Re: [PATCH v2] drm/i915: Move to CPU domain in pwrite/pread

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Nov 12, 2014 at 03:01:30PM +0000, Chris Wilson wrote:
> On Wed, Nov 12, 2014 at 04:57:10PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Currently it's possible to get visible cache dirt on scanout on LLC
> > machines when using pwrite on the future scanout bo if its cache_level
> > is already NONE.
> > 
> > pwrite's "does this need clflush?" checks would decide that no clflush
> > is necessary since the bo isn't currently pinned to the display and LLC
> > makes everything else coherent. The subsequent set_cache_level(NONE)
> > would also do nothing since cache_level is already correct. And hence
> > no clflush will be performed and we flip to a bo which can still have
> > dirty data in the caches.
> > 
> > To correctly track the cache dirtyness move the object to CPU write
> > domain in pwrite. This cures the cache dirt since we explicitly flush
> > the CPU write domain in the pin_to_display path.
> > 
> > Give pread the same treatment simply in the name of symmetry.
> > 
> > v2: Use trace_i915_gem_object_change_domain() and provide some kind
> >     of commit message
> > 
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> > v1 was floated on irc.
> 
> Doesn't it still suffer the same problem that only accessed cache lines
> are clflushed, but we declare the entire object as valid?

Ah in case there's already dirty stuff in the cache before the pwrite?
Hmm, yeah that would be a problem.

How about about clearing write_domain in the end only if write_domain
was != CPU in the start?

-- 
Ville Syrjälä
Intel OTC
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