[Bug 198885] New: amdgpu.dc=1 on CIK (R4 Mullins APU) brightness impossible to change. by Fn or "echo".

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=198885

            Bug ID: 198885
           Summary: amdgpu.dc=1 on CIK (R4 Mullins APU) brightness
                    impossible to change. by Fn or "echo".
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.15.4
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: soprwa@gmail.com
        Regression: No

Created attachment 274359
  --> https://bugzilla.kernel.org/attachment.cgi?id=274359&action=edit
dmesg with "amdgpu.dc=1"

I am using gentoo ~amd64, kernel 4.15.4, amdgpu with amdgpu.dc=1 on CIK A6 APU
- R4 Mullins.

There is no possibility to change brightness level both with Fn keys or
directly by an "echo" command, even as root when "amdgpu.dc=1" is set:

ls -l /sys/class/backlight/amdgpu_bl0/
razem 0
-r--r--r-- 1 root root 4096 02-22 13:41 actual_brightness
-rw-r--r-- 1 root root 4096 02-22 13:41 bl_power
-rw-r--r-- 1 root root 4096 02-22 13:41 brightness
lrwxrwxrwx 1 root root    0 02-22 13:41 device -> ../../../0000:00:01.0
-r--r--r-- 1 root root 4096 02-22 13:40 max_brightness
drwxr-xr-x 2 root root    0 02-22 13:41 power
lrwxrwxrwx 1 root root    0 02-22 13:40 subsystem ->
../../../../../class/backlight
-r--r--r-- 1 root root 4096 02-22 13:40 type
-rw-r--r-- 1 root root 4096 02-22 13:39 uevent


"/sys/class/backlight/amdgpu_bl0 # echo 22 > actual_brightness
bash: actual_brightness: permission denied"

and 

"/sys/class/backlight/amdgpu_bl0 # echo 128 > brightness
bash: echo: błąd zapisu: Bad argument"

Without "amdgpu.dc=1" in kernel command line changing brightness level is
working as expected, both Fn keys, and echoing directly to a file.

I know that there are plans enabling amdgpu.dc by default, so impossibility to
change brightness level IMHO is a big "no go".

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