Accessing the HDMI audio /proc interface causes the screen to turn off temporarily (Latitude 7520)
- System architecture: x86_64
- Kernel version: drm-tip at
3ae2e00290c290713e21118220a817a24b44d39f - Linux distribution: Debian unstable
- Machine: Dell Latitude 7520
Running cat /proc/asound/card0/codec#2 causes the screen to turn off and immediately back on, as if I plugged in an external monitor.
The screen turns off and on twice for every file read, with roughly 1 second in between: off -> on -> ~1 second -> off -> on.
If an external monitor is plugged in, the screen turns off and on only once immediately after reading the file.
These two facts make me think this is related to the driver's power management.
Reading /proc/asound/card0/codec#2 is one way to cause the issue, but probably not the only one. For example, playing with HDA analyzer I found that simply accessing the details of the AUDIO_OUT pins (0x3, 0x5, 0x7, 0x9) can cause the same behavior. If I disable those PINs, accessing them does not trigger the bug, but reading the /proc file will.
This bug was originally reported here: https://bugzilla.kernel.org/show_bug.cgi?id=215885
Update:
cat /sys/module/snd_hda_intel/parameters/power_save returns 1, which explains the delay.
Setting power_save to -1 on boot prevents the screen from turning off.