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The sdparm utility uses the MODE SENSE SCSI command to read device parameters
and the MODE SELECT SCSI command to change them. CD/DVD drives (and their
media) have a large amount of information that either the user can't change
or are characteristics of the media. The MMC set has a GET CONFIGURATION
SCSI command to fetch information (basically capabilities) that the user is
not able to change. To read this information see the sg_get_config utility in
the sg3_utils package. Earlier versions of MMC put information in the "CD/DVD
(MM) capabilities and mechanical status" mode page that is now found in the
GET CONFIGURATION feature and profile pages. The sdparm utility shows the
CD/DVD capabilities and mechanical status mode page which has information up
to about "DVD-R" vintage. Thereafter, (MMC-4 and MMC-5) information
about "DVD+R" and later (e.g. BD and HD-DVD) are in feature and profile pages
fetched with the GET CONFIGURATION command.
There is a C program called chk_sdparm_data.c in the src directory for
checking the integrity of the mode items in the sdparm_data.c file. A simple
example for building chk_sdparm_data is given inside the source file. The
sdparm_data.c file only needs to be checked for integrity after new mode
page items are added or changed.
The linux hdparm utility is usually found in the /sbin and sometimes in
the /usr/sbin directory. That location is not usually on the PATH of a
non-root user. Since hdparm is mainly used on disks, this is an appropriate
place. Currently in Linux sdparm is placed in the /usr/bin directory (by
the sdparm.spec file) or /usr/local/bin (by 'make install' when not
overridden by option given to "./configure"). This allows both root and
non-root users to access sdparm. Permissions on SCSI disks devices should be
sufficient to stop a non-root user changing parameters unless they have both
read and write permissions. Non-root users should be able read SCSI disk (or
other device) parameters if they have read permissions on the appropriate
device. A typical SCSI disk has permissions like this:
# ls -l /dev/sda
brw-r----- 1 root disk 8, 0 Jul 28 2005 /dev/sda
There are also CD/DVD drives to consider. Many distributions give non-root
(GUI) users permissions (and indeed ownership) of these devices. This allows
users to "burn" CDs and DVDs. Almost all CD/DVD drives use MMC which is a
SCSI command set. Hence sdparm is appropriate to read and change parameters
on CD/DVD drives. A typical (ATAPI transport) CD/DVD drive has permissions
like this (where "fred" is the GUI user):
# ls -l /dev/hdc
brw------- 1 fred disk 22, 0 Jul 28 2005 /dev/hdc
Doug Gilbert
24th September 2007
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