Dear Maintainer,

I was trying to control my monitor's backlight with the DDC/CI interface
via the ddcci kernel module provided in the ddcci-dkms package. The
monitor, a Dell P2219H, does support DDC/CI according to the
manufacturer's website[1]. It is plugged into my Thinkpad T440p's
docking station via DisplayPort.

Loading the "ddcci" kernel module with the "dyndbg" option fails with
this error in dmesg:

    [   23.172820] ddcci: initializing ddcci driver
    [   23.492855] ddcci: detecting 7:6e
    [   23.561323] ddcci: detection failed: invalid identification response (2b != 6e)
    [   23.561325] ddcci: received message was 2b c6 47 a5 43 37 ce 15 11 0f 5e ae 89 38 25 c9 55 41 1a 62 ff 9c 42 f6 20 d4 c7 27 3d 6c df f7
    [   23.608904] ddcci: detecting 10:6e
    [   23.608909] ddcci: ddcci driver initialized
    [   23.616410] ddcci: registering driver [ddcci-backlight]
    [   23.616430] ddcci: driver [ddcci-backlight] registered

After loading the ddcci module has completed, /sys/bus/ddcci/devices
does not contain any nodes:

    $ ls -a /sys/bus/ddcci/devices
    .  ..

Loading ddcci-backlight succeeds, but /sys/class/backlight/ does not
contain any new nodes:

    $ ls -al /sys/class/backlight/
    insgesamt 0
    drwxr-xr-x  2 root root 0 Sep  1 19:26 .
    drwxr-xr-x 61 root root 0 Sep  1 18:38 ..
    lrwxrwxrwx  1 root root 0 Sep  1 19:26 intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight

The "intel_backlight" node refers to my laptop's internal monitor's
backlight and is there regardless of whether the ddcci modules are
loaded or not.

To the exact questions:

* What led up to the situation?

I wanted to use the sysfs interface provided by the ddcci-backlight
module to control my monitor's backlight, specifically at the evening
hours.

* What exactly did you do (or not do) that was effective (or
  ineffective)?

I installed the ddcci-dkms package with apt-get and used modprobe(8) to
first load the ddcci module, and then the ddcci-backlight module. Since
that did not cause any change in the sysfs and left no messages in the
dmesg, I added those two modules to /etc/modules and created the file
/etc/modprobe.d/ddcci.conf with the following content:

    options ddcci dyndbg
    options ddcci-backlight dyndbg

Then I rebooted. It resulted in the dmesg output given above.

* What was the outcome of this action?

The dmesg contained the lines given above. No new nodes in sysfs
appeared.

* What outcome did you expect instead?

Instead of the error in the dmesg, nodes to control my monitor's
backlight should have appeared in sysfs.

  -quintus

[1]: https://www.dell.com/de-de/shop/dell-22-monitor-p2219h/apd/210-apwr/monitore-und-monitorzubeh%C3%B6r

By chance I have come today over these two pieces of information:

* https://www.ddcutil.com/faq/
* which links to: https://lists.freedesktop.org/archives/intel-gfx/2017-May/127426.html

I don't use ddcutil, but I do have the i915 kernel module, which appears
to be at fault.

  -quintus

By chance I have come today over these two pieces of information:

* https://www.ddcutil.com/faq/
* which links to: https://lists.freedesktop.org/archives/intel-gfx/2017-May/127426.html

I don't use ddcutil, but I do have the i915 kernel module, which appears
to be at fault.

  -quintus

Hi quintus,

I think there might be a regression in the kernel too; the module used to
work for me (via DP, but not a dock), but it doesn’t with the current kernel
— the i2c layer always gives me an I/O error.

I’ll investigate but it might take me a little while...

Regards,

Stephen

The driver works fine for me now on the current 5.10 kernel in Debian 11; do
you still have this issue?

Regards,

Stephen

Hi,

Am Sonntag, dem 05. Juni 2022 schrieb Stephen Kitt:

I do not use ddcci anymore, so I can't relate. Sorry.

  -quintus

No worries, I’ll close the bug then.

Regards,

Stephen