Re: [Intel-gfx] [PATCH] drm/i915: Unconditionally flush writes before execbuffer

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org,
	Akash Goel <akash.goel@intel.com>,
	stable@vger.kernel.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915: Unconditionally flush writes before execbuffer
Date: Mon, 11 May 2015 16:25:52 +0100	[thread overview]
Message-ID: <20150511152552.GD19022@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <20150511103437.GA15256@phenom.ffwll.local>

On Mon, May 11, 2015 at 12:34:37PM +0200, Daniel Vetter wrote:
> On Mon, May 11, 2015 at 08:51:36AM +0100, Chris Wilson wrote:
> > With the advent of mmap(wc), we have a path to write directly into
> > active GPU buffers. When combined with async updates (i.e. avoiding the
> > explicit domain management along with the memory barriers and GPU
> > stalls) we start to see the GPU read the wrong values from memory - i.e.
> > we have insufficient memory barriers along the execbuffer path. Writes
> > through the GTT should have been naturally serialised with execution
> > through the GTT as well and so the impact only seems to be from the WC
> > paths.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Akash Goel <akash.goel@intel.com>
> > Cc: stable@vger.kernel.org
> 
> Do we have a nasty igt for this? Bugzilla?

I've added igt/gem_streaming_writes.

That wmb() is not enough for !llc. Since the wmb() made piglit happy it
is quite possible I haven't hit the same path exactly, but it's going to
take some investigation to see if igt/gem_streaming_writes can possibly
work on !llc.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre