[Intel-gfx] [PATCH] drm/i915 : Avoid superfluous invalidation of CPU cache lines

[Daniel Vetter]

On Tue, Nov 24, 2015 at 12:04:06PM +0200, Ville Syrjälä wrote:
> On Tue, Nov 24, 2015 at 03:35:24PM +0530, akash.goel at intel.com wrote:
> > From: Akash Goel <akash.goel at intel.com>
> > 
> > When the object is moved out of CPU read domain, the cachelines
> > are not invalidated immediately. The invalidation is deferred till
> > next time the object is brought back into CPU read domain.
> > But the invalidation is done unconditionally, i.e. even for the case
> > where the cachelines were flushed previously, when the object moved out
> > of CPU write domain. This is avoidable and would lead to some optimization.
> > Though this is not a hypothetical case, but is unlikely to occur often.
> > The aim is to detect changes to the backing storage whilst the
> > data is potentially in the CPU cache, and only clflush in those case.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Signed-off-by: Akash Goel <akash.goel at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_drv.h | 1 +
> >  drivers/gpu/drm/i915/i915_gem.c | 9 ++++++++-
> >  2 files changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index df9316f..fedb71d 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -2098,6 +2098,7 @@ struct drm_i915_gem_object {
> >  	unsigned long gt_ro:1;
> >  	unsigned int cache_level:3;
> >  	unsigned int cache_dirty:1;
> > +	unsigned int cache_clean:1;
> 
> So now we have cache_dirty and cache_clean which seems redundant,
> except somehow cache_dirty != !cache_clean?

We also have read_domains & DOMAIN_CPU. Which is which?

Documentation for this stuff would be awesome, and probably should be
included in this patch. With kerneldoc in 4.4 we can do multi-paragraph
kerneldoc comments for individual struct members.
-Daniel

> 
> >  
> >  	unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
> >  
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 19c282b..a13ffd4 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -3552,6 +3552,7 @@ i915_gem_clflush_object(struct drm_i915_gem_object *obj,
> >  	trace_i915_gem_object_clflush(obj);
> >  	drm_clflush_sg(obj->pages);
> >  	obj->cache_dirty = false;
> > +	obj->cache_clean = true;
> >  
> >  	return true;
> >  }
> > @@ -3982,7 +3983,13 @@ i915_gem_object_set_to_cpu_domain(struct drm_i915_gem_object *obj, bool write)
> >  
> >  	/* Flush the CPU cache if it's still invalid. */
> >  	if ((obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0) {
> > -		i915_gem_clflush_object(obj, false);
> > +		/* Invalidation not needed as there should not be any data in
> > +		 * CPU cache lines for this object, since clflush would have
> > +		 * happened when the object last moved out of CPU write domain.
> > +		 */
> > +		if (!obj->cache_clean)
> > +			i915_gem_clflush_object(obj, false);
> > +		obj->cache_clean = false;
> >  
> >  		obj->base.read_domains |= I915_GEM_DOMAIN_CPU;
> >  	}
> > -- 
> > 1.9.2
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Ville Syrjälä
> Intel OTC
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch