GUI Permissions

[p1r473]

Hi Linus!
I have ran the following command:

echo 'SUBSYSTEM=="backlight",RUN+="/bin/chmod 666 /sys/class/backlight/%k/brightness /sys/class/backlight/%k/bl_power /sys/devices/platform/ff150000.i2c/i2c-3/3-0045/tinker_mcu_bl
"' | sudo tee -a /etc/udev/rules.d/backlight-permissions.rules

Note, I ran for both the TinkerBoard and the Pi files, as all files exist on my OS.
This does not fix the permissions for me.

Sidenote, I actually had to replace the %k with rpi_backlight:

echo 'SUBSYSTEM=="backlight",RUN+="/bin/chmod 666 /sys/class/backlight/rpi_backlight/brightness /sys/class/backlight/rpi_backlight/bl_power /sys/devices/platform/ff150000.i2c/i2c-3/3-0045/tinker_mcu_bl
> "' | sudo tee -a /etc/udev/rules.d/backlight-permissions.rules

as I got:

chmod: cannot access '/sys/class/backlight/%k/brightness': No such file or directory
chmod: cannot access '/sys/class/backlight/%k/bl_power': No such file or directory

Just wondering if you have any idea why both the udev rule wouldn't work, nor the %k in that rule?

[p1r473]

I've created a workaround:

cat /etc/systemd/system/setbacklightpermissions.service

[Unit]
Description=Set backlight writable by everybody

[Service]
Type=simple
User=root
ExecStart=/bin/chmod 666 /sys/class/backlight/rpi_backlight/brightness /sys/class/backlight/rpi_backlight/bl_power /sys/devices/platform/ff150000.i2c/i2c-3/3-0045/tinker_mcu_bl

[Install]
WantedBy=multi-user.target

then ran:

$ sudo systemctl enable setbacklightpermissions.service
$ sudo systemctl start setbacklightpermissions.service
$ sudo systemctl status setbacklightpermissions.service

Seems to work. However, would still love your input.

[linusg]

Hi @p1r473,

I'm afraid I have no idea what's going on there. %k should work as described in the udev manpage:

https://linux.die.net/man/8/udev
...
%k - The "kernel name" for the device.

Might be something specific to the Tinker Board OS? Never used it, can't investigate.

Using Systemd for this is a bad workaround IMO 😄
See if there are other rules on your system which do similar stuff, maybe you can spot differences.

This does not fix the permissions for me.

Did you try, without having the Systemd unit enabled, what happens after a reboot? Or sudo udevadm trigger?

[p1r473]

For the %k, my kernel name is rpi_backlight?
Because, it looks like %k is supposed to be rpi_backlight in order for your %k to fill.

Anyways,

 systemctl disable setbacklightpermissions

echo 'SUBSYSTEM=="backlight",RUN+="/bin/chmod 666 /sys/class/backlight/rpi_backlight/brightness /sys/class/backlight/rpi_backlight/bl_power /sys/devices/platform/ff150000.i2c/i2c-3/3-0045/tinker_mcu_bl
> "' | sudo tee -a /etc/udev/rules.d/backlight-permissions.rules
reboot

No luck with GUI permissions
then:

sudo udevadm trigger

Still no luck. Is it possible that my subsystem is not called backlight, so this does not ever run?

Tried to find my subsystem name:

root@TinkerBoardS:~ # udevadm info -q all -a /dev/sdb
Unknown device, absolute path in /dev/ or /sys expected.
root@TinkerBoardS:~ # udevadm info -n /dev/sdb -q property | sed -n 's/SUBSYSTEM=//p'
device node not found
root@TinkerBoardS:~ # udevadm info -n /dev/sdb -q property | sed -n 's/SUBSYSTEM=//p'
device node not found
root@TinkerBoardS:~ # udevadm info -a -n /dev/sdb
device node not found

Also tried, but didn't fix permissions:

mv /etc/udev/rules.d/backlight-permissions.rules /lib/udev/rules.d/backlight-permissions.rules

Also tried chaning SUBSYSTEM to 'graphics' and renaming the rule to 999-backlight-permissions.rules to fit the other rules

[p1r473]

Not sure if it was a problem with the echo or the tee, but just manually creating the file seems to work:
/etc/udev/rules.d/backlight-permissions.rules:

SUBSYSTEM=="backlight",RUN+="/bin/chmod 666 /sys/class/backlight/%k/brightness /sys/class/backlight/%k/bl_power /sys/devices/platform/ff150000.i2c/i2c-3/3-0045/tinker_mcu_bl"

Thanks!

[linusg]

Weird! Glad you got it working though, thanks for reporting back!