devtmpfs

• Mounted at /dev
• Udev is responsible for the dynamic device management needed for hot plugging devices
• Whenever a device is plugged
• The kernel emits an event
• Systemd (systemd-udevd.service) listens the event
• The daemon creates a file in /dev
• The udev daemon searches configured rules to match the event with a rule to identify the device

Devices

input

• /dev/input

# all mice events (touch + mouse)
cat /dev/input/mice

# keyboard
cat /dev/input/by-id/usb-my-keyboard-event-kbd

disk

• /dev/disk
• /dev/disk/by-id
• /dev/disk/by-label
• /dev/disk/by-uuid
• /dev/disk/by-path

Udev Rules

• Location
• /usr/lib/udev/rules.d: default rules (do not edit)
• /etc/udev/rules.d: customized rules (free to edit)
• Lower-numbered rule files are processed first
• That can prevent a rule in a higher-numbered rule to make a change by using the := assignment operator

Match conditions

• Equality operators
• ==: equals
• !=: does not equal
• Types
• KERNEL
• ATTRS
• ENV: by property

Actions

• Assignment operators
• = (overwrite and assign)
• += (add without overwriting, append)
• := (add and prevent other rules from overwriting)

• Types

• ATTR: change attribute

• MODE: change the device file permission
• SYMLINK: create a symbolic link at a location (relative to /dev)
• TAG

• ENV: add a property

• The actions are taken only if all the match conditions are met

Examples

# Assigns the i2c devices to group i2c, and gives that group RW access:
# Assign RW access to /dev/i2c* devices whenever the user is in i2c group
KERNEL=="i2c-[0-9]*", GROUP="i2c", MODE="0660"

# Gives everyone RW access to the /dev/i2c devices
KERNEL=="i2c-[0-9]*", MODE="0666"

# Assign a tag to the device /dev/dri/card1 whenever it's plugged
ENV{DEVNAME}=="/dev/dri/card1", TAG+="mutter-device-preferred-primary"

# Assign a tag to the gpu device that matches the vid and pid
SUBSYSTEM=="drm", KERNEL=="card*", ATTRS{idVendor}=="1002", ATTRS{idProduct}=="73ff", TAG+="mutter-device-preferred-primary"

# Assign tag by ID_PATH
SUBSYSTEM=="drm", ENV{ID_PATH}=="pci-0000:0e:00.0", TAG+="mutter-device-preferred-primary"

# If a env is define then take the run a command
ENV{SOME_ENV}=="?*" IMPORT{program}="ls -la"

# /etc/udev/rules.d/61-gpu.rules
KERNEL=="card?", SUBSYSTEM=="drm", ATTRS{vendor}=="0x8086", ATTRS{device}=="0x3e9b", SYMLINK+="dri/by-name/intel-uhd-graphics-630"
KERNEL=="card?", SUBSYSTEM=="drm", ATTRS{vendor}=="0x1002", ATTRS{device}=="0x7340", SYMLINK+="dri/by-name/amd-radeon-pro-5500m", ATTR{device/power_dpm_force_performance_level}="low"
KERNEL=="card?", SUBSYSTEM=="drm", ATTRS{vendor}=="0x1002", ATTRS{device}=="0x73ff", SYMLINK+="dri/by-name/amd-radeon-rx-6600-xt", TAG+="mutter-device-preferred-primary"