9.2. CDROM driver (sr or scd)
CDROM and DVD drives (and WORM devices) are accessible via the sr upper
level device driver.
While "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:
This diagram glosses over some of the differences between the
protocol stacks. CDROM device names are not maintained
by the uniform CDROM layer but rather by each individual protocol stack.
In the case of the SCSI subsystem, device names are maintained by the
sr driver while the IDE subsystem maintains device names with its
central "ide" driver (i.e. not by the ide-cd driver). USB and IEEE1394
cd devices names are maintained by their respective stacks. This may
partially explain why the /dev/cdrom is often a
symbolic link to the appropriate subsystem's device name.
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.
People often use the dd command to read a data CDROM
containing an iso9660 file system. If a count argument is not given then
the dd command will read the number of 2048 byte sectors
indicated by the SCSI Read Capacity command. Unfortunately this can
include unwritten (or "run out") sectors at the end of the image that can
cause I/O errors. Use the isosize command (see its man
page) to find the true length of the iso9660 image and use that in the
"count=" argument given to the dd command.
9.2.1. sr boot parameters
None.
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:
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.]
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
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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.
9.2.4. ATAPI cdroms
Many Linux users have no SCSI devices (or adapters) in their systems. They
become a little perplexed as to why cd writer software (e.g.
cdrecord and cdrdao)
and cd music reading programs (e.g. cdparanoia) use
the Linux SCSI subsystem. The answer is that these programs need lower level
access to these devices. ATAPI (ATA Packet Interface) is essentially
a SCSI command set sent over an ATA
transport. [The discussion in this section is also applicable to ATAPI
tape drives and ATAPI floppy drives.]
Currently both cdrecord and cdparanoia
interface to the SCSI generic driver (sg) and, in the case of ATAPI
cd devices, use the ide-scsi pseudo device driver to access the hardware.
This may change in the future as in the 2.4 series kernels a packet
interface ioctl has been added to the uniform cdrom layer (see the diagram
in Section 9.2 above).
The default action of the IDE subsystem in Linux is to claim all ATA
devices for its built-in drivers. In the case of an ATAPI cd writer, it
will be claimed by the built-in ide-cd driver. Once this has happened,
the SCSI subsystem is unable to get control over an ATAPI device. The
ide-scsi (pseudo lower level SCSI) driver can only register ATAPI devices
in the SCSI subsystem that have not already been
claimed by the IDE subsystem.
Notice the built-in qualification in the previous
paragraph. If both the ide-cd and ide-scsi drivers are modules then the
first one loaded will claim the ATAPI cd devices (e.g. cd/dvd readers and
writers). Furthermore you can switch the controlling driver module by
rmmod-ing one and modprobe-ing the
other.
Probably the most flexible way to instruct the IDE core driver that you
want the cd writer at /dev/hdd accessible to
cdrecord is to use the kernel boot option:
"hdd=ide-scsi". This will cause the ide-cd driver to bypass
/dev/hdd (irrespective of whether ide-cd driver is
built-in or a module). As long as the ide-scsi driver is built-in or a
module then it will "capture" the cd writer
at /dev/hdd (with the IDE core driver loading the
ide-scsi module if required).
The ide-cd driver module can be instructed to ignore certain ATA devices
with the following syntax:
modprobe ide-cd ignore='hdc hdd'
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In this case the ide-cd driver will ignore the devices at
/dev/hdc and
/dev/hdd .
This effect can also be accomplished by placing a line like this:
"options ide-cd ignore=hdd" in the
/etc/modules.conf
file.
A new option added in the lk 2.4 series is of the form "hdd=scsi".
This option seems to have a similar function to the "hdd=ide-scsi"
option discussed above. Furthermore "hdd=scsi" can only be used if
both the SCSI mid-level and the ide-scsi drivers are built into the
kernel (otherwise "BAD OPTION" is reported by the ide_setup function).
To find out whether an ATAPI cd device is "owned" by the SCSI subsystem,
the output of cat /proc/scsi/scsi can be checked.
Another technique is to observe the "drive name:" line of
cat /proc/sys/dev/cdrom/info for "sr" entries. The
following output is from my system:
$ cat /proc/sys/dev/cdrom/info
CD-ROM information, Id: cdrom.c 3.12 2000/10/18
drive name: sr1 sr0
drive speed: 16 0
drive # of slots: 1 1
Can close tray: 1 1
Can open tray: 1 1
Can lock tray: 1 1
Can change speed: 1 1
Can select disk: 0 0
Can read multisession: 1 1
Can read MCN: 1 1
Reports media changed: 1 1
Can play audio: 1 1
Can write CD-R: 1 0
Can write CD-RW: 1 0
Can read DVD: 0 1
Can write DVD-R: 0 0
Can write DVD-RAM: 0 0
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Once an ATAPI cd writer at /dev/hdd has been registered by the SCSI
subsystem, then cdroms should be mounted via the "scd" device name
and cd players should also use the "scd" device. Strangely the
hdparm command should still use the
/dev/hdd device file (or the "echo ... > /proc/ide/hdd/settings"
method described in this section).