From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH] drm/i915/vlv: hack to init backlight regs
 if BIOS fails to do so
Date: Wed, 25 Sep 2013 22:53:14 +0200
Message-ID: <20130925205314.GP12663@phenom.ffwll.local>
References: <1380127240-1310-1-git-send-email-jbarnes@virtuousgeek.org>
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To: Jani Nikula <jani.nikula@linux.intel.com>
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On Wed, Sep 25, 2013 at 08:18:39PM +0300, Jani Nikula wrote:
> On Wed, 25 Sep 2013, Jesse Barnes <jbarnes@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