[Intel-gfx] [PATCH] drm/i915/vlv: hack to init backlight regs if BIOS fails to do so

[Daniel Vetter]

On Wed, Sep 25, 2013 at 08:18:39PM +0300, Jani Nikula wrote:
> On Wed, 25 Sep 2013, Jesse Barnes <jbarnes at virtuousgeek.org> wrote:
> > Still digging up the actual VBT info for this, but wanted to get this
> > out there for testing, or in case others are also bugged by this.
> 
> I had a look at this a few weeks back. The VBT value for max backlight
> is in Hz (as is the value you get through opregion) and transforming
> that into the value the registers eat needs some digging. I tried, but
> none of the real world examples of VBT and PWM freq matched any of that,
> so I moved on...
> 
> > This can happen if you boot with an external display connected.  In that
> > case, the attached eDP backlight modulation frequency may not be
> > programmed, so we need to use something (in this case the value my BIOS
> > normally programs with just the internal display enabled).
> 
> Something similar is required for non-vlv ChromeOS stuff too AFAIK.

Afaik ChromeOS doesn't have a vbt, so I think we need to shovel some
failsafe (yeah, failsafe and backlight doesn't compute, I know) default
into the regs in case all else fails.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch