=====================
SCSI Interfaces Guide
=====================

:Author: James Bottomley
:Author: Rob Landley

Introduction
============

Protocol vs bus
---------------

Once upon a time, the Small Computer Systems Interface defined both a
parallel I/O bus and a data protocol to connect a wide variety of
peripherals (disk drives, tape drives, modems, printers, scanners,
optical drives, test equipment, and medical devices) to a host computer.

Although the old parallel (fast/wide/ultra) SCSI bus has largely fallen
out of use, the SCSI command set is more widely used than ever to
communicate with devices over a number of different busses.

The `SCSI protocol <https://www.t10.org/scsi-3.htm>`__ is a big-endian
peer-to-peer packet based protocol. SCSI commands are 6, 10, 12, or 16
bytes long, often followed by an associated data payload.

SCSI commands can be transported over just about any kind of bus, and
are the default protocol for storage devices attached to USB, SATA, SAS,
Fibre Channel, FireWire, and ATAPI devices. SCSI packets are also
commonly exchanged over Infiniband,
TCP/IP (`iSCSI <https://en.wikipedia.org/wiki/ISCSI>`__), even `Parallel
ports <http://cyberelk.net/tim/parport/parscsi.html>`__.

Design of the Linux SCSI subsystem
----------------------------------

The SCSI subsystem uses a three layer design, with upper, mid, and low
layers. Every operation involving the SCSI subsystem (such as reading a
sector from a disk) uses one driver at each of the 3 levels: one upper
layer driver, one lower layer driver, and the SCSI midlayer.

The SCSI upper layer provides the interface between userspace and the
kernel, in the form of block and char device nodes for I/O and ioctl().
The SCSI lower layer contains drivers for specific hardware devices.

In between is the SCSI mid-layer, analogous to a network routing layer
such as the IPv4 stack. The SCSI mid-layer routes a packet based data
protocol between the upper layer's /dev nodes and the corresponding
devices in the lower layer. It manages command queues, provides error
handling and power management functions, and responds to ioctl()
requests.

SCSI upper layer
================

The upper layer supports the user-kernel interface by providing device
nodes.

sd (SCSI Disk)
--------------

sd (sd_mod.o)

sr (SCSI CD-ROM)
----------------

sr (sr_mod.o)

st (SCSI Tape)
--------------

st (st.o)

sg (SCSI Generic)
-----------------

sg (sg.o)

ch (SCSI Media Changer)
-----------------------

ch (ch.c)

SCSI mid layer
==============

SCSI midlayer implementation
----------------------------

include/scsi/scsi_device.h
~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: include/scsi/scsi_device.h
   :internal:

drivers/scsi/scsi.c
~~~~~~~~~~~~~~~~~~~

Main file for the SCSI midlayer.

.. kernel-doc:: drivers/scsi/scsi.c
   :export:

drivers/scsi/scsicam.c
~~~~~~~~~~~~~~~~~~~~~~

`SCSI Common Access
Method <http://www.t10.org/ftp/t10/drafts/cam/cam-r12b.pdf>`__ support
functions, for use with HDIO_GETGEO, etc.

.. kernel-doc:: drivers/scsi/scsicam.c
   :export:

drivers/scsi/scsi_error.c
~~~~~~~~~~~~~~~~~~~~~~~~~~

Common SCSI error/timeout handling routines.

.. kernel-doc:: drivers/scsi/scsi_error.c
   :export:

drivers/scsi/scsi_devinfo.c
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Manage scsi_dev_info_list, which tracks blacklisted and whitelisted
devices.

.. kernel-doc:: drivers/scsi/scsi_devinfo.c
   :export:

drivers/scsi/scsi_ioctl.c
~~~~~~~~~~~~~~~~~~~~~~~~~~

Handle ioctl() calls for SCSI devices.

.. kernel-doc:: drivers/scsi/scsi_ioctl.c
   :export:

drivers/scsi/scsi_lib.c
~~~~~~~~~~~~~~~~~~~~~~~~

SCSI queuing library.

.. kernel-doc:: drivers/scsi/scsi_lib.c
   :export:

drivers/scsi/scsi_lib_dma.c
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SCSI library functions depending on DMA (map and unmap scatter-gather
lists).

.. kernel-doc:: drivers/scsi/scsi_lib_dma.c
   :export:

drivers/scsi/scsi_proc.c
~~~~~~~~~~~~~~~~~~~~~~~~~

The functions in this file provide an interface between the PROC file
system and the SCSI device drivers It is mainly used for debugging,
statistics and to pass information directly to the lowlevel driver. I.E.
plumbing to manage /proc/scsi/\*

.. kernel-doc:: drivers/scsi/scsi_proc.c

drivers/scsi/scsi_netlink.c
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Infrastructure to provide async events from transports to userspace via
netlink, using a single NETLINK_SCSITRANSPORT protocol for all
transports. See `the original patch submission
<https://lore.kernel.org/linux-scsi/1155070439.6275.5.camel@localhost.localdomain/>`__
for more details.

.. kernel-doc:: drivers/scsi/scsi_netlink.c
   :internal:

drivers/scsi/scsi_scan.c
~~~~~~~~~~~~~~~~~~~~~~~~~

Scan a host to determine which (if any) devices are attached. The
general scanning/probing algorithm is as follows, exceptions are made to
it depending on device specific flags, compilation options, and global
variable (boot or module load time) settings. A specific LUN is scanned
via an INQUIRY command; if the LUN has a device attached, a scsi_device
is allocated and setup for it. For every id of every channel on the
given host, start by scanning LUN 0. Skip hosts that don't respond at
all to a scan of LUN 0. Otherwise, if LUN 0 has a device attached,
allocate and setup a scsi_device for it. If target is SCSI-3 or up,
issue a REPORT LUN, and scan all of the LUNs returned by the REPORT LUN;
else, sequentially scan LUNs up until some maximum is reached, or a LUN
is seen that cannot have a device attached to it.

.. kernel-doc:: drivers/scsi/scsi_scan.c
   :export:

drivers/scsi/scsi_sysctl.c
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Set up the sysctl entry: "/dev/scsi/logging_level"
(DEV_SCSI_LOGGING_LEVEL) which sets/returns scsi_logging_level.

drivers/scsi/scsi_sysfs.c
~~~~~~~~~~~~~~~~~~~~~~~~~~

SCSI sysfs interface routines.

.. kernel-doc:: drivers/scsi/scsi_sysfs.c
   :export:

drivers/scsi/hosts.c
~~~~~~~~~~~~~~~~~~~~

mid to lowlevel SCSI driver interface

.. kernel-doc:: drivers/scsi/hosts.c
   :export:

drivers/scsi/scsi_common.c
~~~~~~~~~~~~~~~~~~~~~~~~~~

general support functions

.. kernel-doc:: drivers/scsi/scsi_common.c
   :export:

Transport classes
-----------------

Transport classes are service libraries for drivers in the SCSI lower
layer, which expose transport attributes in sysfs.

Fibre Channel transport
~~~~~~~~~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_transport_fc.c defines transport attributes
for Fibre Channel.

.. kernel-doc:: drivers/scsi/scsi_transport_fc.c
   :export:

iSCSI transport class
~~~~~~~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_transport_iscsi.c defines transport
attributes for the iSCSI class, which sends SCSI packets over TCP/IP
connections.

.. kernel-doc:: drivers/scsi/scsi_transport_iscsi.c
   :export:

Serial Attached SCSI (SAS) transport class
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_transport_sas.c defines transport
attributes for Serial Attached SCSI, a variant of SATA aimed at large
high-end systems.

The SAS transport class contains common code to deal with SAS HBAs, an
approximated representation of SAS topologies in the driver model, and
various sysfs attributes to expose these topologies and management
interfaces to userspace.

In addition to the basic SCSI core objects this transport class
introduces two additional intermediate objects: The SAS PHY as
represented by struct sas_phy defines an "outgoing" PHY on a SAS HBA or
Expander, and the SAS remote PHY represented by struct sas_rphy defines
an "incoming" PHY on a SAS Expander or end device. Note that this is
purely a software concept, the underlying hardware for a PHY and a
remote PHY is the exactly the same.

There is no concept of a SAS port in this code, users can see what PHYs
form a wide port based on the port_identifier attribute, which is the
same for all PHYs in a port.

.. kernel-doc:: drivers/scsi/scsi_transport_sas.c
   :export:

SATA transport class
~~~~~~~~~~~~~~~~~~~~

The SATA transport is handled by libata, which has its own book of
documentation in this directory.

Parallel SCSI (SPI) transport class
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_transport_spi.c defines transport
attributes for traditional (fast/wide/ultra) SCSI busses.

.. kernel-doc:: drivers/scsi/scsi_transport_spi.c
   :export:

SCSI RDMA (SRP) transport class
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_transport_srp.c defines transport
attributes for SCSI over Remote Direct Memory Access.

.. kernel-doc:: drivers/scsi/scsi_transport_srp.c
   :export:

SCSI lower layer
================

Host Bus Adapter transport types
--------------------------------

Many modern device controllers use the SCSI command set as a protocol to
communicate with their devices through many different types of physical
connections.

In SCSI language a bus capable of carrying SCSI commands is called a
"transport", and a controller connecting to such a bus is called a "host
bus adapter" (HBA).

Debug transport
~~~~~~~~~~~~~~~

The file drivers/scsi/scsi_debug.c simulates a host adapter with a
variable number of disks (or disk like devices) attached, sharing a
common amount of RAM. Does a lot of checking to make sure that we are
not getting blocks mixed up, and panics the kernel if anything out of
the ordinary is seen.

To be more realistic, the simulated devices have the transport
attributes of SAS disks.

For documentation see http://sg.danny.cz/sg/scsi_debug.html

todo
~~~~

Parallel (fast/wide/ultra) SCSI, USB, SATA, SAS, Fibre Channel,
FireWire, ATAPI devices, Infiniband, Parallel ports,
netlink...
