i915 dpcd backlight brightness control stopped working with kernel v5.12
So this is a clevo based laptop P65_67HSHP
XC1507v2 ~ # lspci | grep -i vga
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 630 (rev 04)
01:00.0 VGA compatible controller: NVIDIA Corporation GP104BM [GeForce GTX 1070 Mobile] (rev a1)
where the NVIDIA is the dGPU.
To have backlight brightness control working I have to set:
XC1507v2 ~ # cat /proc/cmdline
root=UUID=c90317de-bed0-4a71-beaf-b5acfbc3d98f crypt_root=UUID=73cc64d2-e973-4e6e-bbf8-31c1d00f6ad8 resume=UUID=1c793108-f3db-4a52-99de-cf24b894dd0b dolvm luks rw initrd=\intel-uc.img initrd=\initramfs-5.12.7.img keymap=de i915.enable_dpcd_backlight=1
where the dpcd thing is the important part.
When I updated from kernel v5.11 to a kernel v 5.12 I discovered the backlight brightness control not working anymore. I then did a bisect, but unfortunately I had to skip a few commits because the kernel won't build since make throws errors.
As far as I can see and understand the commits I read there seems to be several changes in the way the i915 driver addresses the backlight control mechanism.
I also tried enable_dpcd_backlight with the option 2 and 3 as it was mentioned in one of the commits.
I'm kind of lost at this point and I'm looking for help to solve this. Thank you.
Originally I reported this bug over there: https://bugzilla.kernel.org/show_bug.cgi?id=213259 where EDEADLINK blames fe7d52bccab674a22776a2f31236bf4232e85410 - drm/i915/dp: Don't use DPCD backlights that need PWM enable/disable.