Re: [PATCH 4/5] drm/i915: Check live status before reading edid

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Daniel Vetter <daniel@ffwll.ch>,
	Sonika Jindal <sonika.jindal@intel.com>,
	intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 4/5] drm/i915: Check live status before reading edid
Date: Mon, 1 Aug 2016 11:09:28 +0100	[thread overview]
Message-ID: <20160801100928.GA4252@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <20160712113947.GB27260@boom>

On Tue, Jul 12, 2016 at 02:39:47PM +0300, David Weinehall wrote:
> On Thu, Jul 09, 2015 at 07:27:57PM +0200, Daniel Vetter wrote:
> > On Thu, Jul 09, 2015 at 05:34:29PM +0530, Sonika Jindal wrote:
> > > Adding this for SKL onwards.
> > > 
> > > Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
> > 
> > I think this is the critical piece really, and I'd like to roll it out for
> > all platforms. git has the code for all the old ones, so no big deal to
> > digg that out. Also we need your fix for the interrupt handling first ofc,
> > otherwise this won't work.
> > 
> > Then we can apply this and give it some good testing before we start
> > relying on it with caching hdmi probe status. I know this means that
> > things will be slower, but I've been burned a bit too much the last few
> > times we've tried this. And I really think we need the most amount of
> > testing we can get (hence rolling this out for all platforms). If your
> > hpd irq handling bugfix is indeed the bug that will be confirmed quickly,
> > otherwise it means back to debugging.
> 
> "things will be slower" is a bit of an understatement, sadly.
> On machines with no external display connected (the typical usecase for
> any device with an eDP, such as a laptop, tablet, etc.), the approach to
> repeat live status reads until buggy displays can be bothered to reply
> makes us spend twice as long as needed on resume.

Also this is causing stall during runtime hotplug. ~90ms stall causes
3-4 frames to be dropped in 24Hz movie. Even a single frame dropped
during movie playback is enough to be noticed.

This coupled with 

commit f8d03ea0053b23de42c828d559016eabe0b91523
Author: Gary Wang <gary.c.wang@intel.com>
Date:   Wed Dec 23 16:11:35 2015 +0800

    drm/i915: increase the tries for HDMI hotplug live status checking

which purports to "fix" live status for bdw+.

Afaict using live status is as broken as we told you it would be from
earlier experience.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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