The Linux SCSI subsystem in 2.4 HOWTO
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9.2. CDROM driver (sr or scd)

CDROM and DVD drives (and WORM devices) are accessible via the sr upper level device driver. While is "sr" is the device driver name, "sr_mod" is its module name. The device file name is either /dev/sr<n> or /dev/scd<n>.

Following is a diagram illustrating the CDROM subsystem of which sr is a part:

The architecture of the CD-ROM subsystem.

Many modern IDE CDROM players and all DVD players use the ATAPI standard which then allows them to be controlled by the SCSI subsystem with the ide-scsi lower level pseudo driver. The default action is for the IDE subsystem to take ownership of all IDE devices and in this case the default driver would be ide-cd.o . Once the IDE subsystem "owns" an ATAPI CDROM player then this excludes the ide-scsi driver from attaching itself to the same device. In order to change this default action see the following sections on boot and module parameters.

Two types of SCSI devices are accessible via the sr driver:

The sr driver is capable of recognizing 256 CDROM/DVD drives when it is loaded at kernel boot time or later as a module. However, once it is loaded, it will only recognize a fixed number of additional drives. The number of additional drives that can be accommodated is set by the kernel configuration parameter CONFIG_SR_EXTRA_DEVS whose default value is 2.

9.2.1. sr boot parameters

None but the following is related: During the boot sequence of the Linux kernel, the IDE devices are scanned before SCSI devices. This means that if both the ide-cd and ide-scsi drivers are built in, then the ide-cd driver will claim all CDROM devices on the IDE bus(-es). To override this action use the boot parameter "hd<x>=scsi" where <x> is the appropriate drive letter. In this case when the ide-scsi driver is initialized it will find the unclaimed IDE device and "claim" it for the SCSI subsystem.

9.2.2. sr module parameters

Doing a test to find out if a cdrom drive supports XA mode (mode 2) triggers firmware bugs on some drives. Consequently the check for XA mode support is turned off by default. The following module parameter is provided: 
    xa_test=<0|1>
to override the default. [Currently there seems to be no way to turn on XA mode testing when the sr driver is built into the kernel.]

Continuing on with the ATAPI CDROM driver override discussion: for modules this can also be done with insmod ide-cd ignore=hdb to exclude that device. Typically the ide-cd module is loaded automatically so a line in the /etc/modules.conf file like: 
    options ide-cd ignore=hdb
should work. If the ide-scsi module is loaded after that, it will claim hdb. To check if this has happened examine cat /proc/scsi/scsi. Note that any other IDE CDROM player apart from the one connected to /dev/hdb (i.e. first IDE controller, slave device) will still be controlled by the ide-cd module (and hence the IDE subsystem).

9.2.3. sr proc interface

All the following files are readable by all and produce ASCII output when read: 
 /proc/sys/dev/cdrom/autoclose
 /proc/sys/dev/cdrom/autoeject
 /proc/sys/dev/cdrom/check_media
 /proc/sys/dev/cdrom/debug
 /proc/sys/dev/cdrom/info
 /proc/sys/dev/cdrom/lock
They reflect the current state of the CDROM subsystem. This location is part of the procfs's window through to the sysctl configuration mechanism (see man sysctl). All but info are writable by the superuser. There is a column for each CDROM and DVD player in the system in info (not just SCSI devices).

As an example, the auto eject feature can be turned on by the superuser with the command echo "1" > /proc/sys/dev/cdrom/autoeject. This will cause cdroms to be ejected from the drive when unmounted.

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