[Bug 105051] New: Radeon sets max_brightness to -1, breaking GNOME backlight control on Macbook Pro mid-2015 11,5

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=105051

            Bug ID: 105051
           Summary: Radeon sets max_brightness to -1, breaking GNOME
                    backlight control on Macbook Pro mid-2015 11,5
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.2.1
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: tim.sammut@gmail.com
        Regression: No

Created attachment 188601
  --> https://bugzilla.kernel.org/attachment.cgi?id=188601&action=edit
dmesg output

The radeon driver appears to be setting an incorrect value of -1 for
max_brightness in 
/sys/devices/pnp0/00:03/backlight/gmux_backlight/max_brightness. This computer
is a Macbook Pro Retina mid-2015 model 11,5 without any video or acpi related
kernel command line options.

This prevents GNOME from correctly managing the backlight. You can see this
using the following commands:

$ cat /sys/devices/pnp0/00:03/backlight/gmux_backlight/max_brightness
-1

$ /usr/libexec/gsd-backlight-helper --get-max-brightness
Could not get the maximum value of the backlight: got invalid backlight value
from /sys/devices/pnp0/00:03/backlight/gmux_backlight/max_brightness

$ sudo /usr/libexec/gsd-backlight-helper --set-brightness 500
Could not get the maximum value of the backlight: got invalid backlight value
from /sys/devices/pnp0/00:03/backlight/gmux_backlight/max_brightness


This looks to be the same issue reported into the RedHat bugzilla at:

https://bugzilla.redhat.com/show_bug.cgi?id=1263387

I've attached the dmesg from my machine.

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