Ack: [SRU][pqrs] drm/i915: i915.disable_pch_pwm overrides PCH_PWM_ENABLE quirk

[Seth Forshee]

On Tue, Nov 19, 2013 at 11:58:30AM -0800, Kamal Mostafa wrote:
> BugLink: https://bugs.launchpad.net/bugs/1163720
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=47941
> 
> [Impact]
> 
> It has been determined that on the Dell XPS13, enabling some UEFI options
> causes backlight problems since the introduction of
>   e85843b ("drm/i915: quirk no PCH_PWM_ENABLE for Dell XPS13 backlight")
> which exists upstream and in ubuntu-[pqrst].
> 
> Depending on the model of XPS13 and the UEFI configuration, some systems
> experience black-screen on boot with no way to increase brightness; others
> just lose brightness controls.  The exact matrix of configurations and
> behaviors is not well understood.
> 
> This patch provides a boot parameter to allow XPS13 UEFI users to override
> the change introduced by e85843b, restoring the (reported good) behavior
> they had before while still retaining the beneficial fix for the standard
> BIOS XPS13 users.
> 
> This patch was submitted upstream, but they've chosen to look into a
> long-term solution to this and other problems involving backlight vs. UEFI
> instead.  That solution (when it lands) may not be back-portable to our
> earier releases.
> 
> 
> [Test Case]
> 
> Boot with i915.disable_pch_pwm=0 on an affected XPS13 with UEFI.  Observe
> that backlight is no longer stuck at minimum on boot.  Brightness controls
> may also be restored by this, depending on UEFI settings.
> 
> The patch has been tested by several users.
> 
> 
> [Regression Potential]
> 
> Very low.  This patch does not change any behavior by default (only does
> when the boot param is used), and only targets Dell XPS13 models.
> 
> 
> [Other info]
> 
> For the time being, I'd like to apply this fix to ubuntu-[pqrs], but wait
> for the upstream fix to land in ubuntu-trusty.

Seems like a reasonable interim solution with low regression potential.

Acked-by: Seth Forshee <seth.forshee at canonical.com>