[Bug 93643] New: backlight: min value is always 0

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93643

            Bug ID: 93643
           Summary: backlight: min value is always 0
           Product: xorg
           Version: 7.7 (2012.06)
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: aykevanlaethem at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Symptom: backlight turns off completely in the MATE desktop environment when
turning the brightness way down. I would expect the screen to stay lit, even at
the lowest value. I see no use case in turning the screen off completely with
the brightness keys, and it is certainly not good for the user experience when
you actually just want a very dim screen (e.g. in a dark room).

I have this system:

Laptop: Lenovo L450
Graphics (lspci): Intel Corporation Broadwell-U Integrated Graphics (rev 09)
Distribution: Debian stretch (testing)
Kernel: 4.3.0-1-amd64

In /sys/class/backlight, there's just intel_backlight.

/sys/class/backlight/intel_backlight/max_brightness is 937.

Digging in the MATE sources (mate-power-manager), I found it requests minimum
and maximum brightness values using XRRQueryOutputProperty. Unfortunately, this
call returns max=937 (correct) and min=0 (IMHO wrong, should be 1). With the
result that the screen backlight turns off completely when setting the
brightness to the lowest value.

I assume this minimum value is reported by the Intel driver. And indeed, at the
bottom of the function intel_output_create_resources in the file
src/intel_display.c in xf86-video-intel, I found this:

        intel_output_create_ranged_atom(output, &backlight_atom,
                    BACKLIGHT_NAME, 0,
                    intel_output->backlight_max,
                    intel_output->backlight_active_level,
                    FALSE);

I don't really understand what this is doing, but to me it looks like the '0'
here is the minimum value, hardcoded in the source.

This is how far I've gotten digging in the various sources looking for the
issue.

As a reference:
 - https://github.com/mate-desktop/mate-power-manager/issues/179
 - http://marc.info/?l=linux-kernel&m=145225412911583&w=2

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