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    SubjectRe: [PATCH] i915: Quirk out disconnected backlight
    On Fri, 14 Sep 2012 14:12:19 +0100, David Woodhouse <> wrote:
    > On Fri, 2012-09-14 at 14:09 +0100, Grant Likely wrote:
    > > On Fri, Sep 14, 2012 at 2:01 PM, Chris Wilson <> wrote:
    > > > On Fri, 14 Sep 2012 13:57:06 +0100, Grant Likely <> wrote:
    > > >> Some platforms (for instance MacbookPros) have custom backlight drivers
    > > >> and don't use the integrated i915 backlight control. This patch adds a
    > > >> quirk to disable registering the intel backlight when unused on a
    > > >> platform.
    > > >>
    > > >> Tested on MacbookPro8,3. Without this patch both the intel_backlight and
    > > >> gmux_backlight devices get registered and userspace doesn't know which
    > > >> it should use.
    > > >
    > > > Userspace is informed throught the backlight/type property.
    > >
    > > Perhaps, but userspace (Ubuntu) isn't doing anything with it, and it
    > > still remains that it makes no sense whatsoever to register a
    > > backlight device that doesn't exist.
    > Indeed. Userspace (well, gnome-settings-daemon) will use the backlight
    > provided by X, in preference to anything it finds
    > in /sys/class/backlight. So if the Intel one is present (and thus
    > exposed via X) then userspace will never bother with comparing types and
    > choosing the sanest backlight to use.
    > See

    And if you look at that bug, it starts off by complaining that a
    workaround is required in order to use the intel_backlight. By the time
    you hit the issue with apple_gmux, the upstream ddx already carried the
    fix for a couple of months and even had it in a release. And more
    recently we took a patch to allow the user to override which backlight is
    preferred to handle the case of a broken platform/firmware interface
    being selected instead of intel_backlight.

    Userspace is indeed trying to do the right thing with the information
    provided by the kernel. apple_gmux is not the only device with a phantom
    PWM BLC, which is why the default preference is to use the
    platform/firmware provided backlight interface, if any.

    Chris Wilson, Intel Open Source Technology Centre

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