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338 lines
9.4 KiB
338 lines
9.4 KiB
===================== |
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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 <http://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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`I2O <http://i2o.shadowconnect.com/faq.php>`__, TCP/IP |
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(`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 |
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submission <http://marc.info/?l=linux-scsi&m=115507374832500&w=2>`__ for |
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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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aproximated 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/sdebug26.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, I2O, Parallel ports, |
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netlink...
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