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Custom udev rules

udev (the device manager for the Linux kernel) uses rules to manage device events (e.g., when a USB is plugged in). Rules are typically stored in /etc/udev/rules.d/ or /lib/udev/rules.d/ as files with a .rules extension (e.g., 10-my-rule.rules). Lower-numbered files are processed first.

udev Rule

A rule consists of match keys (conditions) and assignment keys (actions), written in the format:

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KEY=="value", ACTION="value"
  • Match keys: Conditions to identify a device (e.g., SUBSYSTEM, ATTR).
  • Assignment keys: Actions to take (e.g., NAME, RUN, SYMLINK).

demos

Set symbolic name

Set symbolic name every time my arduino mega connected

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udevadm info -a -p /sys/class/tty/ttyACM0
  • udevadm: A utility for managing and querying udev, the Linux device manager.
  • info: A subcommand that provides information about a device.
  • -a : Walks up the device tree (from the specified device to its parents) and shows all attributes available for matching in udev rules.
  • -p : Specifies the sysfs path of the device to query /sys/class/tty/ttyACM0.
result: Show only the part i use for the rule 
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looking at parent device '/devices/pci0000:00/0000:00:14.0/usb3/3-4/3-4.1/3-4.1.5/3-4.1.5.3':
    KERNELS=="3-4.1.5.3"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ...
    ATTRS{configuration}==""
    ATTRS{devnum}=="24"
    ATTRS{devpath}=="4.1.5.3"
    ATTRS{idProduct}=="0010"
    ATTRS{idVendor}=="2341"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="Arduino (www.arduino.cc)"
    ...
    ATTRS{product}=="Arduino Mega 2560"
    ATTRS{quirks}=="0x0"
/etc/udev/rules.d/91-arduino.rules
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ACTION=="add",SUBSYSTEMS=="usb",ATTRS{idVendor}=="2341",ATTRS{product}=="Arduino Mega 2560", SYMLINK+="arduino_mega"

debug and monitor

monitor service using

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sudo journalctl -f -u systemd-udev.service

check

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sudo udevadm control --reload-rules && sudo udevadm trigger
  • Disconnect / Connect the device for add event 
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    ll /dev/arduino_mega 
#
lrwxrwxrwx 1 root root 7 Apr  2 06:56 /dev/arduino_mega -> ttyACM0

TODO

  • explain monitor
  • explain device tree
  • explain ATTR ATTRS ENV
  • explain SUBSYSTEMS

Set permission

Set permission to usb device

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SUBSYSTEMS=="i2c-dev", ATTRS{name}=="CP2112 SMBus Bridge on hidraw5", MODE="0666"