scsi: docs: convert scsi_mid_low_api.txt to ReST

   scsi_eh

   scsi_fc_transport

   scsi-generic

   scsi_mid_low_api

   scsi_transport_srp/figures

=======

This document describes SCSI midlayer error handling infrastructure.

Please refer to Documentation/scsi/scsi_mid_low_api.txt for more

Please refer to Documentation/scsi/scsi_mid_low_api.rst for more

information regarding SCSI midlayer.

.. TABLE OF CONTENTS

                          Linux Kernel 2.6 series

.. SPDX-License-Identifier: GPL-2.0

=============================================

SCSI mid_level - lower_level driver interface

=============================================

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

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 

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

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 */ }"

``static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }``

** the scsi_host_alloc() function is a replacement for the rather vaguely

.. [#] the scsi_host_alloc() function is a replacement for the rather vaguely

       named scsi_register() function in most situations.

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:

scsi devices of which only the first 2 respond::

	HBA PROBE: assume 2 SCSI devices found in scan

    LLD                   mid level                    LLD

			    |

			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.

If the LLD wants to adjust the default queue settings, it can invoke

scsi_change_queue_depth() in its slave_configure() routine.

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:

same::

	    HBA REMOVE: assume 2 SCSI devices attached

    LLD                      mid level                 LLD

			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"

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:

An LLD can use this sequence to make the mid level aware of a SCSI device::

		    SCSI DEVICE hotplug

    LLD                   mid level                    LLD

			    |

			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

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:

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

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_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

  - 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

use scsi_device_put() to decrement its reference count (and potentially

delete it).

^^ struct Scsi_Host actually has 2 reference counts which are manipulated

.. Note::

   struct Scsi_Host actually has 2 reference counts which are manipulated

   in parallel by these functions.

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.

``//`` 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

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_".

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

  - 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

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:

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

The interface functions are listed below in alphabetical order.

Summary:

   bios_param - fetch head, sector, cylinder info for a disk

   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

   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

  - 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

  - 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

===============

struct scsi_host_template

-------------------------

There is one "struct scsi_host_template" instance per LLD ***. It is

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

    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

    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

   ``(*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

.. [#] 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).

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

    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

    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.

    this_id

		 - scsi id of host (scsi initiator) or -1 if not known

    sg_tablesize

		 - maximum scatter gather elements allowed by host.

                   Set this to SG_ALL or less to avoid chained SG lists.

                   Must be at least 1.

    max_sectors  - maximum number of sectors (usually 512 bytes) allowed

    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

    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

    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

    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

    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

    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

    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 

    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

    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

    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.

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

    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

    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

    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

    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

    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

                   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

    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

                   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

    device

		 - pointer to scsi_device object that this command is

                   associated with.

    resid        - an LLD should set this signed integer to the requested

    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

                   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

    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

(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:

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:

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

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

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>

	- 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

# to mention a past documentation file, for example, to give credits for

# the original work.

my %false_positives = (

	"Documentation/scsi/scsi_mid_low_api.txt" => "Documentation/Configure.help",

	"Documentation/scsi/scsi_mid_low_api.rst" => "Documentation/Configure.help",

	"drivers/vhost/vhost.c" => "Documentation/virtual/lguest/lguest.c",

);