Re: [PATCH 1/4] drm/i915: save/restore the legacy backlight control

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 28, 2012 at 08:16:54AM +0100, Chris Wilson wrote:
> On Tue, 28 Aug 2012 09:53:36 +0300, Jani Nikula <jani.nikula@intel.com> wrote:
> > From: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > This is a prep patch to stop drm/i915 from changing the LBPC registers
> > itself - but we still need to properly save/restore it on
> > suspend/resume.
> 
> My presumption was that there were BIOSes out there that only adjusted
> the LBPC values, and so any device setting that flag would likely only
> with it through the BIOS hotkeys. I couldn't see any other rational
> reason for the hardware maintaining multiple interfaces...

I've run a hackish version of this through a few bug reports and they
reported that there's no more black screen at boot-up, but also that the
backlight doesn't work. My conclusion was:
- there are machines that are only controlled through the lbpc reg
- some of those use lbpc in an inverted sense
- we have now idea how to figure out the sense of lbpc

Hence my thinking is that we should avoid to touch lbpc (but just restore
it across suspend/resume since some machines need that) and rely on any
firmware madness to properly adjust the backlight. I also realize that
this will regress machines without working firmware, but for which the
current lbpc-adjusting mess seems to do the right thing.

But if the only regression this causes is some non-working backlight
adjusting I'll ignore that, since it looks like we fundamentally can't do
the right thing with lbpc, and this series here should kill a few black
screen bugs.

Cheers, Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48