Package: ddcci-dkms Version: 0.3.2-1 Severity: normal 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 -- System Information: Debian Release: 10.5 APT prefers stable-updates APT policy: (500, 'stable-updates'), (500, 'stable') Architecture: amd64 (x86_64) Kernel: Linux 4.19.0-10-amd64 (SMP w/8 CPU cores) Kernel taint flags: TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8), LANGUAGE=de_DE.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /usr/bin/dash Init: sysvinit (via /sbin/init) Versions of packages ddcci-dkms depends on: ii dkms 2.6.1-4 ddcci-dkms recommends no packages. ddcci-dkms suggests no packages. -- no debconf information