Re: [PATCH v2] agp/intel: Serialise after GTT updates

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Jan 27, 2015 at 09:44:01PM +0000, Chris Wilson wrote:
> On Tue, Jan 27, 2015 at 03:58:05PM +0100, Daniel Vetter wrote:
> > On Mon, Jan 26, 2015 at 10:47:10AM +0000, Chris Wilson wrote:
> > > An interesting bug occurs on Pineview through which the root cause is
> > > that the writes of the PTE values into the GTT is not serialised with
> > > subsequent memory access through the GTT (when using WC updates of the
> > > PTE values). This is despite there being a posting read after the GTT
> > > update. However, by changing the address of the posting read, the memory
> > > access is indeed serialised correctly.
> > > 
> > > Whilst we are manipulating the memory barriers, we can remove the
> > > compiler :memory restraint on the intermediate PTE writes knowing that
> > > we explicitly perform a posting read afterwards.
> > > 
> > > v2: Replace posting reads with explicit write memory barriers - in
> > > particular this is advantages in case of single page objects. Update
> > > comments to mention this issue is only with WC writes.
> > > 
> > > Testcase: igt/gem_exec_big #pnv
> > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88191
> > > Tested-by: huax.lu@intel.com (v1)
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > Shouldn't we Cc: stable@vger.kernel.org too?
> 
> Yes, if we can narrow down a user bug it fixes, definitely.

Right makes sense, so meanwhile queued for next only.
-Daneil
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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