[Intel-gfx] [PATCH] drm/i915: Remove Braswell GGTT update w/a

[Chris Wilson]

On Wed, Feb 22, 2017 at 11:39:30AM +0000, Chris Wilson wrote:
> On Wed, Feb 22, 2017 at 01:27:41PM +0200, Joonas Lahtinen wrote:
> > On ma, 2017-02-20 at 12:47 +0000, Chris Wilson wrote:
> > > Testing with concurrent GGTT accesses no longer show the coherency
> > > problems from yonder, commit 5bab6f60cb4d ("drm/i915: Serialise updates
> > > to GGTT with access through GGTT on Braswell"). My presumption is that
> > > the root cause was more likely fixed by commit 3b5724d702ef ("drm/i915:
> > > Wait for writes through the GTT to land before reading back"), along
> > > with the use of WC updates to the global gTT in commit 8448661d65f6
> > > ("drm/i915: Convert clflushed pagetables over to WC maps". Given
> > > that the original symptoms can no longer be reproduced, time to remove
> > > the workaround.
> > > 
> > > Testcase: igt/gem_concurrenct_blit
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > 
> > Makes one think if the original fix has been appropriate, when adding
> > stop_machine for a software bug :P
> 
> Depends if you consider the months of hair pulling trying to find where
> the flush/stall was missing. It was a desperate patch to fix an annoying
> corruption issue - and since it seems that we now just avoid the
> dangerous path by taking a different route through hw, I don't think
> it is was wholly a sw bug.

Bah, after a few days of continuous testing, I've hit a workload that
shows the bug again. (Small numbers of large GTT objects, rather than
large number of small GTT objects.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre