[Bug 72959] New: [Backlight] Very slow response changing backlight on intel video

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=72959

          Priority: medium
            Bug ID: 72959
          Assignee: chris at chris-wilson.co.uk
           Summary: [Backlight] Very slow response changing backlight on
                    intel video
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: cheetah-freedesktop at fastcat.org
          Hardware: x86-64 (AMD64)
            Status: NEW
           Version: unspecified
         Component: Driver/intel
           Product: xorg

References to other reports of this issue:

https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/847001
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/828296
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=732753

When changing the backlight in the default setup on many Dell and at least some
HP laptops, while the backlight does change, the X environment becomes
unresponsive for several seconds per backlight change (thus changing from
minimum to maximum brighness can cause X to be unresponsive for 30+ seconds). 
This seems to be caused by the server preferring the acpi backlight over the
intel backlight.

The second launchpad link has a patch and a workaround that both purport to fix
this.  The patch (which is 2 years old, but should still apply after
adjustments for files being moved in the source tree) addresses the issue by
making the intel_backlight sysfs interface be preferred over the acpi
interfaces.

The workaround given (which I can confirm works) is to add
acpi_backlight=vendor to the kernel boot, which effectively disables the acpi
backlight entries and causes the driver to pick (for me) the dell_backlight
entry.  From the linked reports, for others the dell backlight entry doesn't
exist, so the intel backlight is picked.

With the server having chosen one of the non-acpi backlight entries, brightness
changes work and are nearly instantaneous.

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