7.10. Creating Custom Symlinks to Devices

7.10.1. 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, and to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put references to those symlinks into /etc/fstab. Udev comes with a script that will generate rules files to create these symlinks for you, depending on the capabilities of each device, but you need to decide which of two modes of operation you wish to have the script use.

First, the script can operate in “by-path” mode (used by default for USB and FireWire devices), where the rules it creates depend on the physical path to the CD or DVD device. Second, it can operate in “by-id” mode (default for IDE and SCSI devices), where the rules it creates depend on identification strings stored in the CD or DVD device itself. The path is determined by Udev's path_id script, and the identification strings are read from the hardware by its ata_id or scsi_id programs, depending on which type of device you have.

There are advantages to each approach; the correct approach to use will depend on what kinds of device changes may happen. If you expect the physical path to the device (that is, the ports and/or slots that it plugs into) to change, for example because you plan on moving the drive to a different IDE port or a different USB connector, then you should use the “by-id” mode. On the other hand, if you expect the device's identification to change, for example because it may die, and you would replace it with a different device with the same capabilities and which is plugged into the same connectors, then you should use the “by-path” mode.

If either type of change is possible with your drive, then choose a mode based on the type of change you expect to happen more often.

If you wish to see the values that the Udev scripts will use, then for the appropriate CD-ROM device, 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. The “by-id” mode will use the ID_SERIAL value if it exists and is not empty, otherwise it will use a combination of ID_MODEL and ID_REVISION. The “by-path” mode will use the ID_PATH value.

If the default mode is not suitable for your situation, then the following modification can be made to the /lib/udev/rules.d/75-cd-aliases-generator.rules file, as follows (where mode is one of “by-id” or “by-path”):

sed -i -e 's/write_cd_rules/& mode/' \
    /lib/udev/rules.d/75-cd-aliases-generator.rules

Note that it is not necessary to create the rules files or symlinks at this time, because you have bind-mounted the host's /dev directory into the LFS system, and we assume the symlinks exist on the host. The rules and symlinks will be created the first time you boot your LFS system.

However, if you have multiple CD-ROM devices, then the symlinks generated at that time may point to different devices than they point to on your host, because devices are not discovered in a predictable order. The assignments created when you first boot the LFS system will be stable, so this is only an issue if you need the symlinks on both systems to point to the same device. If you need that, then inspect (and possibly edit) the generated /etc/udev/rules.d/70-persistent-cd.rules file after booting, to make sure the assigned symlinks match what you need.

7.10.2. Dealing with duplicate devices

As explained in Section 7.9, “Device and Module Handling on an LFS 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 Section 7.13, “Configuring the network Script”, and sound card configuration can be found in BLFS.

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*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", \
    SYMLINK+="webcam"
KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{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-145/writing_udev_rules/index.html.