[Bug 108703] New: Screen sometimes jumps to full brightness on resume from suspend

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108703

            Bug ID: 108703
           Summary: Screen sometimes jumps to full brightness on resume
                    from suspend
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: luke.schlather at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

At night, I usually turn my brightness down as far as it will go using the
brightness keys. This makes it dim, but visible and not off. When I walk away
for a while, the screen goes to sleep, and when I come back and move the mouse,
the screen jumps to full brightness, which is very unpleasant at night.
Investigating, it seems that the brightness in intel_backlight is getting reset
to 4437, but the one in acpi_video0 reads 0.

This can be triggered by running xset dpms force standby. Here's the
conflicting brightness values:

    $ for file in brightness max_brightness actual_brightness;
    > do
    > for directory in /sys/class/backlight/intel_backlight/
/sys/class/backlight/acpi_video0/
    > do
    > echo $directory$file : $(cat $directory$file)
    > done
    > done
    /sys/class/backlight/intel_backlight/brightness : 4437
    /sys/class/backlight/acpi_video0/brightness : 0
    /sys/class/backlight/intel_backlight/max_brightness : 4437
    /sys/class/backlight/acpi_video0/max_brightness : 15
    /sys/class/backlight/intel_backlight/actual_brightness : 4437
    /sys/class/backlight/acpi_video0/actual_brightness : 0

After I press the button to increase the brightness one notch, they are back in
sync, and then I can turn it back down as well.

    $ for file in brightness max_brightness actual_brightness;
    > do
    > for directory in /sys/class/backlight/intel_backlight/
/sys/class/backlight/acpi_video0/
    > do
    > echo $directory$file : $(cat $directory$file)
    > done
    > done
    /sys/class/backlight/intel_backlight/brightness : 53
    /sys/class/backlight/acpi_video0/brightness : 1
    /sys/class/backlight/intel_backlight/max_brightness : 4437
    /sys/class/backlight/acpi_video0/max_brightness : 15
    /sys/class/backlight/intel_backlight/actual_brightness : 53
    /sys/class/backlight/acpi_video0/actual_brightness : 1

This is on a fresh install of Xubuntu 18.04 with default packages including the
latest version of the i915 Intel Driver which is active. Seems like a
regression from Xubuntu 16.04, which I ran on the same laptop without this
issue.

I initially filed this as a bug in XFCE power manager but after some
troubleshooting there I can reproduce the same issue under Gnome as well.

Original bug with troubleshooting:
https://bugs.launchpad.net/ubuntu/+source/xfce4-power-manager/+bug/1782177

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