11.7. Creating custom symlinks to devices

11.7.1. CD-ROM symlinks

Some software that you may want to install later (e.g., various media players) expect the /dev/cdrom and /dev/dvd symlinks to exist. Also, it may be convenient to put references to those symlinks into /etc/fstab. For each of your CD-ROM devices, find the corresponding directory under /sys (e.g., this can be /sys/block/hdd) and run a command similar to the following:

udevadm test /sys/block/hdd

Look at the lines containing the output of various *_id programs.

There are two approaches to creating symlinks. The first one is to use the model name and the serial number, the second one is based on the location of the device on the bus. If you are going to use the first approach, create a file similar to the following:

cat >/etc/udev/rules.d/82-cdrom.rules << EOF

# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", \
    ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", \
    ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd"

EOF

This way, the symlinks will stay correct even if you move the drives to different positions on the IDE bus, but the /dev/cdrom symlink won't be created if you replace the old SAMSUNG CD-ROM with a new drive.

The SUBSYSTEM=="block" key is needed in order to avoid matching SCSI generic devices. Without it, in the case with SCSI CD-ROMs, the symlinks will sometimes point to the correct /dev/srX devices, and sometimes to /dev/sgX, which is wrong.

The second approach yields:

cat >/etc/udev/rules.d/82-cdrom.rules << EOF

# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
    ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
    ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd"

EOF

This way, the symlinks will stay correct even if you replace drives with different models, but place them to the old positions on the IDE bus. The ENV{ID_TYPE}=="cd" key makes sure that the symlink disappears if you put something other than a CD-ROM in that position on the bus.

Of course, it is possible to mix the two approaches.

11.7.2. Dealing with duplicate devices

As explained in Section 11.6, “Device and Module Handling on a CLFS System”, the order in which devices with the same function appear in /dev is essentially random. E.g., if you have a USB web camera and a TV tuner, sometimes /dev/video0 refers to the camera and /dev/video1 refers to the tuner, and sometimes after a reboot the order changes to the opposite one. For all classes of hardware except sound cards and network cards, this is fixable by creating udev rules for custom persistent symlinks. The case of network cards is covered separately in Networking Configuration, and sound card configuration can be found in CBLFS.

For each of your devices that is likely to have this problem (even if the problem doesn't exist in your current Linux distribution), find the corresponding directory under /sys/class or /sys/block. For video devices, this may be /sys/class/video4linux/videoX. Figure out the attributes that identify the device uniquely (usually, vendor and product IDs and/or serial numbers work):

udevadm info -a -p /sys/class/video4linux/video0

Then write rules that create the symlinks, e.g.:

cat >/etc/udev/rules.d/83-duplicate_devs.rules << EOF

# Persistent symlinks for webcam and tuner
KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", \
    SYMLINK+="webcam"
KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", \
    SYMLINK+="tvtuner"

EOF

The result is that /dev/video0 and /dev/video1 devices still refer randomly to the tuner and the web camera (and thus should never be used directly), but there are symlinks /dev/tvtuner and /dev/webcam that always point to the correct device.

More information on writing Udev rules can be found in /usr/share/doc/udev/writing_udev_rules/index.html.