# SCSI Subsystem Interfaces

Douglas
Gilbert

          dgilbert@interlog.com

2003-08-11
2002
2003
Douglas Gilbert

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.

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.

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

For more details see the file COPYING in the source distribution of Linux.

## Introduction

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).

This document can been found in an ASCII text file in the linux kernel source: `Documentation/scsi/scsi_mid_low_api.txt` . It currently hold a little more information than this document. The `drivers/scsi/hosts.h` and `drivers/scsi/scsi.h` headers contain descriptions of members of important structures for the scsi subsystem.

## Driver structure

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.

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:

        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"

The scsi_module.c assumes the name "driver_template" is appropriately defined. It contains 2 functions:

1. init_this_scsi_driver() called during builtin and module driver initialization: invokes mid level's scsi_register_host()

2. exit_this_scsi_driver() called during closedown: invokes mid level's scsi_unregister_host()

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.

## Interface Functions

!EDocumentation/scsi/scsi_mid_low_api.txt

## Locks

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.

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().

## Changes since lk 2.4 series

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.

The older error handling mechanism has been removed. This means the lower level interface functions abort() and reset() have been removed.

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.

## Credits

The following people have contributed to this document:

1. Mike Anderson andmike@us.ibm.com

2. James Bottomley James.Bottomley@steeleye.com

3. Patrick Mansfield patmans@us.ibm.com