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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" []> | ||
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<book id="scsidrivers"> | ||
<bookinfo> | ||
<title>SCSI Subsystem Interfaces</title> | ||
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<authorgroup> | ||
<author> | ||
<firstname>Douglas</firstname> | ||
<surname>Gilbert</surname> | ||
<affiliation> | ||
<address> | ||
<email>dgilbert@interlog.com</email> | ||
</address> | ||
</affiliation> | ||
</author> | ||
</authorgroup> | ||
<pubdate>2003-08-11</pubdate> | ||
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<copyright> | ||
<year>2002</year> | ||
<year>2003</year> | ||
<holder>Douglas Gilbert</holder> | ||
</copyright> | ||
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<legalnotice> | ||
<para> | ||
This documentation is free software; you can redistribute | ||
it and/or modify it under the terms of the GNU General Public | ||
License as published by the Free Software Foundation; either | ||
version 2 of the License, or (at your option) any later | ||
version. | ||
</para> | ||
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||
<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. | ||
See the GNU General Public License for more details. | ||
</para> | ||
|
||
<para> | ||
You should have received a copy of the GNU General Public | ||
License along with this program; if not, write to the Free | ||
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, | ||
MA 02111-1307 USA | ||
</para> | ||
|
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<para> | ||
For more details see the file COPYING in the source | ||
distribution of Linux. | ||
</para> | ||
</legalnotice> | ||
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</bookinfo> | ||
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<toc></toc> | ||
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<chapter id="intro"> | ||
<title>Introduction</title> | ||
<para> | ||
This document outlines the interface between the Linux scsi mid level | ||
and lower level drivers. Lower level drivers are variously called HBA | ||
(host bus adapter) drivers, host drivers (HD) or pseudo adapter drivers. | ||
The latter alludes to the fact that a lower level driver may be a | ||
bridge to another IO subsystem (and the "ide-scsi" driver is an example | ||
of this). There can be many lower level drivers active in a running | ||
system, but only one per hardware type. For example, the aic7xxx driver | ||
controls adaptec controllers based on the 7xxx chip series. Most lower | ||
level drivers can control one or more scsi hosts (a.k.a. scsi initiators). | ||
</para> | ||
<para> | ||
This document can been found in an ASCII text file in the linux kernel | ||
source: <filename>Documentation/scsi/scsi_mid_low_api.txt</filename> . | ||
It currently hold a little more information than this document. The | ||
<filename>drivers/scsi/hosts.h</filename> and <filename> | ||
drivers/scsi/scsi.h</filename> headers contain descriptions of members | ||
of important structures for the scsi subsystem. | ||
</para> | ||
</chapter> | ||
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<chapter id="driver-struct"> | ||
<title>Driver structure</title> | ||
<para> | ||
Traditionally a lower level driver for the scsi subsystem has been | ||
at least two files in the drivers/scsi directory. For example, a | ||
driver called "xyz" has a header file "xyz.h" and a source file | ||
"xyz.c". [Actually there is no good reason why this couldn't all | ||
be in one file.] Some drivers that have been ported to several operating | ||
systems (e.g. aic7xxx which has separate files for generic and | ||
OS-specific code) have more than two files. Such drivers tend to have | ||
their own directory under the drivers/scsi directory. | ||
</para> | ||
<para> | ||
scsi_module.c is normally included at the end of a lower | ||
level driver. For it to work a declaration like this is needed before | ||
it is included: | ||
<programlisting> | ||
static Scsi_Host_Template driver_template = DRIVER_TEMPLATE; | ||
/* DRIVER_TEMPLATE should contain pointers to supported interface | ||
functions. Scsi_Host_Template is defined hosts.h */ | ||
#include "scsi_module.c" | ||
</programlisting> | ||
</para> | ||
<para> | ||
The scsi_module.c assumes the name "driver_template" is appropriately | ||
defined. It contains 2 functions: | ||
<orderedlist> | ||
<listitem><para> | ||
init_this_scsi_driver() called during builtin and module driver | ||
initialization: invokes mid level's scsi_register_host() | ||
</para></listitem> | ||
<listitem><para> | ||
exit_this_scsi_driver() called during closedown: invokes | ||
mid level's scsi_unregister_host() | ||
</para></listitem> | ||
</orderedlist> | ||
</para> | ||
<para> | ||
When a new, lower level driver is being added to Linux, the following | ||
files (all found in the drivers/scsi directory) will need some attention: | ||
Makefile, Config.help and Config.in . It is probably best to look at what | ||
an existing lower level driver does in this regard. | ||
</para> | ||
</chapter> | ||
|
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<chapter id="intfunctions"> | ||
<title>Interface Functions</title> | ||
!EDocumentation/scsi/scsi_mid_low_api.txt | ||
</chapter> | ||
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<chapter id="locks"> | ||
<title>Locks</title> | ||
<para> | ||
Each Scsi_Host instance has a spin_lock called Scsi_Host::default_lock | ||
which is initialized in scsi_register() [found in hosts.c]. Within the | ||
same function the Scsi_Host::host_lock pointer is initialized to point | ||
at default_lock with the scsi_assign_lock() function. Thereafter | ||
lock and unlock operations performed by the mid level use the | ||
Scsi_Host::host_lock pointer. | ||
</para> | ||
<para> | ||
Lower level drivers can override the use of Scsi_Host::default_lock by | ||
using scsi_assign_lock(). The earliest opportunity to do this would | ||
be in the detect() function after it has invoked scsi_register(). It | ||
could be replaced by a coarser grain lock (e.g. per driver) or a | ||
lock of equal granularity (i.e. per host). Using finer grain locks | ||
(e.g. per scsi device) may be possible by juggling locks in | ||
queuecommand(). | ||
</para> | ||
</chapter> | ||
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<chapter id="changes"> | ||
<title>Changes since lk 2.4 series</title> | ||
<para> | ||
io_request_lock has been replaced by several finer grained locks. The lock | ||
relevant to lower level drivers is Scsi_Host::host_lock and there is one | ||
per scsi host. | ||
</para> | ||
<para> | ||
The older error handling mechanism has been removed. This means the | ||
lower level interface functions abort() and reset() have been removed. | ||
</para> | ||
<para> | ||
In the 2.4 series the scsi subsystem configuration descriptions were | ||
aggregated with the configuration descriptions from all other Linux | ||
subsystems in the Documentation/Configure.help file. In the 2.5 series, | ||
the scsi subsystem now has its own (much smaller) drivers/scsi/Config.help | ||
file. | ||
</para> | ||
</chapter> | ||
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<chapter id="credits"> | ||
<title>Credits</title> | ||
<para> | ||
The following people have contributed to this document: | ||
<orderedlist> | ||
<listitem><para> | ||
Mike Anderson <email>andmike@us.ibm.com</email> | ||
</para></listitem> | ||
<listitem><para> | ||
James Bottomley <email>James.Bottomley@steeleye.com</email> | ||
</para></listitem> | ||
<listitem><para> | ||
Patrick Mansfield <email>patmans@us.ibm.com</email> | ||
</para></listitem> | ||
</orderedlist> | ||
</para> | ||
</chapter> | ||
|
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</book> |
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