drm/i915: Fix DPMS and suspend interaction for intel_panel.c

[Indan Zupancic]

On Fri, March 11, 2011 18:27, Jesse Barnes wrote:
> On Fri, 11 Mar 2011 02:35:45 +0100 (CET)
> "Indan Zupancic" <indan at nul.nu> wrote:
>
>> drm/i915: Fix DPMS and suspend interaction for intel_panel.c
>>
>> When suspending intel_panel_disable_backlight() is never called,
>> but intel_panel_enable_backlight() is called at resume. With the
>> effect that if the brightness was ever changed after screen
>> blanking, the wrong brightness gets restored at resume time.
>>
>> Nothing guarantees that those calls will be balanced, so having
>> backlight_enabled makes no sense, as the real state can change
>> without the panel code noticing. So keep things as stateless as
>> possible.
>>
>> Signed-off-by: Indan Zupancic <indan at nul.nu>
>
> Chris is right that we don't always control the backlight brightness
> directly, so we'll want a more complete solution at any rate.

Well, not having control basically means not caching any register
values, or as little as possible. For gen >=4 there's the top bit
of BLC_PWM_CTL2, so the brightness handling can be cleanly separated
from the panel disabling/enabling. For older gens we need to save
the brightness and then set it to zero, as happens now.

For brightness control we only need to set the new brightness on
systems with ASLE.

> I don't think this one is urgent enough to send upstream now, and it
> would be good to make a couple of other fixes as well, while you're
> fixing things up. :)  Comments below.

It's not urgent, it's just a bugfix. I did my best to resist the
urge to do other cleanups as well, so close to the release cycle.