linux/Documentation/scsi/scsi_mid_low_api.txt
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   1                          Linux Kernel 2.6 series
   2                 SCSI mid_level - lower_level driver interface
   3                 =============================================
   4
   5Introduction
   6============
   7This document outlines the interface between the Linux SCSI mid level and
   8SCSI lower level drivers. Lower level drivers (LLDs) are variously called 
   9host bus adapter (HBA) drivers and host drivers (HD). A "host" in this
  10context is a bridge between a computer IO bus (e.g. PCI or ISA) and a
  11single SCSI initiator port on a SCSI transport. An "initiator" port
  12(SCSI terminology, see SAM-3 at http://www.t10.org) sends SCSI commands
  13to "target" SCSI ports (e.g. disks). There can be many LLDs in a running
  14system, but only one per hardware type. Most LLDs can control one or more
  15SCSI HBAs. Some HBAs contain multiple hosts.
  16
  17In some cases the SCSI transport is an external bus that already has
  18its own subsystem in Linux (e.g. USB and ieee1394). In such cases the
  19SCSI subsystem LLD is a software bridge to the other driver subsystem.
  20Examples are the usb-storage driver (found in the drivers/usb/storage
  21directory) and the ieee1394/sbp2 driver (found in the drivers/ieee1394
  22directory).
  23
  24For example, the aic7xxx LLD controls Adaptec SCSI parallel interface
  25(SPI) controllers based on that company's 7xxx chip series. The aic7xxx
  26LLD can be built into the kernel or loaded as a module. There can only be
  27one aic7xxx LLD running in a Linux system but it may be controlling many 
  28HBAs. These HBAs might be either on PCI daughter-boards or built into 
  29the motherboard (or both). Some aic7xxx based HBAs are dual controllers
  30and thus represent two hosts. Like most modern HBAs, each aic7xxx host
  31has its own PCI device address. [The one-to-one correspondence between
  32a SCSI host and a PCI device is common but not required (e.g. with
  33ISA or MCA adapters).]
  34
  35The SCSI mid level isolates an LLD from other layers such as the SCSI
  36upper layer drivers and the block layer.
  37
  38This version of the document roughly matches linux kernel version 2.6.8 .
  39
  40Documentation
  41=============
  42There is a SCSI documentation directory within the kernel source tree, 
  43typically Documentation/scsi . Most documents are in plain
  44(i.e. ASCII) text. This file is named scsi_mid_low_api.txt and can be 
  45found in that directory. A more recent copy of this document may be found
  46at http://www.torque.net/scsi/scsi_mid_low_api.txt.gz . 
  47Many LLDs are documented there (e.g. aic7xxx.txt). The SCSI mid-level is
  48briefly described in scsi.txt which contains a url to a document 
  49describing the SCSI subsystem in the lk 2.4 series. Two upper level 
  50drivers have documents in that directory: st.txt (SCSI tape driver) and 
  51scsi-generic.txt (for the sg driver).
  52
  53Some documentation (or urls) for LLDs may be found in the C source code
  54or in the same directory as the C source code. For example to find a url
  55about the USB mass storage driver see the 
  56/usr/src/linux/drivers/usb/storage directory.
  57
  58The Linux kernel source Documentation/DocBook/scsidrivers.tmpl file
  59refers to this file. With the appropriate DocBook tool-set, this permits
  60users to generate html, ps and pdf renderings of information within this
  61file (e.g. the interface functions).
  62
  63Driver structure
  64================
  65Traditionally an LLD for the SCSI subsystem has been at least two files in
  66the drivers/scsi directory. For example, a driver called "xyz" has a header
  67file "xyz.h" and a source file "xyz.c". [Actually there is no good reason
  68why this couldn't all be in one file; the header file is superfluous.] Some
  69drivers that have been ported to several operating systems have more than
  70two files. For example the aic7xxx driver has separate files for generic 
  71and OS-specific code (e.g. FreeBSD and Linux). Such drivers tend to have
  72their own directory under the drivers/scsi directory.
  73
  74When a new LLD is being added to Linux, the following files (found in the
  75drivers/scsi directory) will need some attention: Makefile and Kconfig .
  76It is probably best to study how existing LLDs are organized.
  77
  78As the 2.5 series development kernels evolve into the 2.6 series
  79production series, changes are being introduced into this interface. An
  80example of this is driver initialization code where there are now 2 models
  81available. The older one, similar to what was found in the lk 2.4 series,
  82is based on hosts that are detected at HBA driver load time. This will be
  83referred to the "passive" initialization model. The newer model allows HBAs
  84to be hot plugged (and unplugged) during the lifetime of the LLD and will
  85be referred to as the "hotplug" initialization model. The newer model is
  86preferred as it can handle both traditional SCSI equipment that is
  87permanently connected as well as modern "SCSI" devices (e.g. USB or
  88IEEE 1394 connected digital cameras) that are hotplugged. Both 
  89initialization models are discussed in the following sections.
  90
  91An LLD interfaces to the SCSI subsystem several ways:
  92  a) directly invoking functions supplied by the mid level
  93  b) passing a set of function pointers to a registration function
  94     supplied by the mid level. The mid level will then invoke these
  95     functions at some point in the future. The LLD will supply
  96     implementations of these functions.
  97  c) direct access to instances of well known data structures maintained
  98     by the mid level
  99
 100Those functions in group a) are listed in a section entitled "Mid level
 101supplied functions" below.
 102
 103Those functions in group b) are listed in a section entitled "Interface
 104functions" below. Their function pointers are placed in the members of
 105"struct scsi_host_template", an instance of which is passed to
 106scsi_host_alloc() ** .  Those interface functions that the LLD does not 
 107wish to supply should have NULL placed in the corresponding member of 
 108struct scsi_host_template.  Defining an instance of struct 
 109scsi_host_template at file scope will cause NULL to be  placed in function
 110 pointer members not explicitly initialized.
 111
 112Those usages in group c) should be handled with care, especially in a
 113"hotplug" environment. LLDs should be aware of the lifetime of instances
 114that are shared with the mid level and other layers.
 115
 116All functions defined within an LLD and all data defined at file scope
 117should be static. For example the slave_alloc() function in an LLD
 118called "xxx" could be defined as 
 119"static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }"
 120
 121** the scsi_host_alloc() function is a replacement for the rather vaguely
 122named scsi_register() function in most situations. The scsi_register()
 123and scsi_unregister() functions remain to support legacy LLDs that use
 124the passive initialization model.
 125
 126
 127Hotplug initialization model
 128============================
 129In this model an LLD controls when SCSI hosts are introduced and removed
 130from the SCSI subsystem. Hosts can be introduced as early as driver
 131initialization and removed as late as driver shutdown. Typically a driver
 132will respond to a sysfs probe() callback that indicates an HBA has been
 133detected. After confirming that the new device is one that the LLD wants
 134to control, the LLD will initialize the HBA and then register a new host
 135with the SCSI mid level.
 136
 137During LLD initialization the driver should register itself with the
 138appropriate IO bus on which it expects to find HBA(s) (e.g. the PCI bus).
 139This can probably be done via sysfs. Any driver parameters (especially
 140those that are writable after the driver is loaded) could also be
 141registered with sysfs at this point. The SCSI mid level first becomes
 142aware of an LLD when that LLD registers its first HBA.
 143
 144At some later time, the LLD becomes aware of an HBA and what follows
 145is a typical sequence of calls between the LLD and the mid level.
 146This example shows the mid level scanning the newly introduced HBA for 3 
 147scsi devices of which only the first 2 respond:
 148
 149     HBA PROBE: assume 2 SCSI devices found in scan
 150LLD                   mid level                    LLD
 151===-------------------=========--------------------===------
 152scsi_host_alloc()  -->
 153scsi_add_host()  ---->
 154scsi_scan_host()  -------+
 155                         |
 156                    slave_alloc()
 157                    slave_configure() -->  scsi_adjust_queue_depth()
 158                         |
 159                    slave_alloc()
 160                    slave_configure()
 161                         |
 162                    slave_alloc()   ***
 163                    slave_destroy() ***
 164------------------------------------------------------------
 165
 166If the LLD wants to adjust the default queue settings, it can invoke
 167scsi_adjust_queue_depth() in its slave_configure() routine.
 168
 169*** For scsi devices that the mid level tries to scan but do not
 170    respond, a slave_alloc(), slave_destroy() pair is called.
 171
 172When an HBA is being removed it could be as part of an orderly shutdown
 173associated with the LLD module being unloaded (e.g. with the "rmmod"
 174command) or in response to a "hot unplug" indicated by sysfs()'s
 175remove() callback being invoked. In either case, the sequence is the
 176same:
 177
 178        HBA REMOVE: assume 2 SCSI devices attached
 179LLD                      mid level                 LLD
 180===----------------------=========-----------------===------
 181scsi_remove_host() ---------+
 182                            |
 183                     slave_destroy()
 184                     slave_destroy()
 185scsi_host_put()
 186------------------------------------------------------------
 187                     
 188It may be useful for a LLD to keep track of struct Scsi_Host instances
 189(a pointer is returned by scsi_host_alloc()). Such instances are "owned"
 190by the mid-level.  struct Scsi_Host instances are freed from
 191scsi_host_put() when the reference count hits zero.
 192
 193Hot unplugging an HBA that controls a disk which is processing SCSI
 194commands on a mounted file system is an interesting situation. Reference
 195counting logic is being introduced into the mid level to cope with many
 196of the issues involved. See the section on reference counting below.
 197
 198
 199The hotplug concept may be extended to SCSI devices. Currently, when an
 200HBA is added, the scsi_scan_host() function causes a scan for SCSI devices
 201attached to the HBA's SCSI transport. On newer SCSI transports the HBA
 202may become aware of a new SCSI device _after_ the scan has completed.
 203An LLD can use this sequence to make the mid level aware of a SCSI device:
 204
 205                 SCSI DEVICE hotplug
 206LLD                   mid level                    LLD
 207===-------------------=========--------------------===------
 208scsi_add_device()  ------+
 209                         |
 210                    slave_alloc()
 211                    slave_configure()   [--> scsi_adjust_queue_depth()]
 212------------------------------------------------------------
 213
 214In a similar fashion, an LLD may become aware that a SCSI device has been
 215removed (unplugged) or the connection to it has been interrupted. Some
 216existing SCSI transports (e.g. SPI) may not become aware that a SCSI
 217device has been removed until a subsequent SCSI command fails which will
 218probably cause that device to be set offline by the mid level. An LLD that
 219detects the removal of a SCSI device can instigate its removal from
 220upper layers with this sequence:
 221
 222                  SCSI DEVICE hot unplug
 223LLD                      mid level                 LLD
 224===----------------------=========-----------------===------
 225scsi_remove_device() -------+
 226                            |
 227                     slave_destroy()
 228------------------------------------------------------------
 229
 230It may be useful for an LLD to keep track of struct scsi_device instances
 231(a pointer is passed as the parameter to slave_alloc() and
 232slave_configure() callbacks). Such instances are "owned" by the mid-level.
 233struct scsi_device instances are freed after slave_destroy().
 234
 235
 236Passive initialization model
 237============================
 238These older LLDs include a file called "scsi_module.c" [yes the ".c" is a
 239little surprising] in their source code. For that file to work an
 240instance of struct scsi_host_template with the name "driver_template"
 241needs to be defined. Here is a typical code sequence used in this model:
 242    static struct scsi_host_template driver_template = {
 243        ...
 244    };
 245    #include "scsi_module.c"
 246
 247The scsi_module.c file contains two functions:
 248  - init_this_scsi_driver() which is executed when the LLD is
 249    initialized (i.e. boot time or module load time)
 250  - exit_this_scsi_driver() which is executed when the LLD is shut
 251    down (i.e. module unload time)
 252Note: since these functions are tagged with __init and __exit qualifiers
 253an LLD should not call them explicitly (since the kernel does that).
 254
 255Here is an example of an initialization sequence when two hosts are
 256detected (so detect() returns 2) and the SCSI bus scan on each host
 257finds 1 SCSI device (and a second device does not respond).
 258
 259LLD                      mid level                 LLD
 260===----------------------=========-----------------===------
 261init_this_scsi_driver() ----+
 262                            |
 263                         detect()  -----------------+
 264                            |                       |
 265                            |                scsi_register()
 266                            |                scsi_register()
 267                            |
 268                      slave_alloc()
 269                      slave_configure()  -->  scsi_adjust_queue_depth()
 270                      slave_alloc()   ***
 271                      slave_destroy() ***
 272                            |
 273                      slave_alloc()
 274                      slave_configure()
 275                      slave_alloc()   ***
 276                      slave_destroy() ***
 277------------------------------------------------------------
 278
 279The mid level invokes scsi_adjust_queue_depth() with tagged queuing off and
 280"cmd_per_lun" for that host as the queue length. These settings can be
 281overridden by a slave_configure() supplied by the LLD.
 282
 283*** For scsi devices that the mid level tries to scan but do not
 284    respond, a slave_alloc(), slave_destroy() pair is called.
 285
 286Here is an LLD shutdown sequence:
 287
 288LLD                      mid level                 LLD
 289===----------------------=========-----------------===------
 290exit_this_scsi_driver() ----+
 291                            |
 292                     slave_destroy()
 293                        release()   -->   scsi_unregister()
 294                            |
 295                     slave_destroy()
 296                        release()   -->   scsi_unregister()
 297------------------------------------------------------------
 298
 299An LLD need not define slave_destroy() (i.e. it is optional). 
 300
 301The shortcoming of the "passive initialization model" is that host
 302registration and de-registration are (typically) tied to LLD initialization
 303and shutdown. Once the LLD is initialized then a new host that appears
 304(e.g. via hotplugging) cannot easily be added without a redundant
 305driver shutdown and re-initialization. It may be possible to write an LLD
 306that uses both initialization models.
 307
 308
 309Reference Counting
 310==================
 311The Scsi_Host structure has had reference counting infrastructure added.
 312This effectively spreads the ownership of struct Scsi_Host instances
 313across the various SCSI layers which use them. Previously such instances
 314were exclusively owned by the mid level. LLDs would not usually need to
 315directly manipulate these reference counts but there may be some cases
 316where they do.
 317
 318There are 3 reference counting functions of interest associated with
 319struct Scsi_Host:
 320  - scsi_host_alloc(): returns a pointer to new instance of struct 
 321        Scsi_Host which has its reference count ^^ set to 1
 322  - scsi_host_get(): adds 1 to the reference count of the given instance
 323  - scsi_host_put(): decrements 1 from the reference count of the given
 324        instance. If the reference count reaches 0 then the given instance
 325        is freed
 326
 327The Scsi_device structure has had reference counting infrastructure added.
 328This effectively spreads the ownership of struct Scsi_device instances
 329across the various SCSI layers which use them. Previously such instances
 330were exclusively owned by the mid level. See the access functions declared
 331towards the end of include/scsi/scsi_device.h . If an LLD wants to keep
 332a copy of a pointer to a Scsi_device instance it should use scsi_device_get()
 333to bump its reference count. When it is finished with the pointer it can
 334use scsi_device_put() to decrement its reference count (and potentially
 335delete it).
 336
 337^^ struct Scsi_Host actually has 2 reference counts which are manipulated
 338in parallel by these functions.
 339
 340
 341Conventions
 342===========
 343First, Linus Torvalds's thoughts on C coding style can be found in the
 344Documentation/CodingStyle file. 
 345
 346Next, there is a movement to "outlaw" typedefs introducing synonyms for 
 347struct tags. Both can be still found in the SCSI subsystem, but
 348the typedefs have been moved to a single file, scsi_typedefs.h to
 349make their future removal easier, for example: 
 350"typedef struct scsi_cmnd Scsi_Cmnd;"
 351
 352Also, most C99 enhancements are encouraged to the extent they are supported
 353by the relevant gcc compilers. So C99 style structure and array
 354initializers are encouraged where appropriate. Don't go too far,
 355VLAs are not properly supported yet.  An exception to this is the use of
 356"//" style comments; /*...*/ comments are still preferred in Linux.
 357
 358Well written, tested and documented code, need not be re-formatted to
 359comply with the above conventions. For example, the aic7xxx driver
 360comes to Linux from FreeBSD and Adaptec's own labs. No doubt FreeBSD
 361and Adaptec have their own coding conventions.
 362
 363
 364Mid level supplied functions
 365============================
 366These functions are supplied by the SCSI mid level for use by LLDs.
 367The names (i.e. entry points) of these functions are exported 
 368so an LLD that is a module can access them. The kernel will
 369arrange for the SCSI mid level to be loaded and initialized before any LLD
 370is initialized. The functions below are listed alphabetically and their
 371names all start with "scsi_".
 372
 373Summary:
 374   scsi_activate_tcq - turn on tag command queueing
 375   scsi_add_device - creates new scsi device (lu) instance
 376   scsi_add_host - perform sysfs registration and set up transport class
 377   scsi_adjust_queue_depth - change the queue depth on a SCSI device
 378   scsi_bios_ptable - return copy of block device's partition table
 379   scsi_block_requests - prevent further commands being queued to given host
 380   scsi_deactivate_tcq - turn off tag command queueing
 381   scsi_host_alloc - return a new scsi_host instance whose refcount==1
 382   scsi_host_get - increments Scsi_Host instance's refcount
 383   scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
 384   scsi_partsize - parse partition table into cylinders, heads + sectors
 385   scsi_register - create and register a scsi host adapter instance.
 386   scsi_remove_device - detach and remove a SCSI device
 387   scsi_remove_host - detach and remove all SCSI devices owned by host
 388   scsi_report_bus_reset - report scsi _bus_ reset observed
 389   scsi_scan_host - scan SCSI bus
 390   scsi_track_queue_full - track successive QUEUE_FULL events 
 391   scsi_unblock_requests - allow further commands to be queued to given host
 392   scsi_unregister - [calls scsi_host_put()]
 393
 394
 395Details:
 396
 397/**
 398 * scsi_activate_tcq - turn on tag command queueing ("ordered" task attribute)
 399 * @sdev:       device to turn on TCQ for
 400 * @depth:      queue depth
 401 *
 402 *      Returns nothing
 403 *
 404 *      Might block: no
 405 *
 406 *      Notes: Eventually, it is hoped depth would be the maximum depth
 407 *      the device could cope with and the real queue depth
 408 *      would be adjustable from 0 to depth.
 409 *
 410 *      Defined (inline) in: include/scsi/scsi_tcq.h
 411 **/
 412void scsi_activate_tcq(struct scsi_device *sdev, int depth)
 413
 414
 415/**
 416 * scsi_add_device - creates new scsi device (lu) instance
 417 * @shost:   pointer to scsi host instance
 418 * @channel: channel number (rarely other than 0)
 419 * @id:      target id number
 420 * @lun:     logical unit number
 421 *
 422 *      Returns pointer to new struct scsi_device instance or 
 423 *      ERR_PTR(-ENODEV) (or some other bent pointer) if something is
 424 *      wrong (e.g. no lu responds at given address)
 425 *
 426 *      Might block: yes
 427 *
 428 *      Notes: This call is usually performed internally during a scsi
 429 *      bus scan when an HBA is added (i.e. scsi_scan_host()). So it
 430 *      should only be called if the HBA becomes aware of a new scsi
 431 *      device (lu) after scsi_scan_host() has completed. If successful
 432 *      this call can lead to slave_alloc() and slave_configure() callbacks
 433 *      into the LLD.
 434 *
 435 *      Defined in: drivers/scsi/scsi_scan.c
 436 **/
 437struct scsi_device * scsi_add_device(struct Scsi_Host *shost, 
 438                                     unsigned int channel,
 439                                     unsigned int id, unsigned int lun)
 440
 441
 442/**
 443 * scsi_add_host - perform sysfs registration and set up transport class
 444 * @shost:   pointer to scsi host instance
 445 * @dev:     pointer to struct device of type scsi class
 446 *
 447 *      Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
 448 *
 449 *      Might block: no
 450 *
 451 *      Notes: Only required in "hotplug initialization model" after a
 452 *      successful call to scsi_host_alloc().  This function does not
 453 *      scan the bus; this can be done by calling scsi_scan_host() or
 454 *      in some other transport-specific way.  The LLD must set up
 455 *      the transport template before calling this function and may only
 456 *      access the transport class data after this function has been called.
 457 *
 458 *      Defined in: drivers/scsi/hosts.c
 459 **/
 460int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
 461
 462
 463/**
 464 * scsi_adjust_queue_depth - allow LLD to change queue depth on a SCSI device
 465 * @sdev:       pointer to SCSI device to change queue depth on
 466 * @tagged:     0 - no tagged queuing
 467 *              MSG_SIMPLE_TAG - simple tagged queuing
 468 *              MSG_ORDERED_TAG - ordered tagged queuing
 469 * @tags        Number of tags allowed if tagged queuing enabled,
 470 *              or number of commands the LLD can queue up
 471 *              in non-tagged mode (as per cmd_per_lun).
 472 *
 473 *      Returns nothing
 474 *
 475 *      Might block: no
 476 *
 477 *      Notes: Can be invoked any time on a SCSI device controlled by this
 478 *      LLD. [Specifically during and after slave_configure() and prior to
 479 *      slave_destroy().] Can safely be invoked from interrupt code. Actual
 480 *      queue depth change may be delayed until the next command is being
 481 *      processed. See also scsi_activate_tcq() and scsi_deactivate_tcq().
 482 *
 483 *      Defined in: drivers/scsi/scsi.c [see source code for more notes]
 484 *
 485 **/
 486void scsi_adjust_queue_depth(struct scsi_device * sdev, int tagged, 
 487                             int tags)
 488
 489
 490/**
 491 * scsi_bios_ptable - return copy of block device's partition table
 492 * @dev:        pointer to block device
 493 *
 494 *      Returns pointer to partition table, or NULL for failure
 495 *
 496 *      Might block: yes
 497 *
 498 *      Notes: Caller owns memory returned (free with kfree() )
 499 *
 500 *      Defined in: drivers/scsi/scsicam.c
 501 **/
 502unsigned char *scsi_bios_ptable(struct block_device *dev)
 503
 504
 505/**
 506 * scsi_block_requests - prevent further commands being queued to given host
 507 *
 508 * @shost: pointer to host to block commands on
 509 *
 510 *      Returns nothing
 511 *
 512 *      Might block: no
 513 *
 514 *      Notes: There is no timer nor any other means by which the requests
 515 *      get unblocked other than the LLD calling scsi_unblock_requests().
 516 *
 517 *      Defined in: drivers/scsi/scsi_lib.c
 518**/
 519void scsi_block_requests(struct Scsi_Host * shost)
 520
 521
 522/**
 523 * scsi_deactivate_tcq - turn off tag command queueing
 524 * @sdev:       device to turn off TCQ for
 525 * @depth:      queue depth (stored in sdev)
 526 *
 527 *      Returns nothing
 528 *
 529 *      Might block: no
 530 *
 531 *      Defined (inline) in: include/scsi/scsi_tcq.h
 532 **/
 533void scsi_deactivate_tcq(struct scsi_device *sdev, int depth)
 534
 535
 536/**
 537 * scsi_host_alloc - create a scsi host adapter instance and perform basic
 538 *                   initialization.
 539 * @sht:        pointer to scsi host template
 540 * @privsize:   extra bytes to allocate in hostdata array (which is the
 541 *              last member of the returned Scsi_Host instance)
 542 *
 543 *      Returns pointer to new Scsi_Host instance or NULL on failure
 544 *
 545 *      Might block: yes
 546 *
 547 *      Notes: When this call returns to the LLD, the SCSI bus scan on
 548 *      this host has _not_ yet been done.
 549 *      The hostdata array (by default zero length) is a per host scratch 
 550 *      area for the LLD's exclusive use.
 551 *      Both associated refcounting objects have their refcount set to 1.
 552 *      Full registration (in sysfs) and a bus scan are performed later when
 553 *      scsi_add_host() and scsi_scan_host() are called.
 554 *
 555 *      Defined in: drivers/scsi/hosts.c .
 556 **/
 557struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
 558                                   int privsize)
 559
 560
 561/**
 562 * scsi_host_get - increment Scsi_Host instance refcount
 563 * @shost:   pointer to struct Scsi_Host instance
 564 *
 565 *      Returns nothing
 566 *
 567 *      Might block: currently may block but may be changed to not block
 568 *
 569 *      Notes: Actually increments the counts in two sub-objects
 570 *
 571 *      Defined in: drivers/scsi/hosts.c
 572 **/
 573void scsi_host_get(struct Scsi_Host *shost)
 574
 575
 576/**
 577 * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
 578 * @shost:   pointer to struct Scsi_Host instance
 579 *
 580 *      Returns nothing
 581 *
 582 *      Might block: currently may block but may be changed to not block
 583 *
 584 *      Notes: Actually decrements the counts in two sub-objects. If the
 585 *      latter refcount reaches 0, the Scsi_Host instance is freed.
 586 *      The LLD need not worry exactly when the Scsi_Host instance is
 587 *      freed, it just shouldn't access the instance after it has balanced
 588 *      out its refcount usage.
 589 *
 590 *      Defined in: drivers/scsi/hosts.c
 591 **/
 592void scsi_host_put(struct Scsi_Host *shost)
 593
 594
 595/**
 596 * scsi_partsize - parse partition table into cylinders, heads + sectors
 597 * @buf: pointer to partition table
 598 * @capacity: size of (total) disk in 512 byte sectors
 599 * @cyls: outputs number of cylinders calculated via this pointer
 600 * @hds: outputs number of heads calculated via this pointer
 601 * @secs: outputs number of sectors calculated via this pointer
 602 *
 603 *      Returns 0 on success, -1 on failure
 604 *
 605 *      Might block: no
 606 *
 607 *      Notes: Caller owns memory returned (free with kfree() )
 608 *
 609 *      Defined in: drivers/scsi/scsicam.c
 610 **/
 611int scsi_partsize(unsigned char *buf, unsigned long capacity,
 612                  unsigned int *cyls, unsigned int *hds, unsigned int *secs)
 613
 614
 615/**
 616 * scsi_register - create and register a scsi host adapter instance.
 617 * @sht:        pointer to scsi host template
 618 * @privsize:   extra bytes to allocate in hostdata array (which is the
 619 *              last member of the returned Scsi_Host instance)
 620 *
 621 *      Returns pointer to new Scsi_Host instance or NULL on failure
 622 *
 623 *      Might block: yes
 624 *
 625 *      Notes: When this call returns to the LLD, the SCSI bus scan on
 626 *      this host has _not_ yet been done.
 627 *      The hostdata array (by default zero length) is a per host scratch 
 628 *      area for the LLD.
 629 *
 630 *      Defined in: drivers/scsi/hosts.c .
 631 **/
 632struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
 633                                 int privsize)
 634
 635
 636/**
 637 * scsi_remove_device - detach and remove a SCSI device
 638 * @sdev:      a pointer to a scsi device instance
 639 *
 640 *      Returns value: 0 on success, -EINVAL if device not attached
 641 *
 642 *      Might block: yes
 643 *
 644 *      Notes: If an LLD becomes aware that a scsi device (lu) has
 645 *      been removed but its host is still present then it can request
 646 *      the removal of that scsi device. If successful this call will
 647 *      lead to the slave_destroy() callback being invoked. sdev is an 
 648 *      invalid pointer after this call.
 649 *
 650 *      Defined in: drivers/scsi/scsi_sysfs.c .
 651 **/
 652int scsi_remove_device(struct scsi_device *sdev)
 653
 654
 655/**
 656 * scsi_remove_host - detach and remove all SCSI devices owned by host
 657 * @shost:      a pointer to a scsi host instance
 658 *
 659 *      Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
 660 *
 661 *      Might block: yes
 662 *
 663 *      Notes: Should only be invoked if the "hotplug initialization
 664 *      model" is being used. It should be called _prior_ to  
 665 *      scsi_unregister().
 666 *
 667 *      Defined in: drivers/scsi/hosts.c .
 668 **/
 669int scsi_remove_host(struct Scsi_Host *shost)
 670
 671
 672/**
 673 * scsi_report_bus_reset - report scsi _bus_ reset observed
 674 * @shost: a pointer to a scsi host involved
 675 * @channel: channel (within) host on which scsi bus reset occurred
 676 *
 677 *      Returns nothing
 678 *
 679 *      Might block: no
 680 *
 681 *      Notes: This only needs to be called if the reset is one which
 682 *      originates from an unknown location.  Resets originated by the 
 683 *      mid level itself don't need to call this, but there should be 
 684 *      no harm.  The main purpose of this is to make sure that a
 685 *      CHECK_CONDITION is properly treated.
 686 *
 687 *      Defined in: drivers/scsi/scsi_error.c .
 688 **/
 689void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
 690
 691
 692/**
 693 * scsi_scan_host - scan SCSI bus
 694 * @shost: a pointer to a scsi host instance
 695 *
 696 *      Might block: yes
 697 *
 698 *      Notes: Should be called after scsi_add_host()
 699 *
 700 *      Defined in: drivers/scsi/scsi_scan.c
 701 **/
 702void scsi_scan_host(struct Scsi_Host *shost)
 703
 704
 705/**
 706 * scsi_track_queue_full - track successive QUEUE_FULL events on given
 707 *                      device to determine if and when there is a need
 708 *                      to adjust the queue depth on the device.
 709 * @sdev:  pointer to SCSI device instance
 710 * @depth: Current number of outstanding SCSI commands on this device,
 711 *         not counting the one returned as QUEUE_FULL.
 712 *
 713 *      Returns 0  - no change needed
 714 *              >0 - adjust queue depth to this new depth
 715 *              -1 - drop back to untagged operation using host->cmd_per_lun
 716 *                   as the untagged command depth
 717 *
 718 *      Might block: no
 719 *
 720 *      Notes: LLDs may call this at any time and we will do "The Right
 721 *              Thing"; interrupt context safe. 
 722 *
 723 *      Defined in: drivers/scsi/scsi.c .
 724 **/
 725int scsi_track_queue_full(struct scsi_device *sdev, int depth)
 726
 727
 728/**
 729 * scsi_unblock_requests - allow further commands to be queued to given host
 730 *
 731 * @shost: pointer to host to unblock commands on
 732 *
 733 *      Returns nothing
 734 *
 735 *      Might block: no
 736 *
 737 *      Defined in: drivers/scsi/scsi_lib.c .
 738**/
 739void scsi_unblock_requests(struct Scsi_Host * shost)
 740
 741
 742/**
 743 * scsi_unregister - unregister and free memory used by host instance
 744 * @shp:        pointer to scsi host instance to unregister.
 745 *
 746 *      Returns nothing
 747 *
 748 *      Might block: no
 749 *
 750 *      Notes: Should not be invoked if the "hotplug initialization
 751 *      model" is being used. Called internally by exit_this_scsi_driver()
 752 *      in the "passive initialization model". Hence a LLD has no need to
 753 *      call this function directly.
 754 *
 755 *      Defined in: drivers/scsi/hosts.c .
 756 **/
 757void scsi_unregister(struct Scsi_Host * shp)
 758
 759
 760
 761
 762Interface Functions
 763===================
 764Interface functions are supplied (defined) by LLDs and their function
 765pointers are placed in an instance of struct scsi_host_template which
 766is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
 767Some are mandatory. Interface functions should be declared static. The
 768accepted convention is that driver "xyz" will declare its slave_configure() 
 769function as:
 770    static int xyz_slave_configure(struct scsi_device * sdev);
 771and so forth for all interface functions listed below.
 772
 773A pointer to this function should be placed in the 'slave_configure' member
 774of a "struct scsi_host_template" instance. A pointer to such an instance
 775should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
 776init_this_scsi_driver()].
 777
 778The interface functions are also described in the include/scsi/scsi_host.h
 779file immediately above their definition point in "struct scsi_host_template".
 780In some cases more detail is given in scsi_host.h than below.
 781
 782The interface functions are listed below in alphabetical order.
 783
 784Summary:
 785   bios_param - fetch head, sector, cylinder info for a disk
 786   detect - detects HBAs this driver wants to control
 787   eh_timed_out - notify the host that a command timer expired
 788   eh_abort_handler - abort given command
 789   eh_bus_reset_handler - issue SCSI bus reset
 790   eh_device_reset_handler - issue SCSI device reset
 791   eh_host_reset_handler - reset host (host bus adapter)
 792   info - supply information about given host
 793   ioctl - driver can respond to ioctls
 794   proc_info - supports /proc/scsi/{driver_name}/{host_no}
 795   queuecommand - queue scsi command, invoke 'done' on completion
 796   release - release all resources associated with given host
 797   slave_alloc - prior to any commands being sent to a new device 
 798   slave_configure - driver fine tuning for given device after attach
 799   slave_destroy - given device is about to be shut down
 800
 801
 802Details:
 803
 804/**
 805 *      bios_param - fetch head, sector, cylinder info for a disk
 806 *      @sdev: pointer to scsi device context (defined in 
 807 *             include/scsi/scsi_device.h)
 808 *      @bdev: pointer to block device context (defined in fs.h)
 809 *      @capacity:  device size (in 512 byte sectors)
 810 *      @params: three element array to place output:
 811 *              params[0] number of heads (max 255)
 812 *              params[1] number of sectors (max 63)
 813 *              params[2] number of cylinders 
 814 *
 815 *      Return value is ignored
 816 *
 817 *      Locks: none
 818 *
 819 *      Calling context: process (sd)
 820 *
 821 *      Notes: an arbitrary geometry (based on READ CAPACITY) is used
 822 *      if this function is not provided. The params array is
 823 *      pre-initialized with made up values just in case this function 
 824 *      doesn't output anything.
 825 *
 826 *      Optionally defined in: LLD
 827 **/
 828    int bios_param(struct scsi_device * sdev, struct block_device *bdev,
 829                   sector_t capacity, int params[3])
 830
 831
 832/**
 833 *      detect - detects HBAs this driver wants to control
 834 *      @shtp: host template for this driver.
 835 *
 836 *      Returns number of hosts this driver wants to control. 0 means no
 837 *      suitable hosts found.
 838 *
 839 *      Locks: none held
 840 *
 841 *      Calling context: process [invoked from init_this_scsi_driver()]
 842 *
 843 *      Notes: First function called from the SCSI mid level on this
 844 *      driver. Upper level drivers (e.g. sd) may not (yet) be present.
 845 *      For each host found, this method should call scsi_register() 
 846 *      [see hosts.c].
 847 *
 848 *      Defined in: LLD (required if "passive initialization mode" is used,
 849 *                       not invoked in "hotplug initialization mode")
 850 **/
 851    int detect(struct scsi_host_template * shtp)
 852
 853
 854/**
 855 *      eh_timed_out - The timer for the command has just fired
 856 *      @scp: identifies command timing out
 857 *
 858 *      Returns:
 859 *
 860 *      EH_HANDLED:             I fixed the error, please complete the command
 861 *      EH_RESET_TIMER:         I need more time, reset the timer and
 862 *                              begin counting again
 863 *      EH_NOT_HANDLED          Begin normal error recovery
 864 *
 865 *
 866 *      Locks: None held
 867 *
 868 *      Calling context: interrupt
 869 *
 870 *      Notes: This is to give the LLD an opportunity to do local recovery.
 871 *      This recovery is limited to determining if the outstanding command
 872 *      will ever complete.  You may not abort and restart the command from
 873 *      this callback.
 874 *
 875 *      Optionally defined in: LLD
 876 **/
 877     int eh_timed_out(struct scsi_cmnd * scp)
 878
 879
 880/**
 881 *      eh_abort_handler - abort command associated with scp
 882 *      @scp: identifies command to be aborted
 883 *
 884 *      Returns SUCCESS if command aborted else FAILED
 885 *
 886 *      Locks: None held
 887 *
 888 *      Calling context: kernel thread
 889 *
 890 *      Notes: Invoked from scsi_eh thread. No other commands will be
 891 *      queued on current host during eh.
 892 *
 893 *      Optionally defined in: LLD
 894 **/
 895     int eh_abort_handler(struct scsi_cmnd * scp)
 896
 897
 898/**
 899 *      eh_bus_reset_handler - issue SCSI bus reset
 900 *      @scp: SCSI bus that contains this device should be reset
 901 *
 902 *      Returns SUCCESS if command aborted else FAILED
 903 *
 904 *      Locks: None held
 905 *
 906 *      Calling context: kernel thread
 907 *
 908 *      Notes: Invoked from scsi_eh thread. No other commands will be
 909 *      queued on current host during eh.
 910 *
 911 *      Optionally defined in: LLD
 912 **/
 913     int eh_bus_reset_handler(struct scsi_cmnd * scp)
 914
 915
 916/**
 917 *      eh_device_reset_handler - issue SCSI device reset
 918 *      @scp: identifies SCSI device to be reset
 919 *
 920 *      Returns SUCCESS if command aborted else FAILED
 921 *
 922 *      Locks: None held
 923 *
 924 *      Calling context: kernel thread
 925 *
 926 *      Notes: Invoked from scsi_eh thread. No other commands will be
 927 *      queued on current host during eh.
 928 *
 929 *      Optionally defined in: LLD
 930 **/
 931     int eh_device_reset_handler(struct scsi_cmnd * scp)
 932
 933
 934/**
 935 *      eh_host_reset_handler - reset host (host bus adapter)
 936 *      @scp: SCSI host that contains this device should be reset
 937 *
 938 *      Returns SUCCESS if command aborted else FAILED
 939 *
 940 *      Locks: None held
 941 *
 942 *      Calling context: kernel thread
 943 *
 944 *      Notes: Invoked from scsi_eh thread. No other commands will be
 945 *      queued on current host during eh. 
 946 *      With the default eh_strategy in place, if none of the _abort_, 
 947 *      _device_reset_, _bus_reset_ or this eh handler function are 
 948 *      defined (or they all return FAILED) then the device in question 
 949 *      will be set offline whenever eh is invoked.
 950 *
 951 *      Optionally defined in: LLD
 952 **/
 953     int eh_host_reset_handler(struct scsi_cmnd * scp)
 954
 955
 956/**
 957 *      info - supply information about given host: driver name plus data
 958 *             to distinguish given host
 959 *      @shp: host to supply information about
 960 *
 961 *      Return ASCII null terminated string. [This driver is assumed to
 962 *      manage the memory pointed to and maintain it, typically for the
 963 *      lifetime of this host.]
 964 *
 965 *      Locks: none
 966 *
 967 *      Calling context: process
 968 *
 969 *      Notes: Often supplies PCI or ISA information such as IO addresses
 970 *      and interrupt numbers. If not supplied struct Scsi_Host::name used
 971 *      instead. It is assumed the returned information fits on one line 
 972 *      (i.e. does not included embedded newlines).
 973 *      The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
 974 *      function (or struct Scsi_Host::name if this function is not
 975 *      available).
 976 *      In a similar manner, init_this_scsi_driver() outputs to the console
 977 *      each host's "info" (or name) for the driver it is registering.
 978 *      Also if proc_info() is not supplied, the output of this function
 979 *      is used instead.
 980 *
 981 *      Optionally defined in: LLD
 982 **/
 983    const char * info(struct Scsi_Host * shp)
 984
 985
 986/**
 987 *      ioctl - driver can respond to ioctls
 988 *      @sdp: device that ioctl was issued for
 989 *      @cmd: ioctl number
 990 *      @arg: pointer to read or write data from. Since it points to
 991 *            user space, should use appropriate kernel functions
 992 *            (e.g. copy_from_user() ). In the Unix style this argument
 993 *            can also be viewed as an unsigned long.
 994 *
 995 *      Returns negative "errno" value when there is a problem. 0 or a
 996 *      positive value indicates success and is returned to the user space.
 997 *
 998 *      Locks: none
 999 *
1000 *      Calling context: process
1001 *
1002 *      Notes: The SCSI subsystem uses a "trickle down" ioctl model.
1003 *      The user issues an ioctl() against an upper level driver
1004 *      (e.g. /dev/sdc) and if the upper level driver doesn't recognize
1005 *      the 'cmd' then it is passed to the SCSI mid level. If the SCSI
1006 *      mid level does not recognize it, then the LLD that controls
1007 *      the device receives the ioctl. According to recent Unix standards
1008 *      unsupported ioctl() 'cmd' numbers should return -ENOTTY.
1009 *
1010 *      Optionally defined in: LLD
1011 **/
1012    int ioctl(struct scsi_device *sdp, int cmd, void *arg)
1013
1014
1015/**
1016 *      proc_info - supports /proc/scsi/{driver_name}/{host_no}
1017 *      @buffer: anchor point to output to (0==writeto1_read0) or fetch from
1018 *               (1==writeto1_read0).
1019 *      @start: where "interesting" data is written to. Ignored when
1020 *              1==writeto1_read0.
1021 *      @offset: offset within buffer 0==writeto1_read0 is actually
1022 *               interested in. Ignored when 1==writeto1_read0 .
1023 *      @length: maximum (or actual) extent of buffer
1024 *      @host_no: host number of interest (struct Scsi_Host::host_no)
1025 *      @writeto1_read0: 1 -> data coming from user space towards driver
1026 *                            (e.g. "echo some_string > /proc/scsi/xyz/2")
1027 *                       0 -> user what data from this driver
1028 *                            (e.g. "cat /proc/scsi/xyz/2")
1029 *
1030 *      Returns length when 1==writeto1_read0. Otherwise number of chars
1031 *      output to buffer past offset.
1032 *
1033 *      Locks: none held
1034 *
1035 *      Calling context: process
1036 *
1037 *      Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
1038 *      support can now be configured out of the scsi subsystem.
1039 *
1040 *      Optionally defined in: LLD
1041 **/
1042    int proc_info(char * buffer, char ** start, off_t offset, 
1043                  int length, int host_no, int writeto1_read0)
1044
1045
1046/**
1047 *      queuecommand - queue scsi command, invoke 'done' on completion
1048 *      @scp: pointer to scsi command object
1049 *      @done: function pointer to be invoked on completion
1050 *
1051 *      Returns 0 on success.
1052 *
1053 *      If there's a failure, return either:
1054 *
1055 *      SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
1056 *      SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
1057 *
1058 *      On both of these returns, the mid-layer will requeue the I/O
1059 *
1060 *      - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
1061 *      device will be paused, and it will be unpaused when a command to
1062 *      the device returns (or after a brief delay if there are no more
1063 *      outstanding commands to it).  Commands to other devices continue
1064 *      to be processed normally.
1065 *
1066 *      - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
1067 *      is paused and will be unpaused when any command returns from
1068 *      the host (or after a brief delay if there are no outstanding
1069 *      commands to the host).
1070 *
1071 *      For compatibility with earlier versions of queuecommand, any
1072 *      other return value is treated the same as
1073 *      SCSI_MLQUEUE_HOST_BUSY.
1074 *
1075 *      Other types of errors that are detected immediately may be
1076 *      flagged by setting scp->result to an appropriate value,
1077 *      invoking the 'done' callback, and then returning 0 from this
1078 *      function. If the command is not performed immediately (and the
1079 *      LLD is starting (or will start) the given command) then this
1080 *      function should place 0 in scp->result and return 0.
1081 *
1082 *      Command ownership.  If the driver returns zero, it owns the
1083 *      command and must take responsibility for ensuring the 'done'
1084 *      callback is executed.  Note: the driver may call done before
1085 *      returning zero, but after it has called done, it may not
1086 *      return any value other than zero.  If the driver makes a
1087 *      non-zero return, it must not execute the command's done
1088 *      callback at any time.
1089 *
1090 *      Locks: struct Scsi_Host::host_lock held on entry (with "irqsave")
1091 *             and is expected to be held on return.
1092 *
1093 *      Calling context: in interrupt (soft irq) or process context
1094 *
1095 *      Notes: This function should be relatively fast. Normally it will
1096 *      not wait for IO to complete. Hence the 'done' callback is invoked 
1097 *      (often directly from an interrupt service routine) some time after
1098 *      this function has returned. In some cases (e.g. pseudo adapter 
1099 *      drivers that manufacture the response to a SCSI INQUIRY)
1100 *      the 'done' callback may be invoked before this function returns.
1101 *      If the 'done' callback is not invoked within a certain period
1102 *      the SCSI mid level will commence error processing.
1103 *      If a status of CHECK CONDITION is placed in "result" when the
1104 *      'done' callback is invoked, then the LLD driver should 
1105 *      perform autosense and fill in the struct scsi_cmnd::sense_buffer
1106 *      array. The scsi_cmnd::sense_buffer array is zeroed prior to
1107 *      the mid level queuing a command to an LLD.
1108 *
1109 *      Defined in: LLD
1110 **/
1111    int queuecommand(struct scsi_cmnd * scp, 
1112                     void (*done)(struct scsi_cmnd *))
1113
1114
1115/**
1116 *      release - release all resources associated with given host
1117 *      @shp: host to be released.
1118 *
1119 *      Return value ignored (could soon be a function returning void).
1120 *
1121 *      Locks: none held
1122 *
1123 *      Calling context: process
1124 *
1125 *      Notes: Invoked from scsi_module.c's exit_this_scsi_driver().
1126 *      LLD's implementation of this function should call 
1127 *      scsi_unregister(shp) prior to returning.
1128 *      Only needed for old-style host templates.
1129 *
1130 *      Defined in: LLD (required in "passive initialization model",
1131 *                       should not be defined in hotplug model)
1132 **/
1133    int release(struct Scsi_Host * shp)
1134
1135
1136/**
1137 *      slave_alloc -   prior to any commands being sent to a new device 
1138 *                      (i.e. just prior to scan) this call is made
1139 *      @sdp: pointer to new device (about to be scanned)
1140 *
1141 *      Returns 0 if ok. Any other return is assumed to be an error and
1142 *      the device is ignored.
1143 *
1144 *      Locks: none
1145 *
1146 *      Calling context: process
1147 *
1148 *      Notes: Allows the driver to allocate any resources for a device
1149 *      prior to its initial scan. The corresponding scsi device may not
1150 *      exist but the mid level is just about to scan for it (i.e. send
1151 *      and INQUIRY command plus ...). If a device is found then
1152 *      slave_configure() will be called while if a device is not found
1153 *      slave_destroy() is called.
1154 *      For more details see the include/scsi/scsi_host.h file.
1155 *
1156 *      Optionally defined in: LLD
1157 **/
1158    int slave_alloc(struct scsi_device *sdp)
1159
1160
1161/**
1162 *      slave_configure - driver fine tuning for given device just after it
1163 *                     has been first scanned (i.e. it responded to an
1164 *                     INQUIRY)
1165 *      @sdp: device that has just been attached
1166 *
1167 *      Returns 0 if ok. Any other return is assumed to be an error and
1168 *      the device is taken offline. [offline devices will _not_ have
1169 *      slave_destroy() called on them so clean up resources.]
1170 *
1171 *      Locks: none
1172 *
1173 *      Calling context: process
1174 *
1175 *      Notes: Allows the driver to inspect the response to the initial
1176 *      INQUIRY done by the scanning code and take appropriate action.
1177 *      For more details see the include/scsi/scsi_host.h file.
1178 *
1179 *      Optionally defined in: LLD
1180 **/
1181    int slave_configure(struct scsi_device *sdp)
1182
1183
1184/**
1185 *      slave_destroy - given device is about to be shut down. All
1186 *                      activity has ceased on this device.
1187 *      @sdp: device that is about to be shut down
1188 *
1189 *      Returns nothing
1190 *
1191 *      Locks: none
1192 *
1193 *      Calling context: process
1194 *
1195 *      Notes: Mid level structures for given device are still in place
1196 *      but are about to be torn down. Any per device resources allocated
1197 *      by this driver for given device should be freed now. No further
1198 *      commands will be sent for this sdp instance. [However the device
1199 *      could be re-attached in the future in which case a new instance
1200 *      of struct scsi_device would be supplied by future slave_alloc()
1201 *      and slave_configure() calls.]
1202 *
1203 *      Optionally defined in: LLD
1204 **/
1205    void slave_destroy(struct scsi_device *sdp)
1206
1207
1208
1209Data Structures
1210===============
1211struct scsi_host_template
1212-------------------------
1213There is one "struct scsi_host_template" instance per LLD ***. It is
1214typically initialized as a file scope static in a driver's header file. That
1215way members that are not explicitly initialized will be set to 0 or NULL.
1216Member of interest:
1217    name         - name of driver (may contain spaces, please limit to
1218                   less than 80 characters)
1219    proc_name    - name used in "/proc/scsi/<proc_name>/<host_no>" and
1220                   by sysfs in one of its "drivers" directories. Hence
1221                   "proc_name" should only contain characters acceptable
1222                   to a Unix file name.
1223   (*queuecommand)() - primary callback that the mid level uses to inject
1224                   SCSI commands into an LLD.
1225The structure is defined and commented in include/scsi/scsi_host.h
1226
1227*** In extreme situations a single driver may have several instances
1228    if it controls several different classes of hardware (e.g. an LLD
1229    that handles both ISA and PCI cards and has a separate instance of
1230    struct scsi_host_template for each class).
1231
1232struct Scsi_Host
1233----------------
1234There is one struct Scsi_Host instance per host (HBA) that an LLD
1235controls. The struct Scsi_Host structure has many members in common
1236with "struct scsi_host_template". When a new struct Scsi_Host instance
1237is created (in scsi_host_alloc() in hosts.c) those common members are
1238initialized from the driver's struct scsi_host_template instance. Members
1239of interest:
1240    host_no      - system wide unique number that is used for identifying
1241                   this host. Issued in ascending order from 0.
1242    can_queue    - must be greater than 0; do not send more than can_queue
1243                   commands to the adapter.
1244    this_id      - scsi id of host (scsi initiator) or -1 if not known
1245    sg_tablesize - maximum scatter gather elements allowed by host.
1246                   0 implies scatter gather not supported by host
1247    max_sectors  - maximum number of sectors (usually 512 bytes) allowed
1248                   in a single SCSI command. The default value of 0 leads
1249                   to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
1250                   scsi_host.h) which is currently set to 1024. So for a
1251                   disk the maximum transfer size is 512 KB when max_sectors
1252                   is not defined. Note that this size may not be sufficient
1253                   for disk firmware uploads.
1254    cmd_per_lun  - maximum number of commands that can be queued on devices
1255                   controlled by the host. Overridden by LLD calls to
1256                   scsi_adjust_queue_depth().
1257    unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
1258                   restriction), 0=>can use full 32 bit (or better) DMA
1259                   address space
1260    use_clustering - 1=>SCSI commands in mid level's queue can be merged,
1261                     0=>disallow SCSI command merging
1262    hostt        - pointer to driver's struct scsi_host_template from which
1263                   this struct Scsi_Host instance was spawned
1264    hostt->proc_name  - name of LLD. This is the driver name that sysfs uses
1265    transportt   - pointer to driver's struct scsi_transport_template instance
1266                   (if any). FC and SPI transports currently supported.
1267    sh_list      - a double linked list of pointers to all struct Scsi_Host
1268                   instances (currently ordered by ascending host_no)
1269    my_devices   - a double linked list of pointers to struct scsi_device 
1270                   instances that belong to this host.
1271    hostdata[0]  - area reserved for LLD at end of struct Scsi_Host. Size
1272                   is set by the second argument (named 'xtr_bytes') to
1273                   scsi_host_alloc() or scsi_register().
1274
1275The scsi_host structure is defined in include/scsi/scsi_host.h
1276
1277struct scsi_device
1278------------------
1279Generally, there is one instance of this structure for each SCSI logical unit
1280on a host. Scsi devices connected to a host are uniquely identified by a
1281channel number, target id and logical unit number (lun).
1282The structure is defined in include/scsi/scsi_device.h
1283
1284struct scsi_cmnd
1285----------------
1286Instances of this structure convey SCSI commands to the LLD and responses
1287back to the mid level. The SCSI mid level will ensure that no more SCSI
1288commands become queued against the LLD than are indicated by
1289scsi_adjust_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
1290be at least one instance of struct scsi_cmnd available for each SCSI device.
1291Members of interest:
1292    cmnd         - array containing SCSI command
1293    cmnd_len     - length (in bytes) of SCSI command
1294    sc_data_direction - direction of data transfer in data phase. See
1295                "enum dma_data_direction" in include/linux/dma-mapping.h
1296    request_bufflen - number of data bytes to transfer (0 if no data phase)
1297    use_sg       - ==0 -> no scatter gather list, hence transfer data
1298                          to/from request_buffer
1299                 - >0 ->  scatter gather list (actually an array) in
1300                          request_buffer with use_sg elements
1301    request_buffer - either contains data buffer or scatter gather list
1302                     depending on the setting of use_sg. Scatter gather
1303                     elements are defined by 'struct scatterlist' found
1304                     in include/asm/scatterlist.h .
1305    done         - function pointer that should be invoked by LLD when the
1306                   SCSI command is completed (successfully or otherwise).
1307                   Should only be called by an LLD if the LLD has accepted
1308                   the command (i.e. queuecommand() returned or will return
1309                   0). The LLD may invoke 'done'  prior to queuecommand()
1310                   finishing.
1311    result       - should be set by LLD prior to calling 'done'. A value
1312                   of 0 implies a successfully completed command (and all
1313                   data (if any) has been transferred to or from the SCSI
1314                   target device). 'result' is a 32 bit unsigned integer that
1315                   can be viewed as 4 related bytes. The SCSI status value is
1316                   in the LSB. See include/scsi/scsi.h status_byte(),
1317                   msg_byte(), host_byte() and driver_byte() macros and
1318                   related constants.
1319    sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
1320                   should be written when the SCSI status (LSB of 'result')
1321                   is set to CHECK_CONDITION (2). When CHECK_CONDITION is
1322                   set, if the top nibble of sense_buffer[0] has the value 7
1323                   then the mid level will assume the sense_buffer array
1324                   contains a valid SCSI sense buffer; otherwise the mid
1325                   level will issue a REQUEST_SENSE SCSI command to
1326                   retrieve the sense buffer. The latter strategy is error
1327                   prone in the presence of command queuing so the LLD should
1328                   always "auto-sense".
1329    device       - pointer to scsi_device object that this command is
1330                   associated with.
1331    resid        - an LLD should set this signed integer to the requested
1332                   transfer length (i.e. 'request_bufflen') less the number
1333                   of bytes that are actually transferred. 'resid' is
1334                   preset to 0 so an LLD can ignore it if it cannot detect
1335                   underruns (overruns should be rare). If possible an LLD
1336                   should set 'resid' prior to invoking 'done'. The most
1337                   interesting case is data transfers from a SCSI target
1338                   device device (i.e. READs) that underrun. 
1339    underflow    - LLD should place (DID_ERROR << 16) in 'result' if
1340                   actual number of bytes transferred is less than this
1341                   figure. Not many LLDs implement this check and some that
1342                   do just output an error message to the log rather than
1343                   report a DID_ERROR. Better for an LLD to implement
1344                   'resid'.
1345
1346The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
1347
1348
1349Locks
1350=====
1351Each struct Scsi_Host instance has a spin_lock called struct 
1352Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in 
1353hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
1354is initialized to point at default_lock.  Thereafter lock and unlock
1355operations performed by the mid level use the struct Scsi_Host::host_lock
1356pointer.  Previously drivers could override the host_lock pointer but
1357this is not allowed anymore.
1358
1359
1360Autosense
1361=========
1362Autosense (or auto-sense) is defined in the SAM-2 document as "the
1363automatic return of sense data to the application client coincident
1364with the completion of a SCSI command" when a status of CHECK CONDITION
1365occurs. LLDs should perform autosense. This should be done when the LLD
1366detects a CHECK CONDITION status by either: 
1367    a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
1368       to perform an extra data in phase on such responses
1369    b) or, the LLD issuing a REQUEST SENSE command itself
1370
1371Either way, when a status of CHECK CONDITION is detected, the mid level
1372decides whether the LLD has performed autosense by checking struct 
1373scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
1374then autosense is assumed to have taken place. If it has another value (and
1375this byte is initialized to 0 before each command) then the mid level will
1376issue a REQUEST SENSE command.
1377
1378In the presence of queued commands the "nexus" that maintains sense
1379buffer data from the command that failed until a following REQUEST SENSE
1380may get out of synchronization. This is why it is best for the LLD
1381to perform autosense.
1382
1383
1384Changes since lk 2.4 series
1385===========================
1386io_request_lock has been replaced by several finer grained locks. The lock 
1387relevant to LLDs is struct Scsi_Host::host_lock and there is
1388one per SCSI host.
1389
1390The older error handling mechanism has been removed. This means the
1391LLD interface functions abort() and reset() have been removed.
1392The struct scsi_host_template::use_new_eh_code flag has been removed.
1393
1394In the 2.4 series the SCSI subsystem configuration descriptions were 
1395aggregated with the configuration descriptions from all other Linux 
1396subsystems in the Documentation/Configure.help file. In the 2.6 series, 
1397the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
1398file that contains both configuration and help information.
1399
1400struct SHT has been renamed to struct scsi_host_template.
1401
1402Addition of the "hotplug initialization model" and many extra functions
1403to support it.
1404
1405
1406Credits
1407=======
1408The following people have contributed to this document:
1409        Mike Anderson <andmike at us dot ibm dot com>
1410        James Bottomley <James dot Bottomley at hansenpartnership dot com>
1411        Patrick Mansfield <patmans at us dot ibm dot com> 
1412        Christoph Hellwig <hch at infradead dot org>
1413        Doug Ledford <dledford at redhat dot com>
1414        Andries Brouwer <Andries dot Brouwer at cwi dot nl>
1415        Randy Dunlap <rdunlap at xenotime dot net>
1416        Alan Stern <stern at rowland dot harvard dot edu>
1417
1418
1419Douglas Gilbert
1420dgilbert at interlog dot com
142121st September 2004
1422