linux-kernel - Re: [Intel-gfx] [PATCH 2/2] drm/i915: Get rid of intel_dp_dpcd_read

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Date:	Mon, 21 Mar 2016 15:38:31 +0200
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Jani Nikula <jani.nikula@...ux.intel.com>
Cc:	Lyude <cpaul@...hat.com>, Daniel Vetter <daniel.vetter@...el.com>,
	intel-gfx@...ts.freedesktop.org, arthur.j.runyan@...el.com,
	open list <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 2/2] drm/i915: Get rid of
 intel_dp_dpcd_read_wake()

On Mon, Mar 21, 2016 at 12:30:54PM +0200, Jani Nikula wrote:
> On Fri, 18 Mar 2016, Ville Syrjälä <ville.syrjala@...ux.intel.com> wrote:
> > On Fri, Mar 18, 2016 at 06:12:35PM +0200, Ville Syrjälä wrote:
> >> On Fri, Mar 18, 2016 at 04:13:45PM +0200, Ville Syrjälä wrote:
> >> > On Thu, Mar 17, 2016 at 11:40:45AM -0400, Lyude wrote:
> >> > > -	drm_dp_dpcd_read(aux, DP_DPCD_REV, buffer, 1);
> >> > 
> >> > NAK
> >> > 
> >> > If people keep intentionally breaking my shit I'm going to become
> >> > really grumpy soon.
> >> 
> >> Oh, and just in case someone wants to come up with a better kludge,
> >> I just spent a few minutes analyzing the behavior of this crappy
> >> monitor a.
> >> 
> >> What happens is that when the monitor is fully powered up (LED is blue)
> >> things are fine. After the monitor goes to sleep (LED turns orange)
> >> the first DPCD read will produce garbage. Further DPCD reads are fine,
> >> even if I wait a significant amount of time between the reads, as long
> >> as the monitor didn't do a power on->off cycle in between. So it looks
> >> like it's always just the first read after power down that gets
> >> corrupted.
> >> 
> >> Now I think I'll go and test how writes behave, assuming I can find a
> >> decently sized chunk of DPCD address space I can write. And maybe I
> >> should also try i2c-over-aux...
> >
> > The first DPCD write after powerdown also got corrupted. But i2c-over-aux
> > seems unaffected for whatever reason.
> 
> Did the display go to sleep on its own, or did we do something? In
> particular, does DPCD DP_SET_POWER register play a role? What if we skip
> writing D3 to it? What if we do that write as the first thing (every
> time)?

User pressing any of the buttons on the monitor is enough to wake it,
and after a short timeout it will power down on its own, leading to
the corrupted access.

Keeping DP_SET_POWER at D0 doesn't change anything.

-- 
Ville Syrjälä
Intel OTC