bisected: Dell XPS 17 ignores ACPI intel_backlight sysfs values greater than 1
Summary
In the following regression, values of /sys/class/backlight/intel_backlight/brightness no longer correspond with my screen brightness.
The offending commit (for my system) is 98e497e203a5, "[1/2] drm/i915/dpcd_bl: uncheck PWM_PIN_CAP when detect eDP backlight capabilities". This affects the 5.11 branch in stable, which affected me when I upgraded from 5.10.16.arch1-1 to 5.11.2-arch1-1. As of the current mainline 5.12-rc2 (which I just tested) the problem is not resolved.
How to reproduce:
Change the value of /sys/class/backlight/intel_backlight/brightness to an integer from 0-255 (where 255 is the max_brightness).
- Expected result: At value 0, the screen is off, and at values 1-255, the screen gradually goes from dim to full brightness.
- Actual result: At value 0, the screen is off, and at values 1-255, the screen is the same brightness.
System information:
Dell XPS 17 9700 with an NVIDIA Optimus graphics hardware. The optimus setup uses i915 for the intel modesetting and nvidia for the main nvidia setting.
Workaround
@jani has suggested the workaround to set i915.enable_dpcd_backlight=0 instead of i915.enable_dpcd_backlight=1 in my kernel parameters. I have done this, and indeed it is a valid workaround. With this new kernel parameter, there are a number of errors and warnings from ACPI; see my dmesg_after_enable_dpcd_backlight_eq_0.txt.