How to find reasons why an udev rule is not applied?

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I added an udev rule to generate a separate link to a special tty device. I took the vendor and product id from lsusb:

Bus 001 Device 016: ID abcd:1234 Foo Device

The dmesg output for the device is:

[  369.384850] usb 1-1.1: new full-speed USB device number 4 using xhci_hcd
[  369.470492] usb 1-1.1: New USB device found, idVendor=09d8, idProduct=0420
[  369.470506] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[  369.470515] usb 1-1.1: Product: TWN4/B1.06/CCL3.12/PRS1.04
[  369.470522] usb 1-1.1: Manufacturer: OEM
[  369.475188] cdc_acm 1-1.1:1.0: ttyACM0: USB ACM device

Normally the Debian system generates an ACM* tty for this device:

$ ls /dev/ttyACM*
/dev/ttyACM0

So I created /ect/udev/rules.d/99-bizrfid.rules:

ACTION=="add", ATTRS{idVendor}=="09d8", ATTRS{idProduct}=="0420", SYMLINK+="ttyMyDevice"

But ls /dev/tty* does not show ttyMyDevice.

What I tried so far without success:

  1. Running udevadm trigger. This should apply the new udev rule.
  2. I did a reboot.
  3. I had a look to udevadm test /dev/ttyACM0. My udevrule is shown at the end. So I am sure that it had not been overridden. But I wonder about the 'unable to open' hint. Nevertheless this hint is present for other ttys, such as tty0 or ttyS0, too.
...    
Reading rules file: /lib/udev/rules.d/98-systemd.rules
Reading rules file: /etc/udev/rules.d/99-bizrfid.rules
rules contain 49152 bytes tokens (4096 * 12 bytes), 12883 bytes strings
4240 strings (40079 bytes), 3515 de-duplicated (27922 bytes), 726 trie nodes used
unable to open device '/sys/dev/ttyACM0'
Unload module index
Unloaded link configuration context.
  1. I reconnected the USB device.
  2. I tried another udev rule. This rule is 100% correct because I use this rule already fo other devices. But it does not work for the ttyACM0:
ACTION=="add", KERNEL=="ttyACM*", MODE="0666"

I wonder why this is not working. How to find reasons why an udev rule is not applied?

udev tty usb linux-kernel linux
2 Answers

According to the comments I the following can be tried to debug the problem:

Check the driver

Use dmesg to check the driver you are using and ensure that it is a generic one. If it is a custom compiled one, the problem could be inside your custom implementation. If it is a generic one, go on to step two. I found the driver name in the last line: cdc_acm

[  369.384850] usb 1-1.1: new full-speed USB device number 4 using xhci_hcd
[  369.470492] usb 1-1.1: New USB device found, idVendor=09d8, idProduct=0420
[  369.470506] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[  369.470515] usb 1-1.1: Product: TWN4/B1.06/CCL3.12/PRS1.04
[  369.470522] usb 1-1.1: Manufacturer: OEM
[  369.475188] cdc_acm 1-1.1:1.0: ttyACM0: USB ACM device

Check the user permissions

Ensure that the permission of your udev rule is correct:

chmod 644 99-myrule.rules
chown root:root 99-myrule.rules

Do a test run for the add-action

Check the output of udevadm test -a add $(udevadm info -q path -n /dev/ttyACM0). My output looks like this:

calling: test

version 232
This program is for debugging only, it does not run any program
specified by a RUN key. It may show incorrect results, because
some values may be different, or not available at a simulation run.

=== trie on-disk ===
tool version:          232
file size:         8447696 bytes
header size             80 bytes
strings            1849040 bytes
nodes              6598576 bytes
Load module index
Found container virtualization none
timestamp of '/etc/systemd/network' changed
timestamp of '/usr/lib/systemd/network' changed
timestamp of '/lib/systemd/network' changed
Parsed configuration file /lib/systemd/network/99-default.link
Created link configuration context.
timestamp of '/etc/udev/rules.d' changed
Skipping overridden file: /lib/udev/rules.d/99-bizrfid.rules.
Reading rules file: /lib/udev/rules.d/50-firmware.rules

...

Reading rules file: /lib/udev/rules.d/98-systemd.rules
Reading rules file: /etc/udev/rules.d/99-bizrfid.rules
rules contain 49152 bytes tokens (4096 * 12 bytes), 12900 bytes strings
4244 strings (40114 bytes), 3517 de-duplicated (27942 bytes), 728 trie nodes used
GROUP 20 /lib/udev/rules.d/50-udev-default.rules:26
IMPORT builtin 'hwdb' /lib/udev/rules.d/60-serial.rules:7
IMPORT builtin 'usb_id' /lib/udev/rules.d/60-serial.rules:8
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-1/1-1.1/1-1.1:1.0: if_class 2 protocol 0
IMPORT builtin 'hwdb' /lib/udev/rules.d/60-serial.rules:8
IMPORT builtin 'path_id' /lib/udev/rules.d/60-serial.rules:15
LINK 'serial/by-path/pci-0000:00:14.0-usb-0:1.1:1.0' /lib/udev/rules.d/60-serial.rules:16
IMPORT builtin skip 'usb_id' /lib/udev/rules.d/60-serial.rules:19
LINK 'serial/by-id/usb-OEM_TWN4_B1.06_CCF3.00_PRS1.04-if00' /lib/udev/rules.d/60-serial.rules:23
LINK 'ttyMyDevice' /etc/udev/rules.d/99-bizrfid.rules:1
handling device node '/dev/ttyACM0', devnum=c166:0, mode=0660, uid=0, gid=20
preserve permissions /dev/ttyACM0, 020660, uid=0, gid=20
preserve already existing symlink '/dev/char/166:0' to '../ttyACM0'
found 'c166:0' claiming '/run/udev/links/\x2fserial\x2fby-id\x2fusb-OEM_TWN4_B1.06_CCF3.00_PRS1.04-if00'
creating link '/dev/serial/by-id/usb-OEM_TWN4_B1.06_CCF3.00_PRS1.04-if00' to '/dev/ttyACM0'
preserve already existing symlink '/dev/serial/by-id/usb-OEM_TWN4_B1.06_CCF3.00_PRS1.04-if00' to '../../ttyACM0'
found 'c166:0' claiming '/run/udev/links/\x2fserial\x2fby-path\x2fpci-0000:00:14.0-usb-0:1.1:1.0'
creating link '/dev/serial/by-path/pci-0000:00:14.0-usb-0:1.1:1.0' to '/dev/ttyACM0'
preserve already existing symlink '/dev/serial/by-path/pci-0000:00:14.0-usb-0:1.1:1.0' to '../../ttyACM0'
creating link '/dev/ttyMyDevice' to '/dev/ttyACM0'
creating symlink '/dev/ttyMyDevice' to 'ttyACM0'
created db file '/run/udev/data/c166:0' for '/devices/pci0000:00/0000:00:14.0/usb1/1-1/1-1.1/1-1.1:1.0/tty/ttyACM0'
ACTION=add
DEVLINKS=/dev/serial/by-path/pci-0000:00:14.0-usb-0:1.1:1.0 /dev/serial/by-id/usb-OEM_TWN4_B1.06_CCF3.00_PRS1.04-if00 /dev/ttyMyDevice
DEVNAME=/dev/ttyACM0
DEVPATH=/devices/pci0000:00/0000:00:14.0/usb1/1-1/1-1.1/1-1.1:1.0/tty/ttyACM0
ID_BUS=usb
ID_MODEL=TWN4_B1.06_CCF3.00_PRS1.04
ID_MODEL_ENC=TWN4\x2fB1.06\x2fCCF3.00\x2fPRS1.04
ID_MODEL_FROM_DATABASE=Atom Processor Z36xxx/Z37xxx, Celeron N2000 Series USB xHCI
ID_MODEL_ID=0420
ID_PATH=pci-0000:00:14.0-usb-0:1.1:1.0
ID_PATH_TAG=pci-0000_00_14_0-usb-0_1_1_1_0
ID_PCI_CLASS_FROM_DATABASE=Serial bus controller
ID_PCI_INTERFACE_FROM_DATABASE=XHCI
ID_PCI_SUBCLASS_FROM_DATABASE=USB controller
ID_REVISION=0200
ID_SERIAL=OEM_TWN4_B1.06_CCF3.00_PRS1.04
ID_TYPE=generic
ID_USB_CLASS_FROM_DATABASE=Communications
ID_USB_DRIVER=cdc_acm
ID_USB_INTERFACES=:020201:0a0000:
ID_USB_INTERFACE_NUM=00
ID_VENDOR=OEM
ID_VENDOR_ENC=OEM
ID_VENDOR_FROM_DATABASE=Intel Corporation
ID_VENDOR_ID=09d8
MAJOR=166
MINOR=0
SUBSYSTEM=tty
TAGS=:systemd:
USEC_INITIALIZED=71480300178
Unload module index
Unloaded link configuration context.
user_66779500

For the udev rule /ect/udev/rules.d/99-bizrfid.rules I would add the SUBSYSTEM:

ACTION=="add", SUBSYSTEM=="tty", ATTRS{idVendor}=="09d8", ATTRS{idProduct}=="0420", SYMLINK+="ttyMyDevice"

You could add also ATTRS{manufacturer}==, but idVendor and idProduct are sufficient.

Otherwise it looks correct and the test run creates the symlink:

creating link '/dev/ttyMyDevice' to '/dev/ttyACM0'

user_60598960
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