 *	scan the bus; this can be done by calling scsi_scan_host() or
429	 *	in some other transport-specific way.  The LLD must set up
430	 *	the transport template before calling this function and may only
431	 *	access the transport class data after this function has been called.
432	 *
433	 *      Defined in: drivers/scsi/hosts.c
434	 **/
435	int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
436	
437	
438	/**
439	 * scsi_change_queue_depth - allow LLD to change queue depth on a SCSI device
440	 * @sdev:       pointer to SCSI device to change queue depth on
441	 * @tags        Number of tags allowed if tagged queuing enabled,
442	 *              or number of commands the LLD can queue up
443	 *              in non-tagged mode (as per cmd_per_lun).
444	 *
445	 *      Returns nothing
446	 *
447	 *      Might block: no
448	 *
449	 *      Notes: Can be invoked any time on a SCSI device controlled by this
450	 *      LLD. [Specifically during and after slave_configure() and prior to
451	 *      slave_destroy().] Can safely be invoked from interrupt code.
452	 *
453	 *      Defined in: drivers/scsi/scsi.c [see source code for more notes]
454	 *
455	 **/
456	int scsi_change_queue_depth(struct scsi_device *sdev, int tags)
457	
458	
459	/**
460	 * scsi_bios_ptable - return copy of block device's partition table
461	 * @dev:        pointer to block device
462	 *
463	 *      Returns pointer to partition table, or NULL for failure
464	 *
465	 *      Might block: yes
466	 *
467	 *      Notes: Caller owns memory returned (free with kfree() )
468	 *
469	 *      Defined in: drivers/scsi/scsicam.c
470	 **/
471	unsigned char *scsi_bios_ptable(struct block_device *dev)
472	
473	
474	/**
475	 * scsi_block_requests - prevent further commands being queued to given host
476	 *
477	 * @shost: pointer to host to block commands on
478	 *
479	 *      Returns nothing
480	 *
481	 *      Might block: no
482	 *
483	 *      Notes: There is no timer nor any other means by which the requests
484	 *      get unblocked other than the LLD calling scsi_unblock_requests().
485	 *
486	 *      Defined in: drivers/scsi/scsi_lib.c
487	**/
488	void scsi_block_requests(struct Scsi_Host * shost)
489	
490	
491	/**
492	 * scsi_host_alloc - create a scsi host adapter instance and perform basic
493	 *                   initialization.
494	 * @sht:        pointer to scsi host template
495	 * @privsize:   extra bytes to allocate in hostdata array (which is the
496	 *              last member of the returned Scsi_Host instance)
497	 *
498	 *      Returns pointer to new Scsi_Host instance or NULL on failure
499	 *
500	 *      Might block: yes
501	 *
502	 *      Notes: When this call returns to the LLD, the SCSI bus scan on
503	 *      this host has _not_ yet been done.
504	 *      The hostdata array (by default zero length) is a per host scratch 
505	 *      area for the LLD's exclusive use.
506	 *      Both associated refcounting objects have their refcount set to 1.
507	 *      Full registration (in sysfs) and a bus scan are performed later when
508	 *      scsi_add_host() and scsi_scan_host() are called.
509	 *
510	 *      Defined in: drivers/scsi/hosts.c .
511	 **/
512	struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
513	                                   int privsize)
514	
515	
516	/**
517	 * scsi_host_get - increment Scsi_Host instance refcount
518	 * @shost:   pointer to struct Scsi_Host instance
519	 *
520	 *      Returns nothing
521	 *
522	 *      Might block: currently may block but may be changed to not block
523	 *
524	 *      Notes: Actually increments the counts in two sub-objects
525	 *
526	 *      Defined in: drivers/scsi/hosts.c
527	 **/
528	void scsi_host_get(struct Scsi_Host *shost)
529	
530	
531	/**
532	 * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
533	 * @shost:   pointer to struct Scsi_Host instance
534	 *
535	 *      Returns nothing
536	 *
537	 *      Might block: currently may block but may be changed to not block
538	 *
539	 *      Notes: Actually decrements the counts in two sub-objects. If the
540	 *      latter refcount reaches 0, the Scsi_Host instance is freed.
541	 *      The LLD need not worry exactly when the Scsi_Host instance is
542	 *      freed, it just shouldn't access the instance after it has balanced
543	 *      out its refcount usage.
544	 *
545	 *      Defined in: drivers/scsi/hosts.c
546	 **/
547	void scsi_host_put(struct Scsi_Host *shost)
548	
549	
550	/**
551	 * scsi_partsize - parse partition table into cylinders, heads + sectors
552	 * @buf: pointer to partition table
553	 * @capacity: size of (total) disk in 512 byte sectors
554	 * @cyls: outputs number of cylinders calculated via this pointer
555	 * @hds: outputs number of heads calculated via this pointer
556	 * @secs: outputs number of sectors calculated via this pointer
557	 *
558	 *      Returns 0 on success, -1 on failure
559	 *
560	 *      Might block: no
561	 *
562	 *      Notes: Caller owns memory returned (free with kfree() )
563	 *
564	 *      Defined in: drivers/scsi/scsicam.c
565	 **/
566	int scsi_partsize(unsigned char *buf, unsigned long capacity,
567	                  unsigned int *cyls, unsigned int *hds, unsigned int *secs)
568	
569	
570	/**
571	 * scsi_register - create and register a scsi host adapter instance.
572	 * @sht:        pointer to scsi host template
573	 * @privsize:   extra bytes to allocate in hostdata array (which is the
574	 *              last member of the returned Scsi_Host instance)
575	 *
576	 *      Returns pointer to new Scsi_Host instance or NULL on failure
577	 *
578	 *      Might block: yes
579	 *
580	 *      Notes: When this call returns to the LLD, the SCSI bus scan on
581	 *      this host has _not_ yet been done.
582	 *      The hostdata array (by default zero length) is a per host scratch 
583	 *      area for the LLD.
584	 *
585	 *      Defined in: drivers/scsi/hosts.c .
586	 **/
587	struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
588	                                 int privsize)
589	
590	
591	/**
592	 * scsi_remove_device - detach and remove a SCSI device
593	 * @sdev:      a pointer to a scsi device instance
594	 *
595	 *      Returns value: 0 on success, -EINVAL if device not attached
596	 *
597	 *      Might block: yes
598	 *
599	 *      Notes: If an LLD becomes aware that a scsi device (lu) has
600	 *      been removed but its host is still present then it can request
601	 *      the removal of that scsi device. If successful this call will
602	 *      lead to the slave_destroy() callback being invoked. sdev is an 
603	 *      invalid pointer after this call.
604	 *
605	 *      Defined in: drivers/scsi/scsi_sysfs.c .
606	 **/
607	int scsi_remove_device(struct scsi_device *sdev)
608	
609	
610	/**
611	 * scsi_remove_host - detach and remove all SCSI devices owned by host
612	 * @shost:      a pointer to a scsi host instance
613	 *
614	 *      Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
615	 *
616	 *      Might block: yes
617	 *
618	 *      Notes: Should only be invoked if the "hotplug initialization
619	 *      model" is being used. It should be called _prior_ to  
620	 *      scsi_unregister().
621	 *
622	 *      Defined in: drivers/scsi/hosts.c .
623	 **/
624	int scsi_remove_host(struct Scsi_Host *shost)
625	
626	
627	/**
628	 * scsi_report_bus_reset - report scsi _bus_ reset observed
629	 * @shost: a pointer to a scsi host involved
630	 * @channel: channel (within) host on which scsi bus reset occurred
631	 *
632	 *      Returns nothing
633	 *
634	 *      Might block: no
635	 *
636	 *      Notes: This only needs to be called if the reset is one which
637	 *      originates from an unknown location.  Resets originated by the 
638	 *      mid level itself don't need to call this, but there should be 
639	 *      no harm.  The main purpose of this is to make sure that a
640	 *      CHECK_CONDITION is properly treated.
641	 *
642	 *      Defined in: drivers/scsi/scsi_error.c .
643	 **/
644	void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
645	
646	
647	/**
648	 * scsi_scan_host - scan SCSI bus
649	 * @shost: a pointer to a scsi host instance
650	 *
651	 *	Might block: yes
652	 *
653	 *	Notes: Should be called after scsi_add_host()
654	 *
655	 *	Defined in: drivers/scsi/scsi_scan.c
656	 **/
657	void scsi_scan_host(struct Scsi_Host *shost)
658	
659	
660	/**
661	 * scsi_track_queue_full - track successive QUEUE_FULL events on given
662	 *                      device to determine if and when there is a need
663	 *                      to adjust the queue depth on the device.
664	 * @sdev:  pointer to SCSI device instance
665	 * @depth: Current number of outstanding SCSI commands on this device,
666	 *         not counting the one returned as QUEUE_FULL.
667	 *
668	 *      Returns 0  - no change needed
669	 *              >0 - adjust queue depth to this new depth
670	 *              -1 - drop back to untagged operation using host->cmd_per_lun
671	 *                   as the untagged command depth
672	 *
673	 *      Might block: no
674	 *
675	 *      Notes: LLDs may call this at any time and we will do "The Right
676	 *              Thing"; interrupt context safe. 
677	 *
678	 *      Defined in: drivers/scsi/scsi.c .
679	 **/
680	int scsi_track_queue_full(struct scsi_device *sdev, int depth)
681	
682	
683	/**
684	 * scsi_unblock_requests - allow further commands to be queued to given host
685	 *
686	 * @shost: pointer to host to unblock commands on
687	 *
688	 *      Returns nothing
689	 *
690	 *      Might block: no
691	 *
692	 *      Defined in: drivers/scsi/scsi_lib.c .
693	**/
694	void scsi_unblock_requests(struct Scsi_Host * shost)
695	
696	
697	/**
698	 * scsi_unregister - unregister and free memory used by host instance
699	 * @shp:        pointer to scsi host instance to unregister.
700	 *
701	 *      Returns nothing
702	 *
703	 *      Might block: no
704	 *
705	 *      Notes: Should not be invoked if the "hotplug initialization
706	 *      model" is being used. Called internally by exit_this_scsi_driver()
707	 *      in the "passive initialization model". Hence a LLD has no need to
708	 *      call this function directly.
709	 *
710	 *      Defined in: drivers/scsi/hosts.c .
711	 **/
712	void scsi_unregister(struct Scsi_Host * shp)
713	
714	
715	
716	
717	Interface Functions
718	===================
719	Interface functions are supplied (defined) by LLDs and their function
720	pointers are placed in an instance of struct scsi_host_template which
721	is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
722	Some are mandatory. Interface functions should be declared static. The
723	accepted convention is that driver "xyz" will declare its slave_configure() 
724	function as:
725	    static int xyz_slave_configure(struct scsi_device * sdev);
726	and so forth for all interface functions listed below.
727	
728	A pointer to this function should be placed in the 'slave_configure' member
729	of a "struct scsi_host_template" instance. A pointer to such an instance
730	should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
731	init_this_scsi_driver()].
732	
733	The interface functions are also described in the include/scsi/scsi_host.h
734	file immediately above their definition point in "struct scsi_host_template".
735	In some cases more detail is given in scsi_host.h than below.
736	
737	The interface functions are listed below in alphabetical order.
738	
739	Summary:
740	   bios_param - fetch head, sector, cylinder info for a disk
741	   detect - detects HBAs this driver wants to control
742	   eh_timed_out - notify the host that a command timer expired
743	   eh_abort_handler - abort given command
744	   eh_bus_reset_handler - issue SCSI bus reset
745	   eh_device_reset_handler - issue SCSI device reset
746	   eh_host_reset_handler - reset host (host bus adapter)
747	   info - supply information about given host
748	   ioctl - driver can respond to ioctls
749	   proc_info - supports /proc/scsi/{driver_name}/{host_no}
750	   queuecommand - queue scsi command, invoke 'done' on completion
751	   release - release all resources associated with given host
752	   slave_alloc - prior to any commands being sent to a new device 
753	   slave_configure - driver fine tuning for given device after attach
754	   slave_destroy - given device is about to be shut down
755	
756	
757	Details:
758	
759	/**
760	 *      bios_param - fetch head, sector, cylinder info for a disk
761	 *      @sdev: pointer to scsi device context (defined in 
762	 *             include/scsi/scsi_device.h)
763	 *      @bdev: pointer to block device context (defined in fs.h)
764	 *      @capacity:  device size (in 512 byte sectors)
765	 *      @params: three element array to place output:
766	 *              params[0] number of heads (max 255)
767	 *              params[1] number of sectors (max 63)
768	 *              params[2] number of cylinders 
769	 *
770	 *      Return value is ignored
771	 *
772	 *      Locks: none
773	 *
774	 *      Calling context: process (sd)
775	 *
776	 *      Notes: an arbitrary geometry (based on READ CAPACITY) is used
777	 *      if this function is not provided. The params array is
778	 *      pre-initialized with made up values just in case this function 
779	 *      doesn't output anything.
780	 *
781	 *      Optionally defined in: LLD
782	 **/
783	    int bios_param(struct scsi_device * sdev, struct block_device *bdev,
784	                   sector_t capacity, int params[3])
785	
786	
787	/**
788	 *      detect - detects HBAs this driver wants to control
789	 *      @shtp: host template for this driver.
790	 *
791	 *      Returns number of hosts this driver wants to control. 0 means no
792	 *      suitable hosts found.
793	 *
794	 *      Locks: none held
795	 *
796	 *      Calling context: process [invoked from init_this_scsi_driver()]
797	 *
798	 *      Notes: First function called from the SCSI mid level on this
799	 *      driver. Upper level drivers (e.g. sd) may not (yet) be present.
800	 *      For each host found, this method should call scsi_register() 
801	 *      [see hosts.c].
802	 *
803	 *      Defined in: LLD (required if "passive initialization mode" is used,
804	 *                       not invoked in "hotplug initialization mode")
805	 **/
806	    int detect(struct scsi_host_template * shtp)
807	
808	
809	/**
810	 *      eh_timed_out - The timer for the command has just fired
811	 *      @scp: identifies command timing out
812	 *
813	 *      Returns:
814	 *
815	 *      EH_HANDLED:             I fixed the error, please complete the command
816	 *      EH_RESET_TIMER:         I need more time, reset the timer and
817	 *                              begin counting again
818	 *      EH_NOT_HANDLED          Begin normal error recovery
819	 *
820	 *
821	 *      Locks: None held
822	 *
823	 *      Calling context: interrupt
824	 *
825	 *      Notes: This is to give the LLD an opportunity to do local recovery.
826	 *      This recovery is limited to determining if the outstanding command
827	 *      will ever complete.  You may not abort and restart the command from
828	 *      this callback.
829	 *
830	 *      Optionally defined in: LLD
831	 **/
832	     int eh_timed_out(struct scsi_cmnd * scp)
833	
834	
835	/**
836	 *      eh_abort_handler - abort command associated with scp
837	 *      @scp: identifies command to be aborted
838	 *
839	 *      Returns SUCCESS if command aborted else FAILED
840	 *
841	 *      Locks: None held
842	 *
843	 *      Calling context: kernel thread
844	 *
845	 *      Notes: If 'no_async_abort' is defined this callback
846	 *  	will be invoked from scsi_eh thread. No other commands
847	 *	will then be queued on current host during eh.
848	 *	Otherwise it will be called whenever scsi_times_out()
849	 *      is called due to a command timeout.
850	 *
851	 *      Optionally defined in: LLD
852	 **/
853	     int eh_abort_handler(struct scsi_cmnd * scp)
854	
855	
856	/**
857	 *      eh_bus_reset_handler - issue SCSI bus reset
858	 *      @scp: SCSI bus that contains this device should be reset
859	 *
860	 *      Returns SUCCESS if command aborted else FAILED
861	 *
862	 *      Locks: None held
863	 *
864	 *      Calling context: kernel thread
865	 *
866	 *      Notes: Invoked from scsi_eh thread. No other commands will be
867	 *      queued on current host during eh.
868	 *
869	 *      Optionally defined in: LLD
870	 **/
871	     int eh_bus_reset_handler(struct scsi_cmnd * scp)
872	
873	
874	/**
875	 *      eh_device_reset_handler - issue SCSI device reset
876	 *      @scp: identifies SCSI device to be reset
877	 *
878	 *      Returns SUCCESS if command aborted else FAILED
879	 *
880	 *      Locks: None held
881	 *
882	 *      Calling context: kernel thread
883	 *
884	 *      Notes: Invoked from scsi_eh thread. No other commands will be
885	 *      queued on current host during eh.
886	 *
887	 *      Optionally defined in: LLD
888	 **/
889	     int eh_device_reset_handler(struct scsi_cmnd * scp)
890	
891	
892	/**
893	 *      eh_host_reset_handler - reset host (host bus adapter)
894	 *      @scp: SCSI host that contains this device should be reset
895	 *
896	 *      Returns SUCCESS if command aborted else FAILED
897	 *
898	 *      Locks: None held
899	 *
900	 *      Calling context: kernel thread
901	 *
902	 *      Notes: Invoked from scsi_eh thread. No other commands will be
903	 *      queued on current host during eh. 
904	 *      With the default eh_strategy in place, if none of the _abort_, 
905	 *      _device_reset_, _bus_reset_ or this eh handler function are 
906	 *      defined (or they all return FAILED) then the device in question 
907	 *      will be set offline whenever eh is invoked.
908	 *
909	 *      Optionally defined in: LLD
910	 **/
911	     int eh_host_reset_handler(struct scsi_cmnd * scp)
912	
913	
914	/**
915	 *      info - supply information about given host: driver name plus data
916	 *             to distinguish given host
917	 *      @shp: host to supply information about
918	 *
919	 *      Return ASCII null terminated string. [This driver is assumed to
920	 *      manage the memory pointed to and maintain it, typically for the
921	 *      lifetime of this host.]
922	 *
923	 *      Locks: none
924	 *
925	 *      Calling context: process
926	 *
927	 *      Notes: Often supplies PCI or ISA information such as IO addresses
928	 *      and interrupt numbers. If not supplied struct Scsi_Host::name used
929	 *      instead. It is assumed the returned information fits on one line 
930	 *      (i.e. does not included embedded newlines).
931	 *      The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
932	 *      function (or struct Scsi_Host::name if this function is not
933	 *      available).
934	 *      In a similar manner, init_this_scsi_driver() outputs to the console
935	 *      each host's "info" (or name) for the driver it is registering.
936	 *      Also if proc_info() is not supplied, the output of this function
937	 *      is used instead.
938	 *
939	 *      Optionally defined in: LLD
940	 **/
941	    const char * info(struct Scsi_Host * shp)
942	
943	
944	/**
945	 *      ioctl - driver can respond to ioctls
946	 *      @sdp: device that ioctl was issued for
947	 *      @cmd: ioctl number
948	 *      @arg: pointer to read or write data from. Since it points to
949	 *            user space, should use appropriate kernel functions
950	 *            (e.g. copy_from_user() ). In the Unix style this argument
951	 *            can also be viewed as an unsigned long.
952	 *
953	 *      Returns negative "errno" value when there is a problem. 0 or a
954	 *      positive value indicates success and is returned to the user space.
955	 *
956	 *      Locks: none
957	 *
958	 *      Calling context: process
959	 *
960	 *      Notes: The SCSI subsystem uses a "trickle down" ioctl model.
961	 *      The user issues an ioctl() against an upper level driver
962	 *      (e.g. /dev/sdc) and if the upper level driver doesn't recognize
963	 *      the 'cmd' then it is passed to the SCSI mid level. If the SCSI
964	 *      mid level does not recognize it, then the LLD that controls
965	 *      the device receives the ioctl. According to recent Unix standards
966	 *      unsupported ioctl() 'cmd' numbers should return -ENOTTY.
967	 *
968	 *      Optionally defined in: LLD
969	 **/
970	    int ioctl(struct scsi_device *sdp, int cmd, void *arg)
971	
972	
973	/**
974	 *      proc_info - supports /proc/scsi/{driver_name}/{host_no}
975	 *      @buffer: anchor point to output to (0==writeto1_read0) or fetch from
976	 *               (1==writeto1_read0).
977	 *      @start: where "interesting" data is written to. Ignored when
978	 *              1==writeto1_read0.
979	 *      @offset: offset within buffer 0==writeto1_read0 is actually
980	 *               interested in. Ignored when 1==writeto1_read0 .
981	 *      @length: maximum (or actual) extent of buffer
982	 *      @host_no: host number of interest (struct Scsi_Host::host_no)
983	 *      @writeto1_read0: 1 -> data coming from user space towards driver
984	 *                            (e.g. "echo some_string > /proc/scsi/xyz/2")
985	 *                       0 -> user what data from this driver
986	 *                            (e.g. "cat /proc/scsi/xyz/2")
987	 *
988	 *      Returns length when 1==writeto1_read0. Otherwise number of chars
989	 *      output to buffer past offset.
990	 *
991	 *      Locks: none held
992	 *
993	 *      Calling context: process
994	 *
995	 *      Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
996	 *      support can now be configured out of the scsi subsystem.
997	 *
998	 *      Optionally defined in: LLD
999	 **/
1000	    int proc_info(char * buffer, char ** start, off_t offset, 
1001	                  int length, int host_no, int writeto1_read0)
1002	
1003	
1004	/**
1005	 *      queuecommand - queue scsi command, invoke scp->scsi_done on completion
1006	 *      @shost: pointer to the scsi host object
1007	 *      @scp: pointer to scsi command object
1008	 *
1009	 *      Returns 0 on success.
1010	 *
1011	 *      If there's a failure, return either:
1012	 *
1013	 *      SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
1014	 *      SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
1015	 *
1016	 *      On both of these returns, the mid-layer will requeue the I/O
1017	 *
1018	 *      - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
1019	 *      device will be paused, and it will be unpaused when a command to
1020	 *      the device returns (or after a brief delay if there are no more
1021	 *      outstanding commands to it).  Commands to other devices continue
1022	 *      to be processed normally.
1023	 *
1024	 *      - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
1025	 *      is paused and will be unpaused when any command returns from
1026	 *      the host (or after a brief delay if there are no outstanding
1027	 *      commands to the host).
1028	 *
1029	 *      For compatibility with earlier versions of queuecommand, any
1030	 *      other return value is treated the same as
1031	 *      SCSI_MLQUEUE_HOST_BUSY.
1032	 *
1033	 *      Other types of errors that are detected immediately may be
1034	 *      flagged by setting scp->result to an appropriate value,
1035	 *      invoking the scp->scsi_done callback, and then returning 0
1036	 *      from this function. If the command is not performed
1037	 *      immediately (and the LLD is starting (or will start) the given
1038	 *      command) then this function should place 0 in scp->result and
1039	 *      return 0.
1040	 *
1041	 *      Command ownership.  If the driver returns zero, it owns the
1042	 *      command and must take responsibility for ensuring the
1043	 *      scp->scsi_done callback is executed.  Note: the driver may
1044	 *      call scp->scsi_done before returning zero, but after it has
1045	 *      called scp->scsi_done, it may not return any value other than
1046	 *      zero.  If the driver makes a non-zero return, it must not
1047	 *      execute the command's scsi_done callback at any time.
1048	 *
1049	 *      Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
1050	 *             held on entry (with "irqsave") and is expected to be
1051	 *             held on return. From 2.6.37 onwards, queuecommand is
1052	 *             called without any locks held.
1053	 *
1054	 *      Calling context: in interrupt (soft irq) or process context
1055	 *
1056	 *      Notes: This function should be relatively fast. Normally it
1057	 *      will not wait for IO to complete. Hence the scp->scsi_done
1058	 *      callback is invoked (often directly from an interrupt service
1059	 *      routine) some time after this function has returned. In some
1060	 *      cases (e.g. pseudo adapter drivers that manufacture the
1061	 *      response to a SCSI INQUIRY) the scp->scsi_done callback may be
1062	 *      invoked before this function returns.  If the scp->scsi_done
1063	 *      callback is not invoked within a certain period the SCSI mid
1064	 *      level will commence error processing.  If a status of CHECK
1065	 *      CONDITION is placed in "result" when the scp->scsi_done
1066	 *      callback is invoked, then the LLD driver should perform
1067	 *      autosense and fill in the struct scsi_cmnd::sense_buffer
1068	 *      array. The scsi_cmnd::sense_buffer array is zeroed prior to
1069	 *      the mid level queuing a command to an LLD.
1070	 *
1071	 *      Defined in: LLD
1072	 **/
1073	    int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
1074	
1075	
1076	/**
1077	 *      release - release all resources associated with given host
1078	 *      @shp: host to be released.
1079	 *
1080	 *      Return value ignored (could soon be a function returning void).
1081	 *
1082	 *      Locks: none held
1083	 *
1084	 *      Calling context: process
1085	 *
1086	 *      Notes: Invoked from scsi_module.c's exit_this_scsi_driver().
1087	 *      LLD's implementation of this function should call 
1088	 *      scsi_unregister(shp) prior to returning.
1089	 *      Only needed for old-style host templates.
1090	 *
1091	 *      Defined in: LLD (required in "passive initialization model",
1092	 *                       should not be defined in hotplug model)
1093	 **/
1094	    int release(struct Scsi_Host * shp)
1095	
1096	
1097	/**
1098	 *      slave_alloc -   prior to any commands being sent to a new device 
1099	 *                      (i.e. just prior to scan) this call is made
1100	 *      @sdp: pointer to new device (about to be scanned)
1101	 *
1102	 *      Returns 0 if ok. Any other return is assumed to be an error and
1103	 *      the device is ignored.
1104	 *
1105	 *      Locks: none
1106	 *
1107	 *      Calling context: process
1108	 *
1109	 *      Notes: Allows the driver to allocate any resources for a device
1110	 *      prior to its initial scan. The corresponding scsi device may not
1111	 *      exist but the mid level is just about to scan for it (i.e. send
1112	 *      and INQUIRY command plus ...). If a device is found then
1113	 *      slave_configure() will be called while if a device is not found
1114	 *      slave_destroy() is called.
1115	 *      For more details see the include/scsi/scsi_host.h file.
1116	 *
1117	 *      Optionally defined in: LLD
1118	 **/
1119	    int slave_alloc(struct scsi_device *sdp)
1120	
1121	
1122	/**
1123	 *      slave_configure - driver fine tuning for given device just after it
1124	 *                     has been first scanned (i.e. it responded to an
1125	 *                     INQUIRY)
1126	 *      @sdp: device that has just been attached
1127	 *
1128	 *      Returns 0 if ok. Any other return is assumed to be an error and
1129	 *      the device is taken offline. [offline devices will _not_ have
1130	 *      slave_destroy() called on them so clean up resources.]
1131	 *
1132	 *      Locks: none
1133	 *
1134	 *      Calling context: process
1135	 *
1136	 *      Notes: Allows the driver to inspect the response to the initial
1137	 *      INQUIRY done by the scanning code and take appropriate action.
1138	 *      For more details see the include/scsi/scsi_host.h file.
1139	 *
1140	 *      Optionally defined in: LLD
1141	 **/
1142	    int slave_configure(struct scsi_device *sdp)
1143	
1144	
1145	/**
1146	 *      slave_destroy - given device is about to be shut down. All
1147	 *                      activity has ceased on this device.
1148	 *      @sdp: device that is about to be shut down
1149	 *
1150	 *      Returns nothing
1151	 *
1152	 *      Locks: none
1153	 *
1154	 *      Calling context: process
1155	 *
1156	 *      Notes: Mid level structures for given device are still in place
1157	 *      but are about to be torn down. Any per device resources allocated
1158	 *      by this driver for given device should be freed now. No further
1159	 *      commands will be sent for this sdp instance. [However the device
1160	 *      could be re-attached in the future in which case a new instance
1161	 *      of struct scsi_device would be supplied by future slave_alloc()
1162	 *      and slave_configure() calls.]
1163	 *
1164	 *      Optionally defined in: LLD
1165	 **/
1166	    void slave_destroy(struct scsi_device *sdp)
1167	
1168	
1169	
1170	Data Structures
1171	===============
1172	struct scsi_host_template
1173	-------------------------
1174	There is one "struct scsi_host_template" instance per LLD ***. It is
1175	typically initialized as a file scope static in a driver's header file. That
1176	way members that are not explicitly initialized will be set to 0 or NULL.
1177	Member of interest:
1178	    name         - name of driver (may contain spaces, please limit to
1179	                   less than 80 characters)
1180	    proc_name    - name used in "/proc/scsi/<proc_name>/<host_no>" and
1181	                   by sysfs in one of its "drivers" directories. Hence
1182	                   "proc_name" should only contain characters acceptable
1183	                   to a Unix file name.
1184	   (*queuecommand)() - primary callback that the mid level uses to inject
1185	                   SCSI commands into an LLD.
1186	The structure is defined and commented in include/scsi/scsi_host.h
1187	
1188	*** In extreme situations a single driver may have several instances
1189	    if it controls several different classes of hardware (e.g. an LLD
1190	    that handles both ISA and PCI cards and has a separate instance of
1191	    struct scsi_host_template for each class).
1192	
1193	struct Scsi_Host
1194	----------------
1195	There is one struct Scsi_Host instance per host (HBA) that an LLD
1196	controls. The struct Scsi_Host structure has many members in common
1197	with "struct scsi_host_template". When a new struct Scsi_Host instance
1198	is created (in scsi_host_alloc() in hosts.c) those common members are
1199	initialized from the driver's struct scsi_host_template instance. Members
1200	of interest:
1201	    host_no      - system wide unique number that is used for identifying
1202	                   this host. Issued in ascending order from 0.
1203	    can_queue    - must be greater than 0; do not send more than can_queue
1204	                   commands to the adapter.
1205	    this_id      - scsi id of host (scsi initiator) or -1 if not known
1206	    sg_tablesize - maximum scatter gather elements allowed by host.
1207	                   0 implies scatter gather not supported by host
1208	    max_sectors  - maximum number of sectors (usually 512 bytes) allowed
1209	                   in a single SCSI command. The default value of 0 leads
1210	                   to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
1211	                   scsi_host.h) which is currently set to 1024. So for a
1212	                   disk the maximum transfer size is 512 KB when max_sectors
1213	                   is not defined. Note that this size may not be sufficient
1214	                   for disk firmware uploads.
1215	    cmd_per_lun  - maximum number of commands that can be queued on devices
1216	                   controlled by the host. Overridden by LLD calls to
1217	                   scsi_change_queue_depth().
1218	    unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
1219	                   restriction), 0=>can use full 32 bit (or better) DMA
1220	                   address space
1221	    use_clustering - 1=>SCSI commands in mid level's queue can be merged,
1222	                     0=>disallow SCSI command merging
1223	    no_async_abort - 1=>Asynchronous aborts are not supported
1224	                     0=>Timed-out commands will be aborted asynchronously
1225	    hostt        - pointer to driver's struct scsi_host_template from which
1226	                   this struct Scsi_Host instance was spawned
1227	    hostt->proc_name  - name of LLD. This is the driver name that sysfs uses
1228	    transportt   - pointer to driver's struct scsi_transport_template instance
1229	                   (if any). FC and SPI transports currently supported.
1230	    sh_list      - a double linked list of pointers to all struct Scsi_Host
1231	                   instances (currently ordered by ascending host_no)
1232	    my_devices   - a double linked list of pointers to struct scsi_device 
1233	                   instances that belong to this host.
1234	    hostdata[0]  - area reserved for LLD at end of struct Scsi_Host. Size
1235	                   is set by the second argument (named 'xtr_bytes') to
1236	                   scsi_host_alloc() or scsi_register().
1237	    vendor_id    - a unique value that identifies the vendor supplying
1238	                   the LLD for the Scsi_Host.  Used most often in validating
1239	                   vendor-specific message requests.  Value consists of an
1240	                   identifier type and a vendor-specific value.
1241	                   See scsi_netlink.h for a description of valid formats.
1242	
1243	The scsi_host structure is defined in include/scsi/scsi_host.h
1244	
1245	struct scsi_device
1246	------------------
1247	Generally, there is one instance of this structure for each SCSI logical unit
1248	on a host. Scsi devices connected to a host are uniquely identified by a
1249	channel number, target id and logical unit number (lun).
1250	The structure is defined in include/scsi/scsi_device.h
1251	
1252	struct scsi_cmnd
1253	----------------
1254	Instances of this structure convey SCSI commands to the LLD and responses
1255	back to the mid level. The SCSI mid level will ensure that no more SCSI
1256	commands become queued against the LLD than are indicated by
1257	scsi_change_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
1258	be at least one instance of struct scsi_cmnd available for each SCSI device.
1259	Members of interest:
1260	    cmnd         - array containing SCSI command
1261	    cmnd_len     - length (in bytes) of SCSI command
1262	    sc_data_direction - direction of data transfer in data phase. See
1263	                "enum dma_data_direction" in include/linux/dma-mapping.h
1264	    request_bufflen - number of data bytes to transfer (0 if no data phase)
1265	    use_sg       - ==0 -> no scatter gather list, hence transfer data
1266	                          to/from request_buffer
1267	                 - >0 ->  scatter gather list (actually an array) in
1268	                          request_buffer with use_sg elements
1269	    request_buffer - either contains data buffer or scatter gather list
1270	                     depending on the setting of use_sg. Scatter gather
1271	                     elements are defined by 'struct scatterlist' found
1272	                     in include/asm/scatterlist.h .
1273	    done         - function pointer that should be invoked by LLD when the
1274	                   SCSI command is completed (successfully or otherwise).
1275	                   Should only be called by an LLD if the LLD has accepted
1276	                   the command (i.e. queuecommand() returned or will return
1277	                   0). The LLD may invoke 'done'  prior to queuecommand()
1278	                   finishing.
1279	    result       - should be set by LLD prior to calling 'done'. A value
1280	                   of 0 implies a successfully completed command (and all
1281	                   data (if any) has been transferred to or from the SCSI
1282	                   target device). 'result' is a 32 bit unsigned integer that
1283	                   can be viewed as 4 related bytes. The SCSI status value is
1284	                   in the LSB. See include/scsi/scsi.h status_byte(),
1285	                   msg_byte(), host_byte() and driver_byte() macros and
1286	                   related constants.
1287	    sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
1288	                   should be written when the SCSI status (LSB of 'result')
1289	                   is set to CHECK_CONDITION (2). When CHECK_CONDITION is
1290	                   set, if the top nibble of sense_buffer[0] has the value 7
1291	                   then the mid level will assume the sense_buffer array
1292	                   contains a valid SCSI sense buffer; otherwise the mid
1293	                   level will issue a REQUEST_SENSE SCSI command to
1294	                   retrieve the sense buffer. The latter strategy is error
1295	                   prone in the presence of command queuing so the LLD should
1296	                   always "auto-sense".
1297	    device       - pointer to scsi_device object that this command is
1298	                   associated with.
1299	    resid        - an LLD should set this signed integer to the requested
1300	                   transfer length (i.e. 'request_bufflen') less the number
1301	                   of bytes that are actually transferred. 'resid' is
1302	                   preset to 0 so an LLD can ignore it if it cannot detect
1303	                   underruns (overruns should be rare). If possible an LLD
1304	                   should set 'resid' prior to invoking 'done'. The most
1305	                   interesting case is data transfers from a SCSI target
1306	                   device (e.g. READs) that underrun.
1307	    underflow    - LLD should place (DID_ERROR << 16) in 'result' if
1308	                   actual number of bytes transferred is less than this
1309	                   figure. Not many LLDs implement this check and some that
1310	                   do just output an error message to the log rather than
1311	                   report a DID_ERROR. Better for an LLD to implement
1312	                   'resid'.
1313	
1314	It is recommended that a LLD set 'resid' on data transfers from a SCSI
1315	target device (e.g. READs). It is especially important that 'resid' is set
1316	when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
1317	(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
1318	data has been received then the safest approach is to indicate no bytes have
1319	been received. For example: to indicate that no valid data has been received
1320	a LLD might use these helpers:
1321	    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
1322	where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
1323	bytes blocks has been received 'resid' could be set like this:
1324	    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
1325	
1326	The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
1327	
1328	
1329	Locks
1330	=====
1331	Each struct Scsi_Host instance has a spin_lock called struct 
1332	Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in 
1333	hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
1334	is initialized to point at default_lock.  Thereafter lock and unlock
1335	operations performed by the mid level use the struct Scsi_Host::host_lock
1336	pointer.  Previously drivers could override the host_lock pointer but
1337	this is not allowed anymore.
1338	
1339	
1340	Autosense
1341	=========
1342	Autosense (or auto-sense) is defined in the SAM-2 document as "the
1343	automatic return of sense data to the application client coincident
1344	with the completion of a SCSI command" when a status of CHECK CONDITION
1345	occurs. LLDs should perform autosense. This should be done when the LLD
1346	detects a CHECK CONDITION status by either: 
1347	    a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
1348	       to perform an extra data in phase on such responses
1349	    b) or, the LLD issuing a REQUEST SENSE command itself
1350	
1351	Either way, when a status of CHECK CONDITION is detected, the mid level
1352	decides whether the LLD has performed autosense by checking struct 
1353	scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
1354	then autosense is assumed to have taken place. If it has another value (and
1355	this byte is initialized to 0 before each command) then the mid level will
1356	issue a REQUEST SENSE command.
1357	
1358	In the presence of queued commands the "nexus" that maintains sense
1359	buffer data from the command that failed until a following REQUEST SENSE
1360	may get out of synchronization. This is why it is best for the LLD
1361	to perform autosense.
1362	
1363	
1364	Changes since lk 2.4 series
1365	===========================
1366	io_request_lock has been replaced by several finer grained locks. The lock 
1367	relevant to LLDs is struct Scsi_Host::host_lock and there is
1368	one per SCSI host.
1369	
1370	The older error handling mechanism has been removed. This means the
1371	LLD interface functions abort() and reset() have been removed.
1372	The struct scsi_host_template::use_new_eh_code flag has been removed.
1373	
1374	In the 2.4 series the SCSI subsystem configuration descriptions were 
1375	aggregated with the configuration descriptions from all other Linux 
1376	subsystems in the Documentation/Configure.help file. In the 2.6 series, 
1377	the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
1378	file that contains both configuration and help information.
1379	
1380	struct SHT has been renamed to struct scsi_host_template.
1381	
1382	Addition of the "hotplug initialization model" and many extra functions
1383	to support it.
1384	
1385	
1386	Credits
1387	=======
1388	The following people have contributed to this document:
1389	        Mike Anderson <andmike at us dot ibm dot com>
1390	        James Bottomley <James dot Bottomley at hansenpartnership dot com>
1391	        Patrick Mansfield <patmans at us dot ibm dot com> 
1392	        Christoph Hellwig <hch at infradead dot org>
1393	        Doug Ledford <dledford at redhat dot com>
1394	        Andries Brouwer <Andries dot Brouwer at cwi dot nl>
1395	        Randy Dunlap <rdunlap at xenotime dot net>
1396	        Alan Stern <stern at rowland dot harvard dot edu>
1397	
1398	
1399	Douglas Gilbert
1400	dgilbert at interlog dot com
1401	21st September 2004
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