blog:2025:03:30:2025-03-30_-_udev_names_per_raspi_pico [BaSzErr]

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====== 2025-03-30 - udev names per RasPi Pico ======

some time ago i found out a [[blog:2013:11:21:avr_programming_as_non-root|way to add udev rules for AVR programmers]]. while i barely ever use AVRs nowadays, the ''udev'' part becomes handy over and over again. :)

recently at work we had a few helper RasPi boards connected a test env, with loads of other TTY-to-USB adapters. it was a mess, as after a reboot one could never tell which device is which. it was time to clean it up, and [[https://github.com/cadsdf|Chris]] pointed me to a neat fact, that RasPi Pico chips actually have a unique serial number to them. that insight was enough to make unique links for each serial device, so that instead of ''/dev/ttuACM3'' you could have ''/dev/my_foobar_device''.


===== per-Pico rule =====

first just check device properties with:
<code bash>
udevadm info --name=/dev/ttyACM3 --attribute-walk 
</code>

then create ''udev'' rule in, say '' /etc/udev/rules.d/50-foobar.rules'' file:
<code>
SUBSYSTEM=="tty", ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="0005", ATTRS{serial}=="1234567890AB", \
  SYMLINK+="my_foobar_device"
</code>
note that the rule is specific to a particular board / µC, as it has a serial number.

then make the change live:
<code bash>
udevadm control --reload-rules
udevadm trigger 
</code>
...and enjoy new symlink! :)


===== multi-device targets =====

to make things more fun, we had one device that reported 4 serial devices (e.g. ''/dev/ttyUSB0'' to ''/dev/ttyUSB3''), but we needed to name / link those differently, to make it readable. ''serial'' is available inside ''tty'' while ''bInterfaceNumber'' is in ''usb''.

the bad news is that ''udev'' does not have built-in support for more than 1 ''SUBSYSTEM''(S) match. the good news is that this can be worked around with labels. let's say we want 3rd of 4 (i.e. number 2):

<code>
# 1st part
SUBSYSTEM=="tty", ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="0005", ATTRS{serial}=="1234567890AB", \
  GOTO="second_match"
GOTO="no_match"

# 2nd part
LABEL="second_match"
SUBSYSTEMS=="usb", ATTRS{bInterfaceNumber}=="02" \
  SYMLINK+="my_device"

# no match - this rule does not apply
LABEL="no_match"
</code>

and voila -- now we can have ''/dev/ttyUSB2'' of our device, differentiated by serial, linked as ''/dev/my_device''.

while this problem could also be addressed by ''SYMLINK+="my_device_%n"'' and have just 4 linked devices, one would still need to remember which-is-which.

blog/2025/03/30/2025-03-30_-_udev_names_per_raspi_pico.txt · Last modified: 2025/03/30 13:54 by basz

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