Date:	Thu, 24 May 2012 16:41:09 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Sean Paul <seanpaul@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	intel-gfx@...ts.freedesktop.org, jbarnes@...tuousgeek.org
Subject: Re: [Intel-gfx] Regression on GMA965 - display seems to have slow
 jump changes in brightness

On Thu, May 24, 2012 at 04:21:48PM +0200, Zdenek Kabelac wrote:
> 2012/5/23 Daniel Vetter <daniel@...ll.ch>:
> > On Wed, May 23, 2012 at 01:48:41PM +0200, Zdenek Kabelac wrote:
> >> 2012/5/23 Daniel Vetter <daniel@...ll.ch>:
> >> > On Wed, May 23, 2012 at 09:07:28AM +0200, Daniel Vetter wrote:
> >> >> On Wed, May 23, 2012 at 08:59:07AM +0200, Zdenek Kabelac wrote:
> >> >> > Hmm I've been using i915.lvds_downclock=1 on grub command line, and
> >> >> > haven't noticed any visible problems with 3.3 kernel. So I'd rather
> >> >> > ask if the problematic patch isn't doing downclocking in a wrong way?
> >> >> > Or maybe detection that downclocking is not supported properly is not
> >> >> > correct now ?
> >> >>
> >> >> Nope, Sean's analysis is pretty much correct, that patch only makes
> >> >> downclocking possible in more circumstances. And downclocking can
> >> >> certainly explain what you're seeing, the backlight pwm signal is driven
> >> >> off the panel dotclock, so if we change that we can very likely cause some
> >> >> funny interference. I guess we could frob the backligth control settings
> >> >> and adjust them for the change in clockspeed, but the current backlight
> >> >> code is a bit a mess. So right now I suggest you just drop that option -
> >> >> there are reasons it's not the default ;-)
> >> >
> >> > Quick question: What's the frequency of the brightness change? And how
> >> > regular are the changes?
> >> > -Daniel
> >>
> >>
> >> Now when it's obvious it's related to powersaving  - it's probably
> >> much easier to explain,
> >> that I've been observing those changes when some activity was happing -
> >> i.e. opening  picture - and after like 1 second image has flashed, -
> >> then I've moved the mouse
> >> stopped - and again image has flashed with brightness a bit.
> >>
> >> The issue would be probably far less noticeable if the period of time
> >> of idle GPU would have to be
> >> much longer (i.e. in minute range)
> >
> > Hm, that sounds more like something ugly is happening when we switch
> > frequencies as opposed to the different frequency causing interference
> > with the backlight. Just to check: You only see a quick flash, then
> > brightness is back to normal?
> 
> In fact I'm not really noticing the moment it goes back to normal -
> I'll notice when it gets darker.
> i.e. when I open  picture with  'qiv'  - after a second I notice that
> picture becomes a bit darker.
> If I do not move with anything - it stays this way.
> 
> >
> > If that's the case, please try Chris' fastboot branch. vsyncing the
> > frequency change might indeed fix this.
> >
> 
> I've tried to compile, but compilation finished with some errors - so
> I've tried to rebase branch on master 3.4 - tried to resolve some
> error - but probably in a wrong thus resulted kernel given me just
> black screen when X has been initialized.  So I'll try to remove some
> config options and I'll try to rebuild original fastboot branch later.

Hm, maybe attach your .config and yell at Chris a bit to fix up his
branch.
-Daniel
-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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