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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+        "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="scsimid">
+  <bookinfo>
+    <title>SCSI Mid Layer Guide</title>
+
+    <authorgroup>
+      <author>
+        <firstname>James</firstname>
+        <surname>Bottomley</surname>
+        <affiliation>
+          <address>
+            <email>James.Bottomley@steeleye.com</email>
+          </address>
+        </affiliation>
+      </author>
+
+      <author>
+        <firstname>Rob</firstname>
+        <surname>Landley</surname>
+        <affiliation>
+          <address>
+            <email>rob@landley.net</email>
+          </address>
+        </affiliation>
+      </author>
+
+    </authorgroup>
+
+    <copyright>
+      <year>2007</year>
+      <holder>Linux Foundation</holder>
+    </copyright>
+
+    <legalnotice>
+      <para>
+        This documentation is free software; you can redistribute
+        it and/or modify it under the terms of the GNU General Public
+        License version 2.
+      </para>
+
+      <para>
+        This program is distributed in the hope that it will be
+        useful, but WITHOUT ANY WARRANTY; without even the implied
+        warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+        For more details see the file COPYING in the source
+        distribution of Linux.
+      </para>
+    </legalnotice>
+  </bookinfo>
+
+  <toc></toc>
+
+  <chapter id="intro">
+    <title>Introduction</title>
+    <sect1 id="protocol_vs_bus">
+      <title>Protocol vs bus</title>
+      <para>
+        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.
+      </para>
+      <para>
+        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.
+      </para>
+      <para>
+        The <ulink url='http://www.t10.org/scsi-3.htm'>SCSI protocol</ulink>
+        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.
+      </para>
+      <para>
+        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,
+        <ulink url='http://i2o.shadowconnect.com/faq.php'>I20</ulink>, TCP/IP
+        (<ulink url='http://en.wikipedia.org/wiki/ISCSI'>iSCSI</ulink>), even
+        <ulink url='http://cyberelk.net/tim/parport/parscsi.html'>Parallel
+        ports</ulink>.
+      </para>
+    </sect1>
+    <sect1 id="subsystem_design">
+      <title>Design of the Linux SCSI subsystem</title>
+      <para>
+        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.
+      </para>
+      <para>
+        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.
+      </para>
+      <para>
+        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.
+      </para>
+    </sect1>
+  </chapter>
+
+  <chapter id="upper_layer">
+    <title>SCSI upper layer</title>
+    <para>
+      The upper layer supports the user-kernel interface by providing
+      device nodes.
+    </para>
+    <sect1 id="sd">
+      <title>sd (SCSI Disk)</title>
+      <para>sd (sd_mod.o)</para>
+<!-- !Idrivers/scsi/sd.c -->
+    </sect1>
+    <sect1 id="sr">
+      <title>sr (SCSI CD-ROM)</title>
+      <para>sr (sr_mod.o)</para>
+    </sect1>
+    <sect1 id="st">
+      <title>st (SCSI Tape)</title>
+      <para>st (st.o)</para>
+    </sect1>
+    <sect1 id="sg">
+      <title>sg (SCSI Generic)</title>
+      <para>sg (sg.o)</para>
+    </sect1>
+    <sect1 id="ch">
+      <title>ch (SCSI Media Changer)</title>
+      <para>ch (ch.c)</para>
+    </sect1>
+  </chapter>
+
+  <chapter id="mid_layer">
+    <title>SCSI mid layer</title>
+
+    <sect1 id="midlayer_implementation">
+      <title>SCSI midlayer implementation</title>
+      <sect2 id="scsi_device.h">
+        <title>include/scsi/scsi_device.h</title>
+        <para>
+        </para>
+!Iinclude/scsi/scsi_device.h
+      </sect2>
+
+      <sect2 id="scsi.c">
+        <title>drivers/scsi/scsi.c</title>
+        <para>Main file for the scsi midlayer.</para>
+!Edrivers/scsi/scsi.c
+      </sect2>
+      <sect2 id="scsicam.c">
+        <title>drivers/scsi/scsicam.c</title>
+        <para>
+          <ulink url='http://www.t10.org/ftp/t10/drafts/cam/cam-r12b.pdf'>SCSI
+          Common Access Method</ulink> support functions, for use with
+          HDIO_GETGEO, etc.
+        </para>
+!Edrivers/scsi/scsicam.c
+      </sect2>
+      <sect2 id="scsi_error.c">
+        <title>drivers/scsi/scsi_error.c</title>
+        <para>Common SCSI error/timeout handling routines.</para>
+!Edrivers/scsi/scsi_error.c
+      </sect2>
+      <sect2 id="scsi_devinfo.c">
+        <title>drivers/scsi/scsi_devinfo.c</title>
+        <para>
+          Manage scsi_dev_info_list, which tracks blacklisted and whitelisted
+          devices.
+        </para>
+!Idrivers/scsi/scsi_devinfo.c
+      </sect2>
+      <sect2 id="scsi_ioctl.c">
+        <title>drivers/scsi/scsi_ioctl.c</title>
+        <para>
+          Handle ioctl() calls for scsi devices.
+        </para>
+!Edrivers/scsi/scsi_ioctl.c
+      </sect2>
+      <sect2 id="scsi_lib.c">
+        <title>drivers/scsi/scsi_lib.c</title>
+        <para>
+          SCSI queuing library.
+        </para>
+!Edrivers/scsi/scsi_lib.c
+      </sect2>
+      <sect2 id="scsi_lib_dma.c">
+        <title>drivers/scsi/scsi_lib_dma.c</title>
+        <para>
+          SCSI library functions depending on DMA
+          (map and unmap scatter-gather lists).
+        </para>
+!Edrivers/scsi/scsi_lib_dma.c
+      </sect2>
+      <sect2 id="scsi_module.c">
+        <title>drivers/scsi/scsi_module.c</title>
+        <para>
+          The file drivers/scsi/scsi_module.c contains legacy support for
+          old-style host templates.  It should never be used by any new driver.
+        </para>
+      </sect2>
+      <sect2 id="scsi_proc.c">
+        <title>drivers/scsi/scsi_proc.c</title>
+        <para>
+          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/*
+        </para>
+!Idrivers/scsi/scsi_proc.c
+      </sect2>
+      <sect2 id="scsi_netlink.c">
+        <title>drivers/scsi/scsi_netlink.c</title>
+        <para>
+          Infrastructure to provide async events from transports to userspace
+          via netlink, using a single NETLINK_SCSITRANSPORT protocol for all
+          transports.
+
+          See <ulink url='http://marc.info/?l=linux-scsi&amp;m=115507374832500&amp;w=2'>the
+          original patch submission</ulink> for more details.
+        </para>
+!Idrivers/scsi/scsi_netlink.c
+      </sect2>
+      <sect2 id="scsi_scan.c">
+        <title>drivers/scsi/scsi_scan.c</title>
+        <para>
+          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.
+        </para>
+!Idrivers/scsi/scsi_scan.c
+      </sect2>
+      <sect2 id="scsi_sysctl.c">
+        <title>drivers/scsi/scsi_sysctl.c</title>
+        <para>
+          Set up the sysctl entry: "/dev/scsi/logging_level"
+          (DEV_SCSI_LOGGING_LEVEL) which sets/returns scsi_logging_level.
+        </para>
+      </sect2>
+      <sect2 id="scsi_sysfs.c">
+        <title>drivers/scsi/scsi_sysfs.c</title>
+        <para>
+          SCSI sysfs interface routines.
+        </para>
+!Edrivers/scsi/scsi_sysfs.c
+      </sect2>
+      <sect2 id="hosts.c">
+        <title>drivers/scsi/hosts.c</title>
+        <para>
+          mid to lowlevel SCSI driver interface
+        </para>
+!Edrivers/scsi/hosts.c
+      </sect2>
+      <sect2 id="constants.c">
+        <title>drivers/scsi/constants.c</title>
+        <para>
+          mid to lowlevel SCSI driver interface
+        </para>
+!Edrivers/scsi/constants.c
+      </sect2>
+    </sect1>
+
+    <sect1 id="Transport_classes">
+      <title>Transport classes</title>
+      <para>
+        Transport classes are service libraries for drivers in the scsi
+        lower layer, which expose transport attributes in sysfs.
+      </para>
+      <sect2 id="Fibre_Channel_transport">
+        <title>Fibre Channel transport</title>
+        <para>
+          The file drivers/scsi/scsi_transport_fc.c defines transport attributes
+          for Fibre Channel.
+        </para>
+!Edrivers/scsi/scsi_transport_fc.c
+      </sect2>
+      <sect2 id="iSCSI_transport">
+        <title>iSCSI transport class</title>
+        <para>
+          The file drivers/scsi/scsi_transport_iscsi.c defines transport
+          attributes for the iSCSI class, which sends SCSI packets over TCP/IP
+          connections.
+        </para>
+!Edrivers/scsi/scsi_transport_iscsi.c
+      </sect2>
+      <sect2 id="SAS_transport">
+        <title>Serial Attached SCSI (SAS) transport class</title>
+        <para>
+          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.
+        </para>
+        <para>
+          The SAS transport class contains common code to deal with SAS HBAs,
+          an aproximated representation of SAS topologies in the driver model,
+          and various sysfs attributes to expose these topologies and managment
+          interfaces to userspace.
+        </para>
+        <para>
+          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.
+        </para>
+        <para>
+          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.
+        </para>
+!Edrivers/scsi/scsi_transport_sas.c
+      </sect2>
+      <sect2 id="SATA_transport">
+        <title>SATA transport class</title>
+        <para>
+          The SATA transport is handled by libata, which has its own book of
+          documentation in this directory.
+        </para>
+      </sect2>
+      <sect2 id="SPI_transport">
+        <title>Parallel SCSI (SPI) transport class</title>
+        <para>
+          The file drivers/scsi/scsi_transport_spi.c defines transport
+          attributes for traditional (fast/wide/ultra) SCSI busses.
+        </para>
+!Edrivers/scsi/scsi_transport_spi.c
+      </sect2>
+      <sect2 id="SRP_transport">
+        <title>SCSI RDMA (SRP) transport class</title>
+        <para>
+          The file drivers/scsi/scsi_transport_srp.c defines transport
+          attributes for SCSI over Remote Direct Memory Access.
+        </para>
+!Edrivers/scsi/scsi_transport_srp.c
+      </sect2>
+    </sect1>
+
+  </chapter>
+
+  <chapter id="lower_layer">
+    <title>SCSI lower layer</title>
+    <sect1 id="hba_drivers">
+      <title>Host Bus Adapter transport types</title>
+      <para>
+        Many modern device controllers use the SCSI command set as a protocol to
+        communicate with their devices through many different types of physical
+        connections.
+      </para>
+      <para>
+        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).
+      </para>
+      <sect2 id="scsi_debug.c">
+        <title>Debug transport</title>
+        <para>
+          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.
+        </para>
+        <para>
+          To be more realistic, the simulated devices have the transport
+          attributes of SAS disks.
+        </para>
+        <para>
+          For documentation see
+          <ulink url='http://www.torque.net/sg/sdebug26.html'>http://www.torque.net/sg/sdebug26.html</ulink>
+        </para>
+<!-- !Edrivers/scsi/scsi_debug.c -->
+      </sect2>
+      <sect2 id="todo">
+        <title>todo</title>
+        <para>Parallel (fast/wide/ultra) SCSI, USB, SATA,
+        SAS, Fibre Channel, FireWire, ATAPI devices, Infiniband,
+        I20, iSCSI, Parallel ports, netlink...
+        </para>
+      </sect2>
+    </sect1>
+  </chapter>
+</book>