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Change-Id: I8a9ee2aea93cd29c52c847d0ce33091a73ae6afe
diff --git a/Documentation/scsi/scsi_mid_low_api.txt b/Documentation/scsi/scsi_mid_low_api.txt
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+ Linux Kernel 2.6 series
+ SCSI mid_level - lower_level driver interface
+ =============================================
+
+Introduction
+============
+This document outlines the interface between the Linux SCSI mid level and
+SCSI lower level drivers. Lower level drivers (LLDs) are variously called
+host bus adapter (HBA) drivers and host drivers (HD). A "host" in this
+context is a bridge between a computer IO bus (e.g. PCI or ISA) and a
+single SCSI initiator port on a SCSI transport. An "initiator" port
+(SCSI terminology, see SAM-3 at http://www.t10.org) sends SCSI commands
+to "target" SCSI ports (e.g. disks). There can be many LLDs in a running
+system, but only one per hardware type. Most LLDs can control one or more
+SCSI HBAs. Some HBAs contain multiple hosts.
+
+In some cases the SCSI transport is an external bus that already has
+its own subsystem in Linux (e.g. USB and ieee1394). In such cases the
+SCSI subsystem LLD is a software bridge to the other driver subsystem.
+Examples are the usb-storage driver (found in the drivers/usb/storage
+directory) and the ieee1394/sbp2 driver (found in the drivers/ieee1394
+directory).
+
+For example, the aic7xxx LLD controls Adaptec SCSI parallel interface
+(SPI) controllers based on that company's 7xxx chip series. The aic7xxx
+LLD can be built into the kernel or loaded as a module. There can only be
+one aic7xxx LLD running in a Linux system but it may be controlling many
+HBAs. These HBAs might be either on PCI daughter-boards or built into
+the motherboard (or both). Some aic7xxx based HBAs are dual controllers
+and thus represent two hosts. Like most modern HBAs, each aic7xxx host
+has its own PCI device address. [The one-to-one correspondence between
+a SCSI host and a PCI device is common but not required (e.g. with
+ISA adapters).]
+
+The SCSI mid level isolates an LLD from other layers such as the SCSI
+upper layer drivers and the block layer.
+
+This version of the document roughly matches linux kernel version 2.6.8 .
+
+Documentation
+=============
+There is a SCSI documentation directory within the kernel source tree,
+typically Documentation/scsi . Most documents are in plain
+(i.e. ASCII) text. This file is named scsi_mid_low_api.txt and can be
+found in that directory. A more recent copy of this document may be found
+at http://web.archive.org/web/20070107183357rn_1/sg.torque.net/scsi/.
+Many LLDs are documented there (e.g. aic7xxx.txt). The SCSI mid-level is
+briefly described in scsi.txt which contains a url to a document
+describing the SCSI subsystem in the lk 2.4 series. Two upper level
+drivers have documents in that directory: st.txt (SCSI tape driver) and
+scsi-generic.txt (for the sg driver).
+
+Some documentation (or urls) for LLDs may be found in the C source code
+or in the same directory as the C source code. For example to find a url
+about the USB mass storage driver see the
+/usr/src/linux/drivers/usb/storage directory.
+
+Driver structure
+================
+Traditionally an LLD for the SCSI subsystem has been at least two files in
+the drivers/scsi directory. For example, a driver called "xyz" has a header
+file "xyz.h" and a source file "xyz.c". [Actually there is no good reason
+why this couldn't all be in one file; the header file is superfluous.] Some
+drivers that have been ported to several operating systems have more than
+two files. For example the aic7xxx driver has separate files for generic
+and OS-specific code (e.g. FreeBSD and Linux). Such drivers tend to have
+their own directory under the drivers/scsi directory.
+
+When a new LLD is being added to Linux, the following files (found in the
+drivers/scsi directory) will need some attention: Makefile and Kconfig .
+It is probably best to study how existing LLDs are organized.
+
+As the 2.5 series development kernels evolve into the 2.6 series
+production series, changes are being introduced into this interface. An
+example of this is driver initialization code where there are now 2 models
+available. The older one, similar to what was found in the lk 2.4 series,
+is based on hosts that are detected at HBA driver load time. This will be
+referred to the "passive" initialization model. The newer model allows HBAs
+to be hot plugged (and unplugged) during the lifetime of the LLD and will
+be referred to as the "hotplug" initialization model. The newer model is
+preferred as it can handle both traditional SCSI equipment that is
+permanently connected as well as modern "SCSI" devices (e.g. USB or
+IEEE 1394 connected digital cameras) that are hotplugged. Both
+initialization models are discussed in the following sections.
+
+An LLD interfaces to the SCSI subsystem several ways:
+ a) directly invoking functions supplied by the mid level
+ b) passing a set of function pointers to a registration function
+ supplied by the mid level. The mid level will then invoke these
+ functions at some point in the future. The LLD will supply
+ implementations of these functions.
+ c) direct access to instances of well known data structures maintained
+ by the mid level
+
+Those functions in group a) are listed in a section entitled "Mid level
+supplied functions" below.
+
+Those functions in group b) are listed in a section entitled "Interface
+functions" below. Their function pointers are placed in the members of
+"struct scsi_host_template", an instance of which is passed to
+scsi_host_alloc() ** . Those interface functions that the LLD does not
+wish to supply should have NULL placed in the corresponding member of
+struct scsi_host_template. Defining an instance of struct
+scsi_host_template at file scope will cause NULL to be placed in function
+ pointer members not explicitly initialized.
+
+Those usages in group c) should be handled with care, especially in a
+"hotplug" environment. LLDs should be aware of the lifetime of instances
+that are shared with the mid level and other layers.
+
+All functions defined within an LLD and all data defined at file scope
+should be static. For example the slave_alloc() function in an LLD
+called "xxx" could be defined as
+"static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }"
+
+** the scsi_host_alloc() function is a replacement for the rather vaguely
+named scsi_register() function in most situations. The scsi_register()
+and scsi_unregister() functions remain to support legacy LLDs that use
+the passive initialization model.
+
+
+Hotplug initialization model
+============================
+In this model an LLD controls when SCSI hosts are introduced and removed
+from the SCSI subsystem. Hosts can be introduced as early as driver
+initialization and removed as late as driver shutdown. Typically a driver
+will respond to a sysfs probe() callback that indicates an HBA has been
+detected. After confirming that the new device is one that the LLD wants
+to control, the LLD will initialize the HBA and then register a new host
+with the SCSI mid level.
+
+During LLD initialization the driver should register itself with the
+appropriate IO bus on which it expects to find HBA(s) (e.g. the PCI bus).
+This can probably be done via sysfs. Any driver parameters (especially
+those that are writable after the driver is loaded) could also be
+registered with sysfs at this point. The SCSI mid level first becomes
+aware of an LLD when that LLD registers its first HBA.
+
+At some later time, the LLD becomes aware of an HBA and what follows
+is a typical sequence of calls between the LLD and the mid level.
+This example shows the mid level scanning the newly introduced HBA for 3
+scsi devices of which only the first 2 respond:
+
+ HBA PROBE: assume 2 SCSI devices found in scan
+LLD mid level LLD
+===-------------------=========--------------------===------
+scsi_host_alloc() -->
+scsi_add_host() ---->
+scsi_scan_host() -------+
+ |
+ slave_alloc()
+ slave_configure() --> scsi_change_queue_depth()
+ |
+ slave_alloc()
+ slave_configure()
+ |
+ slave_alloc() ***
+ slave_destroy() ***
+------------------------------------------------------------
+
+If the LLD wants to adjust the default queue settings, it can invoke
+scsi_change_queue_depth() in its slave_configure() routine.
+
+*** For scsi devices that the mid level tries to scan but do not
+ respond, a slave_alloc(), slave_destroy() pair is called.
+
+When an HBA is being removed it could be as part of an orderly shutdown
+associated with the LLD module being unloaded (e.g. with the "rmmod"
+command) or in response to a "hot unplug" indicated by sysfs()'s
+remove() callback being invoked. In either case, the sequence is the
+same:
+
+ HBA REMOVE: assume 2 SCSI devices attached
+LLD mid level LLD
+===----------------------=========-----------------===------
+scsi_remove_host() ---------+
+ |
+ slave_destroy()
+ slave_destroy()
+scsi_host_put()
+------------------------------------------------------------
+
+It may be useful for a LLD to keep track of struct Scsi_Host instances
+(a pointer is returned by scsi_host_alloc()). Such instances are "owned"
+by the mid-level. struct Scsi_Host instances are freed from
+scsi_host_put() when the reference count hits zero.
+
+Hot unplugging an HBA that controls a disk which is processing SCSI
+commands on a mounted file system is an interesting situation. Reference
+counting logic is being introduced into the mid level to cope with many
+of the issues involved. See the section on reference counting below.
+
+
+The hotplug concept may be extended to SCSI devices. Currently, when an
+HBA is added, the scsi_scan_host() function causes a scan for SCSI devices
+attached to the HBA's SCSI transport. On newer SCSI transports the HBA
+may become aware of a new SCSI device _after_ the scan has completed.
+An LLD can use this sequence to make the mid level aware of a SCSI device:
+
+ SCSI DEVICE hotplug
+LLD mid level LLD
+===-------------------=========--------------------===------
+scsi_add_device() ------+
+ |
+ slave_alloc()
+ slave_configure() [--> scsi_change_queue_depth()]
+------------------------------------------------------------
+
+In a similar fashion, an LLD may become aware that a SCSI device has been
+removed (unplugged) or the connection to it has been interrupted. Some
+existing SCSI transports (e.g. SPI) may not become aware that a SCSI
+device has been removed until a subsequent SCSI command fails which will
+probably cause that device to be set offline by the mid level. An LLD that
+detects the removal of a SCSI device can instigate its removal from
+upper layers with this sequence:
+
+ SCSI DEVICE hot unplug
+LLD mid level LLD
+===----------------------=========-----------------===------
+scsi_remove_device() -------+
+ |
+ slave_destroy()
+------------------------------------------------------------
+
+It may be useful for an LLD to keep track of struct scsi_device instances
+(a pointer is passed as the parameter to slave_alloc() and
+slave_configure() callbacks). Such instances are "owned" by the mid-level.
+struct scsi_device instances are freed after slave_destroy().
+
+
+Passive initialization model
+============================
+These older LLDs include a file called "scsi_module.c" [yes the ".c" is a
+little surprising] in their source code. For that file to work an
+instance of struct scsi_host_template with the name "driver_template"
+needs to be defined. Here is a typical code sequence used in this model:
+ static struct scsi_host_template driver_template = {
+ ...
+ };
+ #include "scsi_module.c"
+
+The scsi_module.c file contains two functions:
+ - init_this_scsi_driver() which is executed when the LLD is
+ initialized (i.e. boot time or module load time)
+ - exit_this_scsi_driver() which is executed when the LLD is shut
+ down (i.e. module unload time)
+Note: since these functions are tagged with __init and __exit qualifiers
+an LLD should not call them explicitly (since the kernel does that).
+
+Here is an example of an initialization sequence when two hosts are
+detected (so detect() returns 2) and the SCSI bus scan on each host
+finds 1 SCSI device (and a second device does not respond).
+
+LLD mid level LLD
+===----------------------=========-----------------===------
+init_this_scsi_driver() ----+
+ |
+ detect() -----------------+
+ | |
+ | scsi_register()
+ | scsi_register()
+ |
+ slave_alloc()
+ slave_configure() --> scsi_change_queue_depth()
+ slave_alloc() ***
+ slave_destroy() ***
+ |
+ slave_alloc()
+ slave_configure()
+ slave_alloc() ***
+ slave_destroy() ***
+------------------------------------------------------------
+
+The mid level invokes scsi_change_queue_depth() with "cmd_per_lun" for that
+host as the queue length. These settings can be overridden by a
+slave_configure() supplied by the LLD.
+
+*** For scsi devices that the mid level tries to scan but do not
+ respond, a slave_alloc(), slave_destroy() pair is called.
+
+Here is an LLD shutdown sequence:
+
+LLD mid level LLD
+===----------------------=========-----------------===------
+exit_this_scsi_driver() ----+
+ |
+ slave_destroy()
+ release() --> scsi_unregister()
+ |
+ slave_destroy()
+ release() --> scsi_unregister()
+------------------------------------------------------------
+
+An LLD need not define slave_destroy() (i.e. it is optional).
+
+The shortcoming of the "passive initialization model" is that host
+registration and de-registration are (typically) tied to LLD initialization
+and shutdown. Once the LLD is initialized then a new host that appears
+(e.g. via hotplugging) cannot easily be added without a redundant
+driver shutdown and re-initialization. It may be possible to write an LLD
+that uses both initialization models.
+
+
+Reference Counting
+==================
+The Scsi_Host structure has had reference counting infrastructure added.
+This effectively spreads the ownership of struct Scsi_Host instances
+across the various SCSI layers which use them. Previously such instances
+were exclusively owned by the mid level. LLDs would not usually need to
+directly manipulate these reference counts but there may be some cases
+where they do.
+
+There are 3 reference counting functions of interest associated with
+struct Scsi_Host:
+ - scsi_host_alloc(): returns a pointer to new instance of struct
+ Scsi_Host which has its reference count ^^ set to 1
+ - scsi_host_get(): adds 1 to the reference count of the given instance
+ - scsi_host_put(): decrements 1 from the reference count of the given
+ instance. If the reference count reaches 0 then the given instance
+ is freed
+
+The Scsi_device structure has had reference counting infrastructure added.
+This effectively spreads the ownership of struct Scsi_device instances
+across the various SCSI layers which use them. Previously such instances
+were exclusively owned by the mid level. See the access functions declared
+towards the end of include/scsi/scsi_device.h . If an LLD wants to keep
+a copy of a pointer to a Scsi_device instance it should use scsi_device_get()
+to bump its reference count. When it is finished with the pointer it can
+use scsi_device_put() to decrement its reference count (and potentially
+delete it).
+
+^^ struct Scsi_Host actually has 2 reference counts which are manipulated
+in parallel by these functions.
+
+
+Conventions
+===========
+First, Linus Torvalds's thoughts on C coding style can be found in the
+Documentation/CodingStyle file.
+
+Next, there is a movement to "outlaw" typedefs introducing synonyms for
+struct tags. Both can be still found in the SCSI subsystem, but
+the typedefs have been moved to a single file, scsi_typedefs.h to
+make their future removal easier, for example:
+"typedef struct scsi_cmnd Scsi_Cmnd;"
+
+Also, most C99 enhancements are encouraged to the extent they are supported
+by the relevant gcc compilers. So C99 style structure and array
+initializers are encouraged where appropriate. Don't go too far,
+VLAs are not properly supported yet. An exception to this is the use of
+"//" style comments; /*...*/ comments are still preferred in Linux.
+
+Well written, tested and documented code, need not be re-formatted to
+comply with the above conventions. For example, the aic7xxx driver
+comes to Linux from FreeBSD and Adaptec's own labs. No doubt FreeBSD
+and Adaptec have their own coding conventions.
+
+
+Mid level supplied functions
+============================
+These functions are supplied by the SCSI mid level for use by LLDs.
+The names (i.e. entry points) of these functions are exported
+so an LLD that is a module can access them. The kernel will
+arrange for the SCSI mid level to be loaded and initialized before any LLD
+is initialized. The functions below are listed alphabetically and their
+names all start with "scsi_".
+
+Summary:
+ scsi_add_device - creates new scsi device (lu) instance
+ scsi_add_host - perform sysfs registration and set up transport class
+ scsi_change_queue_depth - change the queue depth on a SCSI device
+ scsi_bios_ptable - return copy of block device's partition table
+ scsi_block_requests - prevent further commands being queued to given host
+ scsi_host_alloc - return a new scsi_host instance whose refcount==1
+ scsi_host_get - increments Scsi_Host instance's refcount
+ scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
+ scsi_partsize - parse partition table into cylinders, heads + sectors
+ scsi_register - create and register a scsi host adapter instance.
+ scsi_remove_device - detach and remove a SCSI device
+ scsi_remove_host - detach and remove all SCSI devices owned by host
+ scsi_report_bus_reset - report scsi _bus_ reset observed
+ scsi_scan_host - scan SCSI bus
+ scsi_track_queue_full - track successive QUEUE_FULL events
+ scsi_unblock_requests - allow further commands to be queued to given host
+ scsi_unregister - [calls scsi_host_put()]
+
+
+Details:
+
+/**
+ * scsi_add_device - creates new scsi device (lu) instance
+ * @shost: pointer to scsi host instance
+ * @channel: channel number (rarely other than 0)
+ * @id: target id number
+ * @lun: logical unit number
+ *
+ * Returns pointer to new struct scsi_device instance or
+ * ERR_PTR(-ENODEV) (or some other bent pointer) if something is
+ * wrong (e.g. no lu responds at given address)
+ *
+ * Might block: yes
+ *
+ * Notes: This call is usually performed internally during a scsi
+ * bus scan when an HBA is added (i.e. scsi_scan_host()). So it
+ * should only be called if the HBA becomes aware of a new scsi
+ * device (lu) after scsi_scan_host() has completed. If successful
+ * this call can lead to slave_alloc() and slave_configure() callbacks
+ * into the LLD.
+ *
+ * Defined in: drivers/scsi/scsi_scan.c
+ **/
+struct scsi_device * scsi_add_device(struct Scsi_Host *shost,
+ unsigned int channel,
+ unsigned int id, unsigned int lun)
+
+
+/**
+ * scsi_add_host - perform sysfs registration and set up transport class
+ * @shost: pointer to scsi host instance
+ * @dev: pointer to struct device of type scsi class
+ *
+ * Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
+ *
+ * Might block: no
+ *
+ * Notes: Only required in "hotplug initialization model" after a
+ * successful call to scsi_host_alloc(). This function does not
+ * scan the bus; this can be done by calling scsi_scan_host() or
+ * in some other transport-specific way. The LLD must set up
+ * the transport template before calling this function and may only
+ * access the transport class data after this function has been called.
+ *
+ * Defined in: drivers/scsi/hosts.c
+ **/
+int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
+
+
+/**
+ * scsi_change_queue_depth - allow LLD to change queue depth on a SCSI device
+ * @sdev: pointer to SCSI device to change queue depth on
+ * @tags Number of tags allowed if tagged queuing enabled,
+ * or number of commands the LLD can queue up
+ * in non-tagged mode (as per cmd_per_lun).
+ *
+ * Returns nothing
+ *
+ * Might block: no
+ *
+ * Notes: Can be invoked any time on a SCSI device controlled by this
+ * LLD. [Specifically during and after slave_configure() and prior to
+ * slave_destroy().] Can safely be invoked from interrupt code.
+ *
+ * Defined in: drivers/scsi/scsi.c [see source code for more notes]
+ *
+ **/
+int scsi_change_queue_depth(struct scsi_device *sdev, int tags)
+
+
+/**
+ * scsi_bios_ptable - return copy of block device's partition table
+ * @dev: pointer to block device
+ *
+ * Returns pointer to partition table, or NULL for failure
+ *
+ * Might block: yes
+ *
+ * Notes: Caller owns memory returned (free with kfree() )
+ *
+ * Defined in: drivers/scsi/scsicam.c
+ **/
+unsigned char *scsi_bios_ptable(struct block_device *dev)
+
+
+/**
+ * scsi_block_requests - prevent further commands being queued to given host
+ *
+ * @shost: pointer to host to block commands on
+ *
+ * Returns nothing
+ *
+ * Might block: no
+ *
+ * Notes: There is no timer nor any other means by which the requests
+ * get unblocked other than the LLD calling scsi_unblock_requests().
+ *
+ * Defined in: drivers/scsi/scsi_lib.c
+**/
+void scsi_block_requests(struct Scsi_Host * shost)
+
+
+/**
+ * scsi_host_alloc - create a scsi host adapter instance and perform basic
+ * initialization.
+ * @sht: pointer to scsi host template
+ * @privsize: extra bytes to allocate in hostdata array (which is the
+ * last member of the returned Scsi_Host instance)
+ *
+ * Returns pointer to new Scsi_Host instance or NULL on failure
+ *
+ * Might block: yes
+ *
+ * Notes: When this call returns to the LLD, the SCSI bus scan on
+ * this host has _not_ yet been done.
+ * The hostdata array (by default zero length) is a per host scratch
+ * area for the LLD's exclusive use.
+ * Both associated refcounting objects have their refcount set to 1.
+ * Full registration (in sysfs) and a bus scan are performed later when
+ * scsi_add_host() and scsi_scan_host() are called.
+ *
+ * Defined in: drivers/scsi/hosts.c .
+ **/
+struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
+ int privsize)
+
+
+/**
+ * scsi_host_get - increment Scsi_Host instance refcount
+ * @shost: pointer to struct Scsi_Host instance
+ *
+ * Returns nothing
+ *
+ * Might block: currently may block but may be changed to not block
+ *
+ * Notes: Actually increments the counts in two sub-objects
+ *
+ * Defined in: drivers/scsi/hosts.c
+ **/
+void scsi_host_get(struct Scsi_Host *shost)
+
+
+/**
+ * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
+ * @shost: pointer to struct Scsi_Host instance
+ *
+ * Returns nothing
+ *
+ * Might block: currently may block but may be changed to not block
+ *
+ * Notes: Actually decrements the counts in two sub-objects. If the
+ * latter refcount reaches 0, the Scsi_Host instance is freed.
+ * The LLD need not worry exactly when the Scsi_Host instance is
+ * freed, it just shouldn't access the instance after it has balanced
+ * out its refcount usage.
+ *
+ * Defined in: drivers/scsi/hosts.c
+ **/
+void scsi_host_put(struct Scsi_Host *shost)
+
+
+/**
+ * scsi_partsize - parse partition table into cylinders, heads + sectors
+ * @buf: pointer to partition table
+ * @capacity: size of (total) disk in 512 byte sectors
+ * @cyls: outputs number of cylinders calculated via this pointer
+ * @hds: outputs number of heads calculated via this pointer
+ * @secs: outputs number of sectors calculated via this pointer
+ *
+ * Returns 0 on success, -1 on failure
+ *
+ * Might block: no
+ *
+ * Notes: Caller owns memory returned (free with kfree() )
+ *
+ * Defined in: drivers/scsi/scsicam.c
+ **/
+int scsi_partsize(unsigned char *buf, unsigned long capacity,
+ unsigned int *cyls, unsigned int *hds, unsigned int *secs)
+
+
+/**
+ * scsi_register - create and register a scsi host adapter instance.
+ * @sht: pointer to scsi host template
+ * @privsize: extra bytes to allocate in hostdata array (which is the
+ * last member of the returned Scsi_Host instance)
+ *
+ * Returns pointer to new Scsi_Host instance or NULL on failure
+ *
+ * Might block: yes
+ *
+ * Notes: When this call returns to the LLD, the SCSI bus scan on
+ * this host has _not_ yet been done.
+ * The hostdata array (by default zero length) is a per host scratch
+ * area for the LLD.
+ *
+ * Defined in: drivers/scsi/hosts.c .
+ **/
+struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
+ int privsize)
+
+
+/**
+ * scsi_remove_device - detach and remove a SCSI device
+ * @sdev: a pointer to a scsi device instance
+ *
+ * Returns value: 0 on success, -EINVAL if device not attached
+ *
+ * Might block: yes
+ *
+ * Notes: If an LLD becomes aware that a scsi device (lu) has
+ * been removed but its host is still present then it can request
+ * the removal of that scsi device. If successful this call will
+ * lead to the slave_destroy() callback being invoked. sdev is an
+ * invalid pointer after this call.
+ *
+ * Defined in: drivers/scsi/scsi_sysfs.c .
+ **/
+int scsi_remove_device(struct scsi_device *sdev)
+
+
+/**
+ * scsi_remove_host - detach and remove all SCSI devices owned by host
+ * @shost: a pointer to a scsi host instance
+ *
+ * Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
+ *
+ * Might block: yes
+ *
+ * Notes: Should only be invoked if the "hotplug initialization
+ * model" is being used. It should be called _prior_ to
+ * scsi_unregister().
+ *
+ * Defined in: drivers/scsi/hosts.c .
+ **/
+int scsi_remove_host(struct Scsi_Host *shost)
+
+
+/**
+ * scsi_report_bus_reset - report scsi _bus_ reset observed
+ * @shost: a pointer to a scsi host involved
+ * @channel: channel (within) host on which scsi bus reset occurred
+ *
+ * Returns nothing
+ *
+ * Might block: no
+ *
+ * Notes: This only needs to be called if the reset is one which
+ * originates from an unknown location. Resets originated by the
+ * mid level itself don't need to call this, but there should be
+ * no harm. The main purpose of this is to make sure that a
+ * CHECK_CONDITION is properly treated.
+ *
+ * Defined in: drivers/scsi/scsi_error.c .
+ **/
+void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
+
+
+/**
+ * scsi_scan_host - scan SCSI bus
+ * @shost: a pointer to a scsi host instance
+ *
+ * Might block: yes
+ *
+ * Notes: Should be called after scsi_add_host()
+ *
+ * Defined in: drivers/scsi/scsi_scan.c
+ **/
+void scsi_scan_host(struct Scsi_Host *shost)
+
+
+/**
+ * scsi_track_queue_full - track successive QUEUE_FULL events on given
+ * device to determine if and when there is a need
+ * to adjust the queue depth on the device.
+ * @sdev: pointer to SCSI device instance
+ * @depth: Current number of outstanding SCSI commands on this device,
+ * not counting the one returned as QUEUE_FULL.
+ *
+ * Returns 0 - no change needed
+ * >0 - adjust queue depth to this new depth
+ * -1 - drop back to untagged operation using host->cmd_per_lun
+ * as the untagged command depth
+ *
+ * Might block: no
+ *
+ * Notes: LLDs may call this at any time and we will do "The Right
+ * Thing"; interrupt context safe.
+ *
+ * Defined in: drivers/scsi/scsi.c .
+ **/
+int scsi_track_queue_full(struct scsi_device *sdev, int depth)
+
+
+/**
+ * scsi_unblock_requests - allow further commands to be queued to given host
+ *
+ * @shost: pointer to host to unblock commands on
+ *
+ * Returns nothing
+ *
+ * Might block: no
+ *
+ * Defined in: drivers/scsi/scsi_lib.c .
+**/
+void scsi_unblock_requests(struct Scsi_Host * shost)
+
+
+/**
+ * scsi_unregister - unregister and free memory used by host instance
+ * @shp: pointer to scsi host instance to unregister.
+ *
+ * Returns nothing
+ *
+ * Might block: no
+ *
+ * Notes: Should not be invoked if the "hotplug initialization
+ * model" is being used. Called internally by exit_this_scsi_driver()
+ * in the "passive initialization model". Hence a LLD has no need to
+ * call this function directly.
+ *
+ * Defined in: drivers/scsi/hosts.c .
+ **/
+void scsi_unregister(struct Scsi_Host * shp)
+
+
+
+
+Interface Functions
+===================
+Interface functions are supplied (defined) by LLDs and their function
+pointers are placed in an instance of struct scsi_host_template which
+is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
+Some are mandatory. Interface functions should be declared static. The
+accepted convention is that driver "xyz" will declare its slave_configure()
+function as:
+ static int xyz_slave_configure(struct scsi_device * sdev);
+and so forth for all interface functions listed below.
+
+A pointer to this function should be placed in the 'slave_configure' member
+of a "struct scsi_host_template" instance. A pointer to such an instance
+should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
+init_this_scsi_driver()].
+
+The interface functions are also described in the include/scsi/scsi_host.h
+file immediately above their definition point in "struct scsi_host_template".
+In some cases more detail is given in scsi_host.h than below.
+
+The interface functions are listed below in alphabetical order.
+
+Summary:
+ bios_param - fetch head, sector, cylinder info for a disk
+ detect - detects HBAs this driver wants to control
+ eh_timed_out - notify the host that a command timer expired
+ eh_abort_handler - abort given command
+ eh_bus_reset_handler - issue SCSI bus reset
+ eh_device_reset_handler - issue SCSI device reset
+ eh_host_reset_handler - reset host (host bus adapter)
+ info - supply information about given host
+ ioctl - driver can respond to ioctls
+ proc_info - supports /proc/scsi/{driver_name}/{host_no}
+ queuecommand - queue scsi command, invoke 'done' on completion
+ release - release all resources associated with given host
+ slave_alloc - prior to any commands being sent to a new device
+ slave_configure - driver fine tuning for given device after attach
+ slave_destroy - given device is about to be shut down
+
+
+Details:
+
+/**
+ * bios_param - fetch head, sector, cylinder info for a disk
+ * @sdev: pointer to scsi device context (defined in
+ * include/scsi/scsi_device.h)
+ * @bdev: pointer to block device context (defined in fs.h)
+ * @capacity: device size (in 512 byte sectors)
+ * @params: three element array to place output:
+ * params[0] number of heads (max 255)
+ * params[1] number of sectors (max 63)
+ * params[2] number of cylinders
+ *
+ * Return value is ignored
+ *
+ * Locks: none
+ *
+ * Calling context: process (sd)
+ *
+ * Notes: an arbitrary geometry (based on READ CAPACITY) is used
+ * if this function is not provided. The params array is
+ * pre-initialized with made up values just in case this function
+ * doesn't output anything.
+ *
+ * Optionally defined in: LLD
+ **/
+ int bios_param(struct scsi_device * sdev, struct block_device *bdev,
+ sector_t capacity, int params[3])
+
+
+/**
+ * detect - detects HBAs this driver wants to control
+ * @shtp: host template for this driver.
+ *
+ * Returns number of hosts this driver wants to control. 0 means no
+ * suitable hosts found.
+ *
+ * Locks: none held
+ *
+ * Calling context: process [invoked from init_this_scsi_driver()]
+ *
+ * Notes: First function called from the SCSI mid level on this
+ * driver. Upper level drivers (e.g. sd) may not (yet) be present.
+ * For each host found, this method should call scsi_register()
+ * [see hosts.c].
+ *
+ * Defined in: LLD (required if "passive initialization mode" is used,
+ * not invoked in "hotplug initialization mode")
+ **/
+ int detect(struct scsi_host_template * shtp)
+
+
+/**
+ * eh_timed_out - The timer for the command has just fired
+ * @scp: identifies command timing out
+ *
+ * Returns:
+ *
+ * EH_HANDLED: I fixed the error, please complete the command
+ * EH_RESET_TIMER: I need more time, reset the timer and
+ * begin counting again
+ * EH_NOT_HANDLED Begin normal error recovery
+ *
+ *
+ * Locks: None held
+ *
+ * Calling context: interrupt
+ *
+ * Notes: This is to give the LLD an opportunity to do local recovery.
+ * This recovery is limited to determining if the outstanding command
+ * will ever complete. You may not abort and restart the command from
+ * this callback.
+ *
+ * Optionally defined in: LLD
+ **/
+ int eh_timed_out(struct scsi_cmnd * scp)
+
+
+/**
+ * eh_abort_handler - abort command associated with scp
+ * @scp: identifies command to be aborted
+ *
+ * Returns SUCCESS if command aborted else FAILED
+ *
+ * Locks: None held
+ *
+ * Calling context: kernel thread
+ *
+ * Notes: If 'no_async_abort' is defined this callback
+ * will be invoked from scsi_eh thread. No other commands
+ * will then be queued on current host during eh.
+ * Otherwise it will be called whenever scsi_times_out()
+ * is called due to a command timeout.
+ *
+ * Optionally defined in: LLD
+ **/
+ int eh_abort_handler(struct scsi_cmnd * scp)
+
+
+/**
+ * eh_bus_reset_handler - issue SCSI bus reset
+ * @scp: SCSI bus that contains this device should be reset
+ *
+ * Returns SUCCESS if command aborted else FAILED
+ *
+ * Locks: None held
+ *
+ * Calling context: kernel thread
+ *
+ * Notes: Invoked from scsi_eh thread. No other commands will be
+ * queued on current host during eh.
+ *
+ * Optionally defined in: LLD
+ **/
+ int eh_bus_reset_handler(struct scsi_cmnd * scp)
+
+
+/**
+ * eh_device_reset_handler - issue SCSI device reset
+ * @scp: identifies SCSI device to be reset
+ *
+ * Returns SUCCESS if command aborted else FAILED
+ *
+ * Locks: None held
+ *
+ * Calling context: kernel thread
+ *
+ * Notes: Invoked from scsi_eh thread. No other commands will be
+ * queued on current host during eh.
+ *
+ * Optionally defined in: LLD
+ **/
+ int eh_device_reset_handler(struct scsi_cmnd * scp)
+
+
+/**
+ * eh_host_reset_handler - reset host (host bus adapter)
+ * @scp: SCSI host that contains this device should be reset
+ *
+ * Returns SUCCESS if command aborted else FAILED
+ *
+ * Locks: None held
+ *
+ * Calling context: kernel thread
+ *
+ * Notes: Invoked from scsi_eh thread. No other commands will be
+ * queued on current host during eh.
+ * With the default eh_strategy in place, if none of the _abort_,
+ * _device_reset_, _bus_reset_ or this eh handler function are
+ * defined (or they all return FAILED) then the device in question
+ * will be set offline whenever eh is invoked.
+ *
+ * Optionally defined in: LLD
+ **/
+ int eh_host_reset_handler(struct scsi_cmnd * scp)
+
+
+/**
+ * info - supply information about given host: driver name plus data
+ * to distinguish given host
+ * @shp: host to supply information about
+ *
+ * Return ASCII null terminated string. [This driver is assumed to
+ * manage the memory pointed to and maintain it, typically for the
+ * lifetime of this host.]
+ *
+ * Locks: none
+ *
+ * Calling context: process
+ *
+ * Notes: Often supplies PCI or ISA information such as IO addresses
+ * and interrupt numbers. If not supplied struct Scsi_Host::name used
+ * instead. It is assumed the returned information fits on one line
+ * (i.e. does not included embedded newlines).
+ * The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
+ * function (or struct Scsi_Host::name if this function is not
+ * available).
+ * In a similar manner, init_this_scsi_driver() outputs to the console
+ * each host's "info" (or name) for the driver it is registering.
+ * Also if proc_info() is not supplied, the output of this function
+ * is used instead.
+ *
+ * Optionally defined in: LLD
+ **/
+ const char * info(struct Scsi_Host * shp)
+
+
+/**
+ * ioctl - driver can respond to ioctls
+ * @sdp: device that ioctl was issued for
+ * @cmd: ioctl number
+ * @arg: pointer to read or write data from. Since it points to
+ * user space, should use appropriate kernel functions
+ * (e.g. copy_from_user() ). In the Unix style this argument
+ * can also be viewed as an unsigned long.
+ *
+ * Returns negative "errno" value when there is a problem. 0 or a
+ * positive value indicates success and is returned to the user space.
+ *
+ * Locks: none
+ *
+ * Calling context: process
+ *
+ * Notes: The SCSI subsystem uses a "trickle down" ioctl model.
+ * The user issues an ioctl() against an upper level driver
+ * (e.g. /dev/sdc) and if the upper level driver doesn't recognize
+ * the 'cmd' then it is passed to the SCSI mid level. If the SCSI
+ * mid level does not recognize it, then the LLD that controls
+ * the device receives the ioctl. According to recent Unix standards
+ * unsupported ioctl() 'cmd' numbers should return -ENOTTY.
+ *
+ * Optionally defined in: LLD
+ **/
+ int ioctl(struct scsi_device *sdp, int cmd, void *arg)
+
+
+/**
+ * proc_info - supports /proc/scsi/{driver_name}/{host_no}
+ * @buffer: anchor point to output to (0==writeto1_read0) or fetch from
+ * (1==writeto1_read0).
+ * @start: where "interesting" data is written to. Ignored when
+ * 1==writeto1_read0.
+ * @offset: offset within buffer 0==writeto1_read0 is actually
+ * interested in. Ignored when 1==writeto1_read0 .
+ * @length: maximum (or actual) extent of buffer
+ * @host_no: host number of interest (struct Scsi_Host::host_no)
+ * @writeto1_read0: 1 -> data coming from user space towards driver
+ * (e.g. "echo some_string > /proc/scsi/xyz/2")
+ * 0 -> user what data from this driver
+ * (e.g. "cat /proc/scsi/xyz/2")
+ *
+ * Returns length when 1==writeto1_read0. Otherwise number of chars
+ * output to buffer past offset.
+ *
+ * Locks: none held
+ *
+ * Calling context: process
+ *
+ * Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
+ * support can now be configured out of the scsi subsystem.
+ *
+ * Optionally defined in: LLD
+ **/
+ int proc_info(char * buffer, char ** start, off_t offset,
+ int length, int host_no, int writeto1_read0)
+
+
+/**
+ * queuecommand - queue scsi command, invoke scp->scsi_done on completion
+ * @shost: pointer to the scsi host object
+ * @scp: pointer to scsi command object
+ *
+ * Returns 0 on success.
+ *
+ * If there's a failure, return either:
+ *
+ * SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
+ * SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
+ *
+ * On both of these returns, the mid-layer will requeue the I/O
+ *
+ * - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
+ * device will be paused, and it will be unpaused when a command to
+ * the device returns (or after a brief delay if there are no more
+ * outstanding commands to it). Commands to other devices continue
+ * to be processed normally.
+ *
+ * - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
+ * is paused and will be unpaused when any command returns from
+ * the host (or after a brief delay if there are no outstanding
+ * commands to the host).
+ *
+ * For compatibility with earlier versions of queuecommand, any
+ * other return value is treated the same as
+ * SCSI_MLQUEUE_HOST_BUSY.
+ *
+ * Other types of errors that are detected immediately may be
+ * flagged by setting scp->result to an appropriate value,
+ * invoking the scp->scsi_done callback, and then returning 0
+ * from this function. If the command is not performed
+ * immediately (and the LLD is starting (or will start) the given
+ * command) then this function should place 0 in scp->result and
+ * return 0.
+ *
+ * Command ownership. If the driver returns zero, it owns the
+ * command and must take responsibility for ensuring the
+ * scp->scsi_done callback is executed. Note: the driver may
+ * call scp->scsi_done before returning zero, but after it has
+ * called scp->scsi_done, it may not return any value other than
+ * zero. If the driver makes a non-zero return, it must not
+ * execute the command's scsi_done callback at any time.
+ *
+ * Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
+ * held on entry (with "irqsave") and is expected to be
+ * held on return. From 2.6.37 onwards, queuecommand is
+ * called without any locks held.
+ *
+ * Calling context: in interrupt (soft irq) or process context
+ *
+ * Notes: This function should be relatively fast. Normally it
+ * will not wait for IO to complete. Hence the scp->scsi_done
+ * callback is invoked (often directly from an interrupt service
+ * routine) some time after this function has returned. In some
+ * cases (e.g. pseudo adapter drivers that manufacture the
+ * response to a SCSI INQUIRY) the scp->scsi_done callback may be
+ * invoked before this function returns. If the scp->scsi_done
+ * callback is not invoked within a certain period the SCSI mid
+ * level will commence error processing. If a status of CHECK
+ * CONDITION is placed in "result" when the scp->scsi_done
+ * callback is invoked, then the LLD driver should perform
+ * autosense and fill in the struct scsi_cmnd::sense_buffer
+ * array. The scsi_cmnd::sense_buffer array is zeroed prior to
+ * the mid level queuing a command to an LLD.
+ *
+ * Defined in: LLD
+ **/
+ int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
+
+
+/**
+ * release - release all resources associated with given host
+ * @shp: host to be released.
+ *
+ * Return value ignored (could soon be a function returning void).
+ *
+ * Locks: none held
+ *
+ * Calling context: process
+ *
+ * Notes: Invoked from scsi_module.c's exit_this_scsi_driver().
+ * LLD's implementation of this function should call
+ * scsi_unregister(shp) prior to returning.
+ * Only needed for old-style host templates.
+ *
+ * Defined in: LLD (required in "passive initialization model",
+ * should not be defined in hotplug model)
+ **/
+ int release(struct Scsi_Host * shp)
+
+
+/**
+ * slave_alloc - prior to any commands being sent to a new device
+ * (i.e. just prior to scan) this call is made
+ * @sdp: pointer to new device (about to be scanned)
+ *
+ * Returns 0 if ok. Any other return is assumed to be an error and
+ * the device is ignored.
+ *
+ * Locks: none
+ *
+ * Calling context: process
+ *
+ * Notes: Allows the driver to allocate any resources for a device
+ * prior to its initial scan. The corresponding scsi device may not
+ * exist but the mid level is just about to scan for it (i.e. send
+ * and INQUIRY command plus ...). If a device is found then
+ * slave_configure() will be called while if a device is not found
+ * slave_destroy() is called.
+ * For more details see the include/scsi/scsi_host.h file.
+ *
+ * Optionally defined in: LLD
+ **/
+ int slave_alloc(struct scsi_device *sdp)
+
+
+/**
+ * slave_configure - driver fine tuning for given device just after it
+ * has been first scanned (i.e. it responded to an
+ * INQUIRY)
+ * @sdp: device that has just been attached
+ *
+ * Returns 0 if ok. Any other return is assumed to be an error and
+ * the device is taken offline. [offline devices will _not_ have
+ * slave_destroy() called on them so clean up resources.]
+ *
+ * Locks: none
+ *
+ * Calling context: process
+ *
+ * Notes: Allows the driver to inspect the response to the initial
+ * INQUIRY done by the scanning code and take appropriate action.
+ * For more details see the include/scsi/scsi_host.h file.
+ *
+ * Optionally defined in: LLD
+ **/
+ int slave_configure(struct scsi_device *sdp)
+
+
+/**
+ * slave_destroy - given device is about to be shut down. All
+ * activity has ceased on this device.
+ * @sdp: device that is about to be shut down
+ *
+ * Returns nothing
+ *
+ * Locks: none
+ *
+ * Calling context: process
+ *
+ * Notes: Mid level structures for given device are still in place
+ * but are about to be torn down. Any per device resources allocated
+ * by this driver for given device should be freed now. No further
+ * commands will be sent for this sdp instance. [However the device
+ * could be re-attached in the future in which case a new instance
+ * of struct scsi_device would be supplied by future slave_alloc()
+ * and slave_configure() calls.]
+ *
+ * Optionally defined in: LLD
+ **/
+ void slave_destroy(struct scsi_device *sdp)
+
+
+
+Data Structures
+===============
+struct scsi_host_template
+-------------------------
+There is one "struct scsi_host_template" instance per LLD ***. It is
+typically initialized as a file scope static in a driver's header file. That
+way members that are not explicitly initialized will be set to 0 or NULL.
+Member of interest:
+ name - name of driver (may contain spaces, please limit to
+ less than 80 characters)
+ proc_name - name used in "/proc/scsi/<proc_name>/<host_no>" and
+ by sysfs in one of its "drivers" directories. Hence
+ "proc_name" should only contain characters acceptable
+ to a Unix file name.
+ (*queuecommand)() - primary callback that the mid level uses to inject
+ SCSI commands into an LLD.
+The structure is defined and commented in include/scsi/scsi_host.h
+
+*** In extreme situations a single driver may have several instances
+ if it controls several different classes of hardware (e.g. an LLD
+ that handles both ISA and PCI cards and has a separate instance of
+ struct scsi_host_template for each class).
+
+struct Scsi_Host
+----------------
+There is one struct Scsi_Host instance per host (HBA) that an LLD
+controls. The struct Scsi_Host structure has many members in common
+with "struct scsi_host_template". When a new struct Scsi_Host instance
+is created (in scsi_host_alloc() in hosts.c) those common members are
+initialized from the driver's struct scsi_host_template instance. Members
+of interest:
+ host_no - system wide unique number that is used for identifying
+ this host. Issued in ascending order from 0.
+ can_queue - must be greater than 0; do not send more than can_queue
+ commands to the adapter.
+ this_id - scsi id of host (scsi initiator) or -1 if not known
+ sg_tablesize - maximum scatter gather elements allowed by host.
+ 0 implies scatter gather not supported by host
+ max_sectors - maximum number of sectors (usually 512 bytes) allowed
+ in a single SCSI command. The default value of 0 leads
+ to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
+ scsi_host.h) which is currently set to 1024. So for a
+ disk the maximum transfer size is 512 KB when max_sectors
+ is not defined. Note that this size may not be sufficient
+ for disk firmware uploads.
+ cmd_per_lun - maximum number of commands that can be queued on devices
+ controlled by the host. Overridden by LLD calls to
+ scsi_change_queue_depth().
+ unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
+ restriction), 0=>can use full 32 bit (or better) DMA
+ address space
+ use_clustering - 1=>SCSI commands in mid level's queue can be merged,
+ 0=>disallow SCSI command merging
+ no_async_abort - 1=>Asynchronous aborts are not supported
+ 0=>Timed-out commands will be aborted asynchronously
+ hostt - pointer to driver's struct scsi_host_template from which
+ this struct Scsi_Host instance was spawned
+ hostt->proc_name - name of LLD. This is the driver name that sysfs uses
+ transportt - pointer to driver's struct scsi_transport_template instance
+ (if any). FC and SPI transports currently supported.
+ sh_list - a double linked list of pointers to all struct Scsi_Host
+ instances (currently ordered by ascending host_no)
+ my_devices - a double linked list of pointers to struct scsi_device
+ instances that belong to this host.
+ hostdata[0] - area reserved for LLD at end of struct Scsi_Host. Size
+ is set by the second argument (named 'xtr_bytes') to
+ scsi_host_alloc() or scsi_register().
+ vendor_id - a unique value that identifies the vendor supplying
+ the LLD for the Scsi_Host. Used most often in validating
+ vendor-specific message requests. Value consists of an
+ identifier type and a vendor-specific value.
+ See scsi_netlink.h for a description of valid formats.
+
+The scsi_host structure is defined in include/scsi/scsi_host.h
+
+struct scsi_device
+------------------
+Generally, there is one instance of this structure for each SCSI logical unit
+on a host. Scsi devices connected to a host are uniquely identified by a
+channel number, target id and logical unit number (lun).
+The structure is defined in include/scsi/scsi_device.h
+
+struct scsi_cmnd
+----------------
+Instances of this structure convey SCSI commands to the LLD and responses
+back to the mid level. The SCSI mid level will ensure that no more SCSI
+commands become queued against the LLD than are indicated by
+scsi_change_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
+be at least one instance of struct scsi_cmnd available for each SCSI device.
+Members of interest:
+ cmnd - array containing SCSI command
+ cmnd_len - length (in bytes) of SCSI command
+ sc_data_direction - direction of data transfer in data phase. See
+ "enum dma_data_direction" in include/linux/dma-mapping.h
+ request_bufflen - number of data bytes to transfer (0 if no data phase)
+ use_sg - ==0 -> no scatter gather list, hence transfer data
+ to/from request_buffer
+ - >0 -> scatter gather list (actually an array) in
+ request_buffer with use_sg elements
+ request_buffer - either contains data buffer or scatter gather list
+ depending on the setting of use_sg. Scatter gather
+ elements are defined by 'struct scatterlist' found
+ in include/linux/scatterlist.h .
+ done - function pointer that should be invoked by LLD when the
+ SCSI command is completed (successfully or otherwise).
+ Should only be called by an LLD if the LLD has accepted
+ the command (i.e. queuecommand() returned or will return
+ 0). The LLD may invoke 'done' prior to queuecommand()
+ finishing.
+ result - should be set by LLD prior to calling 'done'. A value
+ of 0 implies a successfully completed command (and all
+ data (if any) has been transferred to or from the SCSI
+ target device). 'result' is a 32 bit unsigned integer that
+ can be viewed as 4 related bytes. The SCSI status value is
+ in the LSB. See include/scsi/scsi.h status_byte(),
+ msg_byte(), host_byte() and driver_byte() macros and
+ related constants.
+ sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
+ should be written when the SCSI status (LSB of 'result')
+ is set to CHECK_CONDITION (2). When CHECK_CONDITION is
+ set, if the top nibble of sense_buffer[0] has the value 7
+ then the mid level will assume the sense_buffer array
+ contains a valid SCSI sense buffer; otherwise the mid
+ level will issue a REQUEST_SENSE SCSI command to
+ retrieve the sense buffer. The latter strategy is error
+ prone in the presence of command queuing so the LLD should
+ always "auto-sense".
+ device - pointer to scsi_device object that this command is
+ associated with.
+ resid - an LLD should set this signed integer to the requested
+ transfer length (i.e. 'request_bufflen') less the number
+ of bytes that are actually transferred. 'resid' is
+ preset to 0 so an LLD can ignore it if it cannot detect
+ underruns (overruns should be rare). If possible an LLD
+ should set 'resid' prior to invoking 'done'. The most
+ interesting case is data transfers from a SCSI target
+ device (e.g. READs) that underrun.
+ underflow - LLD should place (DID_ERROR << 16) in 'result' if
+ actual number of bytes transferred is less than this
+ figure. Not many LLDs implement this check and some that
+ do just output an error message to the log rather than
+ report a DID_ERROR. Better for an LLD to implement
+ 'resid'.
+
+It is recommended that a LLD set 'resid' on data transfers from a SCSI
+target device (e.g. READs). It is especially important that 'resid' is set
+when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
+(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
+data has been received then the safest approach is to indicate no bytes have
+been received. For example: to indicate that no valid data has been received
+a LLD might use these helpers:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
+where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
+bytes blocks has been received 'resid' could be set like this:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
+
+The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
+
+
+Locks
+=====
+Each struct Scsi_Host instance has a spin_lock called struct
+Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in
+hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
+is initialized to point at default_lock. Thereafter lock and unlock
+operations performed by the mid level use the struct Scsi_Host::host_lock
+pointer. Previously drivers could override the host_lock pointer but
+this is not allowed anymore.
+
+
+Autosense
+=========
+Autosense (or auto-sense) is defined in the SAM-2 document as "the
+automatic return of sense data to the application client coincident
+with the completion of a SCSI command" when a status of CHECK CONDITION
+occurs. LLDs should perform autosense. This should be done when the LLD
+detects a CHECK CONDITION status by either:
+ a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
+ to perform an extra data in phase on such responses
+ b) or, the LLD issuing a REQUEST SENSE command itself
+
+Either way, when a status of CHECK CONDITION is detected, the mid level
+decides whether the LLD has performed autosense by checking struct
+scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
+then autosense is assumed to have taken place. If it has another value (and
+this byte is initialized to 0 before each command) then the mid level will
+issue a REQUEST SENSE command.
+
+In the presence of queued commands the "nexus" that maintains sense
+buffer data from the command that failed until a following REQUEST SENSE
+may get out of synchronization. This is why it is best for the LLD
+to perform autosense.
+
+
+Changes since lk 2.4 series
+===========================
+io_request_lock has been replaced by several finer grained locks. The lock
+relevant to LLDs is struct Scsi_Host::host_lock and there is
+one per SCSI host.
+
+The older error handling mechanism has been removed. This means the
+LLD interface functions abort() and reset() have been removed.
+The struct scsi_host_template::use_new_eh_code flag has been removed.
+
+In the 2.4 series the SCSI subsystem configuration descriptions were
+aggregated with the configuration descriptions from all other Linux
+subsystems in the Documentation/Configure.help file. In the 2.6 series,
+the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
+file that contains both configuration and help information.
+
+struct SHT has been renamed to struct scsi_host_template.
+
+Addition of the "hotplug initialization model" and many extra functions
+to support it.
+
+
+Credits
+=======
+The following people have contributed to this document:
+ Mike Anderson <andmike at us dot ibm dot com>
+ James Bottomley <James dot Bottomley at hansenpartnership dot com>
+ Patrick Mansfield <patmans at us dot ibm dot com>
+ Christoph Hellwig <hch at infradead dot org>
+ Doug Ledford <dledford at redhat dot com>
+ Andries Brouwer <Andries dot Brouwer at cwi dot nl>
+ Randy Dunlap <rdunlap at xenotime dot net>
+ Alan Stern <stern at rowland dot harvard dot edu>
+
+
+Douglas Gilbert
+dgilbert at interlog dot com
+21st September 2004