338 lines
9.4 KiB
ReStructuredText
338 lines
9.4 KiB
ReStructuredText
=====================
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SCSI Interfaces Guide
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=====================
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:Author: James Bottomley
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:Author: Rob Landley
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Introduction
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============
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Protocol vs bus
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---------------
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Once upon a time, the Small Computer Systems Interface defined both a
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parallel I/O bus and a data protocol to connect a wide variety of
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peripherals (disk drives, tape drives, modems, printers, scanners,
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optical drives, test equipment, and medical devices) to a host computer.
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Although the old parallel (fast/wide/ultra) SCSI bus has largely fallen
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out of use, the SCSI command set is more widely used than ever to
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communicate with devices over a number of different busses.
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The `SCSI protocol <https://www.t10.org/scsi-3.htm>`__ is a big-endian
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peer-to-peer packet based protocol. SCSI commands are 6, 10, 12, or 16
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bytes long, often followed by an associated data payload.
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SCSI commands can be transported over just about any kind of bus, and
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are the default protocol for storage devices attached to USB, SATA, SAS,
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Fibre Channel, FireWire, and ATAPI devices. SCSI packets are also
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commonly exchanged over Infiniband,
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TCP/IP (`iSCSI <https://en.wikipedia.org/wiki/ISCSI>`__), even `Parallel
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ports <http://cyberelk.net/tim/parport/parscsi.html>`__.
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Design of the Linux SCSI subsystem
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----------------------------------
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The SCSI subsystem uses a three layer design, with upper, mid, and low
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layers. Every operation involving the SCSI subsystem (such as reading a
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sector from a disk) uses one driver at each of the 3 levels: one upper
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layer driver, one lower layer driver, and the SCSI midlayer.
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The SCSI upper layer provides the interface between userspace and the
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kernel, in the form of block and char device nodes for I/O and ioctl().
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The SCSI lower layer contains drivers for specific hardware devices.
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In between is the SCSI mid-layer, analogous to a network routing layer
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such as the IPv4 stack. The SCSI mid-layer routes a packet based data
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protocol between the upper layer's /dev nodes and the corresponding
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devices in the lower layer. It manages command queues, provides error
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handling and power management functions, and responds to ioctl()
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requests.
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SCSI upper layer
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================
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The upper layer supports the user-kernel interface by providing device
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nodes.
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sd (SCSI Disk)
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--------------
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sd (sd_mod.o)
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sr (SCSI CD-ROM)
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----------------
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sr (sr_mod.o)
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st (SCSI Tape)
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--------------
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st (st.o)
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sg (SCSI Generic)
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-----------------
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sg (sg.o)
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ch (SCSI Media Changer)
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-----------------------
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ch (ch.c)
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SCSI mid layer
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==============
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SCSI midlayer implementation
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----------------------------
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include/scsi/scsi_device.h
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/scsi/scsi_device.h
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:internal:
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drivers/scsi/scsi.c
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~~~~~~~~~~~~~~~~~~~
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Main file for the SCSI midlayer.
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.. kernel-doc:: drivers/scsi/scsi.c
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:export:
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drivers/scsi/scsicam.c
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~~~~~~~~~~~~~~~~~~~~~~
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`SCSI Common Access
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Method <http://www.t10.org/ftp/t10/drafts/cam/cam-r12b.pdf>`__ support
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functions, for use with HDIO_GETGEO, etc.
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.. kernel-doc:: drivers/scsi/scsicam.c
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:export:
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drivers/scsi/scsi_error.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Common SCSI error/timeout handling routines.
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.. kernel-doc:: drivers/scsi/scsi_error.c
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:export:
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drivers/scsi/scsi_devinfo.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Manage scsi_dev_info_list, which tracks blacklisted and whitelisted
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devices.
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.. kernel-doc:: drivers/scsi/scsi_devinfo.c
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:internal:
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drivers/scsi/scsi_ioctl.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Handle ioctl() calls for SCSI devices.
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.. kernel-doc:: drivers/scsi/scsi_ioctl.c
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:export:
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drivers/scsi/scsi_lib.c
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~~~~~~~~~~~~~~~~~~~~~~~~
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SCSI queuing library.
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.. kernel-doc:: drivers/scsi/scsi_lib.c
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:export:
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drivers/scsi/scsi_lib_dma.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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SCSI library functions depending on DMA (map and unmap scatter-gather
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lists).
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.. kernel-doc:: drivers/scsi/scsi_lib_dma.c
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:export:
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drivers/scsi/scsi_proc.c
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The functions in this file provide an interface between the PROC file
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system and the SCSI device drivers It is mainly used for debugging,
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statistics and to pass information directly to the lowlevel driver. I.E.
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plumbing to manage /proc/scsi/\*
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.. kernel-doc:: drivers/scsi/scsi_proc.c
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:internal:
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drivers/scsi/scsi_netlink.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Infrastructure to provide async events from transports to userspace via
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netlink, using a single NETLINK_SCSITRANSPORT protocol for all
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transports. See `the original patch submission
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<https://lore.kernel.org/linux-scsi/1155070439.6275.5.camel@localhost.localdomain/>`__
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for more details.
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.. kernel-doc:: drivers/scsi/scsi_netlink.c
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:internal:
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drivers/scsi/scsi_scan.c
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Scan a host to determine which (if any) devices are attached. The
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general scanning/probing algorithm is as follows, exceptions are made to
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it depending on device specific flags, compilation options, and global
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variable (boot or module load time) settings. A specific LUN is scanned
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via an INQUIRY command; if the LUN has a device attached, a scsi_device
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is allocated and setup for it. For every id of every channel on the
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given host, start by scanning LUN 0. Skip hosts that don't respond at
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all to a scan of LUN 0. Otherwise, if LUN 0 has a device attached,
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allocate and setup a scsi_device for it. If target is SCSI-3 or up,
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issue a REPORT LUN, and scan all of the LUNs returned by the REPORT LUN;
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else, sequentially scan LUNs up until some maximum is reached, or a LUN
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is seen that cannot have a device attached to it.
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.. kernel-doc:: drivers/scsi/scsi_scan.c
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:internal:
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drivers/scsi/scsi_sysctl.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Set up the sysctl entry: "/dev/scsi/logging_level"
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(DEV_SCSI_LOGGING_LEVEL) which sets/returns scsi_logging_level.
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drivers/scsi/scsi_sysfs.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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SCSI sysfs interface routines.
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.. kernel-doc:: drivers/scsi/scsi_sysfs.c
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:export:
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drivers/scsi/hosts.c
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~~~~~~~~~~~~~~~~~~~~
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mid to lowlevel SCSI driver interface
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.. kernel-doc:: drivers/scsi/hosts.c
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:export:
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drivers/scsi/scsi_common.c
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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general support functions
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.. kernel-doc:: drivers/scsi/scsi_common.c
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:export:
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Transport classes
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-----------------
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Transport classes are service libraries for drivers in the SCSI lower
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layer, which expose transport attributes in sysfs.
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Fibre Channel transport
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~~~~~~~~~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_transport_fc.c defines transport attributes
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for Fibre Channel.
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.. kernel-doc:: drivers/scsi/scsi_transport_fc.c
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:export:
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iSCSI transport class
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~~~~~~~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_transport_iscsi.c defines transport
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attributes for the iSCSI class, which sends SCSI packets over TCP/IP
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connections.
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.. kernel-doc:: drivers/scsi/scsi_transport_iscsi.c
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:export:
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Serial Attached SCSI (SAS) transport class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_transport_sas.c defines transport
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attributes for Serial Attached SCSI, a variant of SATA aimed at large
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high-end systems.
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The SAS transport class contains common code to deal with SAS HBAs, an
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approximated representation of SAS topologies in the driver model, and
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various sysfs attributes to expose these topologies and management
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interfaces to userspace.
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In addition to the basic SCSI core objects this transport class
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introduces two additional intermediate objects: The SAS PHY as
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represented by struct sas_phy defines an "outgoing" PHY on a SAS HBA or
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Expander, and the SAS remote PHY represented by struct sas_rphy defines
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an "incoming" PHY on a SAS Expander or end device. Note that this is
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purely a software concept, the underlying hardware for a PHY and a
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remote PHY is the exactly the same.
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There is no concept of a SAS port in this code, users can see what PHYs
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form a wide port based on the port_identifier attribute, which is the
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same for all PHYs in a port.
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.. kernel-doc:: drivers/scsi/scsi_transport_sas.c
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:export:
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SATA transport class
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~~~~~~~~~~~~~~~~~~~~
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The SATA transport is handled by libata, which has its own book of
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documentation in this directory.
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Parallel SCSI (SPI) transport class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_transport_spi.c defines transport
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attributes for traditional (fast/wide/ultra) SCSI busses.
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.. kernel-doc:: drivers/scsi/scsi_transport_spi.c
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:export:
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SCSI RDMA (SRP) transport class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_transport_srp.c defines transport
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attributes for SCSI over Remote Direct Memory Access.
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.. kernel-doc:: drivers/scsi/scsi_transport_srp.c
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:export:
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SCSI lower layer
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================
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Host Bus Adapter transport types
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--------------------------------
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Many modern device controllers use the SCSI command set as a protocol to
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communicate with their devices through many different types of physical
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connections.
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In SCSI language a bus capable of carrying SCSI commands is called a
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"transport", and a controller connecting to such a bus is called a "host
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bus adapter" (HBA).
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Debug transport
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~~~~~~~~~~~~~~~
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The file drivers/scsi/scsi_debug.c simulates a host adapter with a
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variable number of disks (or disk like devices) attached, sharing a
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common amount of RAM. Does a lot of checking to make sure that we are
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not getting blocks mixed up, and panics the kernel if anything out of
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the ordinary is seen.
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To be more realistic, the simulated devices have the transport
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attributes of SAS disks.
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For documentation see http://sg.danny.cz/sg/scsi_debug.html
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todo
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~~~~
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Parallel (fast/wide/ultra) SCSI, USB, SATA, SAS, Fibre Channel,
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FireWire, ATAPI devices, Infiniband, Parallel ports,
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netlink...
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