                          Linux Kernel 2.5 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, host drivers (HD) or pseudo adapter drivers.
The latter alludes to the fact that a LLD may be a bridge to another IO 
subsystem (e.g. the "usb-storage" driver). There can be many LLDs in a 
running system, but only one per hardware type. Most LLDs can control one
or more SCSI hosts.

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 daughterboards or built into 
the motherboard (or both). Like most modern HBAs, each aic7xxx host
has its own PCI device address. [The one-to-one correspondance between
a SCSI host and a PCI device is common but not required (e.g. with
ISA or MCA adapters).]

The Linux kernel source Documentation/DocBook/scsidrivers.tmpl file
refers to this file. With the appropriate DocBook toolset, this permits
users to generate html, ps and pdf renderings of information within this
file (e.g. the interface functions).

This version of the document roughly matches lk 2.5.50 .

Driver structure
================
Traditionally a 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.] 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, Config.help and
Config.in . SCSI documentation is found in the Documentation/scsi directory
of the kernel source tree. It is probably best to study how existing LLDs 
are organized.

As the 2.5 series development kernels evolve, 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
hotplugged (and unplugged) while the driver is loaded 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" usb-storage devices
(e.g. digital cameras) that are hotplugged. Both initialization models
are discussed in the following sections.

A 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
  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. The function pointers are placed in the members of
"struct SHT", an instance of which is passed to scsi_register() [or
scsi_register_host() in the passive initialization model]. Those interface
functions that are not mandatory and that the LLD does not wish to supply
should have NULL placed in the corresponding member of struct SHT.
[Defining an instance of struct SHT at file scope will cause NULL to be 
placed in function pointer members not explicitly initialized.]

Those instances in group c) are slowly being removed as they tend to be
"racy" especially in a hotplug environment.


Hotplug initialization model
============================
In this model a 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 a HBA is present
(e.g. a PCI device). After confirming it is a device that it wants to
control, it will initialize the HBA and then register a new host with the
SCSI mid level.

Hot unplugging a HBA that controls a disk which is processing SCSI
commands on a mounted file system is an ugly situation. Issues with
this scenario are still being worked through. The primary concern is
the stability of the kernel (specifically the block and SCSI subsystems)
since the effected disk can be "cleaned up" the next time it is seen.

During LLD initialization the driver should register itself with the
appropriate IO bus that it expects to find HBA(s) (e.g. the PCI bus). This
can probably be done via sysfs (formerly known as driverfs). Any driver
parameters (especially those that are writeable after the driver is
loaded) could also be registered with sysfs at this point. At the end of
driver initialization the SCSI mid level is typically not aware of its
presence.

At some later time, the LLD becomes aware of a HBA and what follows
is a typical sequence of calls between the LLD and the mid level.
This example shows 3 devices being found and 1 device not being found:

LLD                   mid level                    LLD
---                   ---------                    ---
scsi_register()  -->
scsi_add_host()  --------+
                         |
		    slave_alloc()
                    slave_configure() -->  scsi_adjust_queue_depth()
			 |
		    slave_alloc()
                    slave_configure() -->  scsi_adjust_queue_depth()
			 |
		    slave_alloc()
                    slave_configure() -->  scsi_adjust_queue_depth()
			 |
		    slave_alloc()   **
                    slave_destroy() **

The invocation of scsi_adjust_queue_depth() by the LLD is required
if slave_configure() is supplied.
** For scsi devices that the mid level tries to scan but do not
   respond, a slave_alloc(), slave_destroy() pair is called.

Here is the corresponding sequence when a host (HBA) is being
removed:

LLD                   mid level  
---                   ---------
scsi_remove_host()  -----+
                         |
                    slave_destroy() 
                    slave_destroy()
                    slave_destroy()
scsi_unregister()  -->

It is practical for a LLD to keep track of struct Scsi_Host instances
(a pointer is returned by scsi_register() ) and struct scsi_device
instances (a pointer is passed as the parameter to slave_alloc() and
slave_configure() ). Both classes of instances are "owned" by the 
mid-level. struct scsi_devices instances are freed after slave_destroy().
struct Scsi_Host instances are freed after scsi_unregister().


Passive initialization model
============================
LLD initialization (both built-in and module) and shutdown are "wired
up" by passing function pointers to the module_init() and module_exit()
macros respectively. In this model the function identified by "module_init"
must call scsi_register_host() and the function identified by "module_exit"
must call scsi_unregister_host().
Most LLDs inherited from the lk 2.4 series include a file called
"scsi_module.c" [yes the ".c" is a little surprising] in their
source code. For that file to work a declaration like this is needed before
it is included:
    static struct SHT driver_template = DRIVER_TEMPLATE;
    #include "scsi_module.c"

In this case "DRIVER_TEMPLATE" is defined to be a structure initializer
that is placed in the driver header file by convention. It contains
pointers to supported interface functions and other values.

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
---                      ---------                 ---
scsi_register_host() -------+
                            |
                         detect()  -----------------+
                            |                       |
                            |                scsi_register()
                            |                scsi_register()
                            |
		      slave_alloc()
                      slave_configure()  -->  scsi_adjust_queue_depth()
		      slave_alloc()   **
		      slave_destroy() **
                            |
		      slave_alloc()
                      slave_configure()  -->  scsi_adjust_queue_depth()
		      slave_alloc()   **
		      slave_destroy() **

If the LLD does not supply a slave_configure() then the mid level invokes
scsi_adjust_queue_depth() itself with tagged queuing off and "cmd_per_lun"
for that host as the queue length.
** For scsi devices that the mid level tries to scan but do not
   respond, a slave_alloc(), slave_destroy() pair is called.

Here is a LLD shutdown sequence:

LLD                      mid level                 LLD
---                      ---------                 ---
scsi_unregister_host() -----+
                            |
                     slave_destroy()
                        release()   -->   scsi_unregister()
                            |
                     slave_destroy()
                        release()   -->   scsi_unregister()

Both slave_destroy() and release() are optional. If they are not supplied
the mid level supplies default actions.

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.


Conventions
===========
First Linus's thoughts on C coding found in file Documentation/CodingStyle . 

Next there is a movement to "outlaw" typedefs introducing synonyms for 
struct tags. Both can be still found in the SCSI subsystem, for example: 
"typedef struct SHT { ...} Scsi_Host_Template;" in hosts.h . In this
case "struct SHT" is preferred to "Scsi_Host_Template".

Also C99 additions are encouraged to the extent they are supported
by the relevant gcc compilers. So "//" style comments are encouraged
were appropriate as are C99 style structure and array initializers.


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 a LLD that is a module can access them when the SCSI
mid level is built into the kernel. 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_".

/**
 * scsi_add_host - perform sysfs registration and SCSI bus scan.
 * @shost:   pointer to scsi host instance
 *
 *      Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
 *
 *	Notes: Only required in "hotplug initialization model" after a
 *	successful call to scsi_register().
 *	Defined in drivers/scsi/hosts.c
 **/
int scsi_add_host(struct Scsi_Host *shost)


/**
 * scsi_adjust_queue_depth - change the queue depth on a SCSI device
 * @SDpnt:	pointer to SCSI device to change queue depth on
 * @tagged:	0 - no tagged queueing
 *		MSG_SIMPLE_TAG - simple (unordered) tagged queueing
 *		MSG_ORDERED_TAG - ordered tagged queueing
 * @tags	Number of tags allowed if tagged queueing enabled,
 *		or number of commands the LLD can queue up
 *		in non-tagged mode (as per cmd_per_lun).
 *
 *	Returns nothing
 *
 * 	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. Actual
 *	queue depth change may be delayed until the next command is being
 *	processed.
 *	Defined in drivers/scsi/scsi.c [see source code for more notes]
 *
 **/
void scsi_adjust_queue_depth(struct scsi_device * SDpnt, int tagged, 
			     int num_tags)


/**
 * scsi_assign_lock - replace default host_lock with given lock
 * @shost: a pointer to a scsi host instance
 * @lock: pointer to lock to replace host_lock for this host
 *
 *      Returns nothing
 *
 *      Notes: Defined in drivers/scsi/hosts.h .
 **/
void scsi_assign_lock(struct Scsi_Host *shost, spinlock_t *lock)


/**
 * scsi_bios_ptable - return copy of block device's partition table
 * @dev:	pointer to block device
 *
 *	Returns pointer to partition table, or NULL or failure
 *
 *	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
 *
 * @SHpnt: pointer to host to block commands on
 *
 *	Returns nothing
 *
 * 	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 * SHpnt)


/**
 * 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
 *
 *	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.
 * @shost_tp:   pointer to scsi host template
 * @xtr_bytes:  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
 *
 *	Notes: When this call returns to the LLDD, 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 SHT *, int xtr_bytes)


/**
 * scsi_register_host - register a low level host driver
 * @shost_tp:   pointer to a scsi host driver template
 *
 *      Returns 0 on Success, 1 on failure
 *
 *      Notes: Should only be invoked if the "passive initialization
 *	model" is being used. Notice this is a _driver_ rather than 
 *	HBA registration function. Most older drivers call this
 *	function by including the scsi_module.c file.
 *	This function is deprecated, use the "hotplug initialization
 *	model" instead.
 *	Defined in drivers/scsi/hosts.c .
 **/
int scsi_register_host(Scsi_Host_Template *shost_tp)


/**
 * 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 ??)
 *
 *      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
 *
 *      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_lib.c .
 **/
void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)


/**
 * scsi_set_pci_device - place PCI device reference in host structure
 * @shost: a pointer to a scsi host instance
 * @pdev: pointer to PCI device instance to assign
 *
 *	Returns nothing
 *
 *      Notes: Defined in drivers/scsi/hosts.h .
 **/
void scsi_set_pci_device(struct Scsi_Host * shost, struct pci_dev * pdev)


/**
 * scsi_to_pci_dma_dir - convert SCSI subsystem direction flag to PCI
 * @scsi_data_direction: SCSI subsystem direction flag
 *
 *	Returns PCI_DMA_TODEVICE given SCSI_DATA_WRITE,
 *		PCI_DMA_FROMDEVICE given SCSI_DATA_READ
 *		PCI_DMA_BIDIRECTIONAL given SCSI_DATA_UNKNOWN
 *		else returns PCI_DMA_NONE
 *
 *      Notes: Defined in drivers/scsi/scsi.h .
 **/
int scsi_to_pci_dma_dir(unsigned char scsi_data_direction)


/**
 * scsi_unblock_requests - allow further commands to be queued to given host
 *
 * @SHpnt: pointer to host to unblock commands on
 *
 *	Returns nothing
 *
 * 	Notes: Defined in drivers/scsi/scsi_lib.c .
**/
void scsi_unblock_requests(struct Scsi_Host * SHpnt)


/**
 * scsi_unregister - unregister and free host
 * @shp:	pointer to scsi host instance to unregister.
 *		N.B. shp points to freed memory on return
 *
 *	Returns nothing
 *
 *      Notes: Should only be invoked if the "hotplug initialization
 *	model" is being used. It should be called _after_  
 *	scsi_remove_host().
 *	Defined in drivers/scsi/hosts.c .
 **/
void scsi_unregister(struct Scsi_Host * shp)


/**
 * scsi_unregister_host - unregister a low level host adapter driver
 * @shost_tp:   scsi host template to unregister.
 *
 *	Returns 0 on Success, 1 on Failure
 *
 *      Notes: Should only be invoked if the "passive initialization
 *	model" is being used. Notice this is a _driver_ rather than 
 *	HBA deregistration function. Most older drivers call this
 *	function by including the scsi_module.c file.
 *	This function is deprecated, use the "hotplug initialization
 *	model" instead.
 *	Defined in drivers/scsi/hosts.c .
 **/
int scsi_unregister_host(Scsi_Host_Template *shost_tp)



Interface Functions
===================
Interface functions are supplied (defined) by LLDs and
their function pointers are placed in an instance of struct SHT which
is passed to scsi_register() [or scsi_register_host()]. 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);

A pointer to this function should be placed in the 'slave_configure' member
of a "struct SHT" instance. A pointer to such an instance should
passed to the mid level's scsi_register() [or scsi_register_host()].

The interface functions are also described in the hosts.h file immediately
above their definition point in "struct SHT". In some cases more detail
is given in hosts.h than below.

Those interface functions marked "Required: yes" must be implemented
by the LLD and a pointer to that function must be placed in the driver's
"struct SHT" instance. Interface functions marked "Required: no" need not
be implemented.

The interface functions are listed below in alphabetical order.


/**
 *      bios_param - fetch head, sector, cylinder info for a disk
 *      @sdev: pointer to scsi device context (defined in scsi.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
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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.
 **/
    int bios_param(struct scsi_device * sdev, struct block_device *bdev,
    		   sector_t capacity, int params[3]);


/**
 *      command - send scsi command to device, wait for reply
 *      @scp: pointer to scsi command object
 *
 *      Returns an int with four component bytes: scsi_byte, msg_byte, 
 *      host_byte, driver_byte (status_byte is in the lsb). A value of
 *      0 is an unqualified success.
 *
 *      Required: if struct Scsi_Host::can_queue can ever by cleared (zero)
 *                then this function is required.
 *
 *      Locks: struct Scsi_Host::host_lock held on entry (with "irqsave")
 *             and is expected to be held on return.
 *
 *      Notes: Drivers tend to be dropping support for this function and
 *      supporting queuecommand() instead.
 **/
    int command(struct scsi_cmnd * scp);


/**
 *      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.
 *
 *      Required: yes, if "passive initialization mode" is used
 *		  [in "hotplug initialization mode" it is not invoked]
 *
 *      Locks: none held
 *
 *      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].
 **/
    int detect(struct SHT * shtp);


/**
 *      eh_abort_handler - abort command associated with scp
 *      @scp: identifies command to be aborted
 *
 *      Returns SUCCESS if command aborted else FAILED
 *
 *      Required: no
 *
 *      Locks: struct Scsi_Host::host_lock held (with irqsave) on entry 
 *	and assumed to be held on return.
 *
 *      Notes: Invoked from scsi_eh thread. No other commands will be
 *      queued on current host during eh.
 **/
     int eh_abort_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
 *
 *      Required: no
 *
 *      Locks: struct Scsi_Host::host_lock held (with irqsave) on entry
 *	and assumed to be held on return.
 *
 *      Notes: Invoked from scsi_eh thread. No other commands will be
 *      queued on current host during eh.
 **/
     int eh_device_reset_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
 *
 *      Required: no
 *
 *      Locks: struct Scsi_Host::host_lock held (with irqsave) on entry 
 *	and assumed to be held on return.
 *
 *      Notes: Invoked from scsi_eh thread. No other commands will be
 *      queued on current host during eh.
 **/
     int eh_bus_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
 *
 *      Required: no
 *
 *      Locks: struct Scsi_Host::host_lock held (with irqsave) on entry
 *	and assumed to be held on return.
 *
 *      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.
 **/
     int eh_host_reset_handler(struct scsi_cmnd * scp);


/**
 *      eh_strategy_handler - driver supplied alternate to scsi_unjam_host()
 *      @shp: host on which error has occurred
 *
 *      Returns TRUE if host unjammed, else FALSE.
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      Notes: Invoked from scsi_eh thread. Driver supplied alternate to 
 *      scsi_unjam_host() found in scsi_error.c
 **/
     int eh_strategy_handler(struct Scsi_Host * shp);


/**
 *      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.]
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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, scsi_register_host() 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.
 **/
    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.
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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.
 *      However the mid level returns -EINVAL for unrecognized 'cmd'
 *      numbers when this function is not supplied by the driver.
 *      Unfortunately some applications expect -EINVAL and react badly
 *	when -ENOTTY is returned; stick with -EINVAL.
 **/
    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.
 *
 *      Required: no
 *
 *      Locks: none held
 *
 *      Notes: Driven from scsi_proc.c which interfaces to proc_fs
 **/
int proc_info(char * buffer, char ** start, off_t offset, 
              int length, int hostno, int writeto1_read0);


/**
 *      queuecommand - queue scsi command, invoke 'done' on completion
 *      @scp: pointer to scsi command object
 *      @done: function pointer to be invoked on completion
 *
 *      Returns 1 if the adapter (host) is busy, else returns 0. One
 *		reason for an adapter to be busy is that the number
 *		of outstanding queued commands is already equal to
 *		struct Scsi_Host::can_queue .
 *
 *      Required: if struct Scsi_Host::can_queue is ever non-zero
 *                then this function is required.
 *
 *      Locks: struct Scsi_Host::host_lock held on entry (with "irqsave")
 *             and is expected to be held on return.
 *
 *      Notes: This function should be relatively fast. Normally it will
 *      not wait for IO to complete. Hence the 'done' callback is invoked 
 *      (often directly from an interrupt service routine) sometime after
 *      this command has returned. In some cases (e.g. pseudo adapter 
 *      drivers that manufacture the response to a SCSI INQUIRY)
 *      the 'done' callback may be invoked before this function returns.
 *      If the 'done' callback is not invoked within a certain period
 *      the SCSI mid level will commence error processing.
 *      The integer with 4 component bytes that command() uses as its
 *      return value should be generated by this function. However, in
 *      this case, it should be placed in scp->result before this function 
 *      returns.
 *      If a status of CHECK CONDITION is placed in "result" when the
 *	'done' callback is invoked, then the LLD driver should 
 *	perform autosense and fill in the struct scsi_cmnd::sense_buffer
 *	array.
 **/
    int queuecommand(struct scsi_cmnd * scp, 
		     void (*done)(struct scsi_cmnd *));


/**
 *      release - release all resources associated with given host
 *      @shp: host to be released.
 *
 *      Return value ignored.
 *
 *      Required: no
 *
 *      Locks: lock_kernel() active on entry and expected to be active
 *      on return.
 *
 *      Notes: Invoked from mid level's scsi_unregister_host().
 *      This function should call scsi_unregister(shp) [found in hosts.c]
 *      prior to returning.
 **/
    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.
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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 hosts.h file.
 **/
    int slave_alloc(struct scsi_device *sdp);


/**
 *      slave_configure - driver fine tuning for give 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.]
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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 hosts.h file.
 *	If this function is not supplied, the mid level will call
 *	scsi_adjust_queue_depth() with the struct Scsi_Host::cmd_per_lun
 *	value on behalf of the given device. If this function is
 *	supplied then its implementation must call
 *	scsi_adjust_queue_depth(). 	
 **/
    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
 *
 *      Required: no
 *
 *      Locks: none
 *
 *      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.]
 **/
    void slave_destroy(struct scsi_device *sdp);



Data Structures
===============
struct SHT
----------
There is one "struct SHT" 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 (should only use characters that are
                   permitted in a unix file name)
    proc_name    - name used in "/proc/scsi/<proc_name>/<host_no>"
The structure is defined and commented in hosts.h

*** In extreme situations a single driver may have several instances
    if it controls several different classes of hardware (e.g. the
    advansys driver handles both ISA and PCI cards).

struct Scsi_Host
----------------
There is one struct Scsi_Host instance per host (HBA) that a LLD
controls. The struct Scsi_Host structure has many members in common
with "struct SHT". When a new struct Scsi_Host instance is created (in
scsi_register() in hosts.c) those common members are initialized from
the driver's struct SHT instance. Members of interest:
    host_no      - system wide unique number that is used for identifying
                   this host. Issued in ascending order from 0 (and the
                   positioning can be influenced by the scsihosts
                   kernel boot (or module) parameter)
    can_queue    - 0->use command(), greater than 0->use queuecommand() and
                   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. 0 implies no maximum.
    cmd_per_lun  - maximum number of command that can be queued on devices
                   controlled by the host. Overridden by LLD calls to
		   scsi_adjust_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
    highmem_io   - 1->can DMA in to or out of high memory,
                   0->use bounce buffers if data is in high memory
    hostt        - pointer to driver's struct SHT from which this
                   struct Scsi_Host instance was spawned
    host_queue   - deceptively named pointer to the start of a double linked
                   list of struct scsi_device instances that belong to this
		   host.
The structure is defined in hosts.h

struct scsi_device
------------------
Generally, there is one instance of this structure for each SCSI logical unit
on a host. Scsi devices are uniquely identified within a host by bus number,
target id and logical unit number (lun).
The structure is defined in scsi.h

struct scsi_cmnd
----------------
Instances of this structure convey SCSI commands to the LLD.
Each SCSI device has a pool of struct scsi_cmnd instances whose size
is determined by scsi_adjust_queue_depth() (or struct Scsi_Host::cmd_per_lun).
There will be at least one instance of struct scsi_cmnd for each SCSI device.
The structure is defined in scsi.h


Locks
=====
Each struct Scsi_Host instance has a spin_lock called struct 
Scsi_Host::default_lock which is initialized in scsi_register() [found in 
hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
is initialized to point at default_lock with the scsi_assign_lock() function.
Thereafter lock and unlock operations performed by the mid level use the
struct Scsi_Host::host_lock pointer.

LLDs can override the use of struct Scsi_Host::default_lock by
using scsi_assign_lock(). The earliest opportunity to do this would
be in the detect() function after it has invoked scsi_register(). It
could be replaced by a coarser grain lock (e.g. per driver) or a
lock of equal granularity (i.e. per host). Using finer grain locks 
(e.g. per SCSI device) may be possible by juggling locks in
queuecommand().

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, 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 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 SHT::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 2.5 series, 
the SCSI subsystem now has its own (much smaller) drivers/scsi/Config.help
file.

Addition of slave_alloc(), slave_configure() and slave_destroy().

Addition of the "hotplug initialization model".


Credits
=======
The following people have contributed to this document:
        Mike Anderson <andmike@us.ibm.com>
        James Bottomley <James.Bottomley@steeleye.com> 
        Patrick Mansfield <patmans@us.ibm.com> 
	Christoph Hellwig <hch@infradead.org>
	Doug Ledford <dledford@redhat.com>


Douglas Gilbert
dgilbert@interlog.com
29th November 2002
