https://bugs.freedesktop.org/show_bug.cgi?id=90151
--- Comment #5 from Alex Deucher alexdeucher@gmail.com --- (In reply to Kevin Sarendranath from comment #0)
The system being used is a Lenovo Thinkpad configured with an R7 M260DX discrete card attached to an A10-7300. Upon booting it, the kernel spits out this message:
[drm:radeon_acpi_init [radeon]] *ERROR* Cannot find a backlight controller
This message is from the dGPU which is not connected to any displays so it can be ignored. The backlight control is properly initialized on the APU which is the chip that controls the backlight.
[ 2.419773] [drm] radeon atom DIG backlight initialized
with the quirk of having the brightness hotkeys not being able to control the brightness. Looking into the radeon source, here is where the call came from (radeon_acpi.c):
What the brightness keys do is up to the OEM. On most modern laptops, they tend to just generate key press events rather than actually changing the brightness or generating acpi events. It's up to the desktop environment to map that event to the brightness controls exposed by the kernel.