Re: [PATCH 2/2] drm/i915: Remove open-coded callers of flush_cpu_write_domain

Daniel Vetter
2015-01-21T14:55:23Z

On Wed, Jan 21, 2015 at 04:12:07PM +0200, Ville Syrjälä wrote:
> On Wed, Jan 21, 2015 at 02:53:49PM +0100, Daniel Vetter wrote:
> > Both places check the cpu domain and through some indirection
> > obj->pin_display and whether cpu access is coherent. And then flush
> > both cpu caches and the chipset cache. Which is what
> > flush_cpu_write_domain does.
> >
> > Only difference is that on top we get a tracepoint, frontbuffer
> > tracking and and domain tracking updates. Which doesn't matter since
> > both paths are slowpaths.
> >
> > So lets go with more shared code for clearer idioms.
> >
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> > drivers/gpu/drm/i915/i915_gem.c | 15 ++++-----------
> > 1 file changed, 4 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 783d1040bf83..b48c39230b80 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1019,11 +1019,8 @@ out:
> > * cachelines in-line while writing and the object moved
> > * out of the cpu write domain while we've dropped the lock.
> > */
> > - if (!needs_clflush_after &&
> > - obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
> > - if (i915_gem_clflush_object(obj, obj->pin_display))
> > - i915_gem_chipset_flush(dev);
> > - }
> > + if (!needs_clflush_after)
> > + i915_gem_object_flush_cpu_write_domain(obj);
>
> Nack. You've just inverted the write domain check.

Indeed, brain must have been offline. And thinking about this more this
really is a low-level clflush from a function playing tricks, so doesn't
make sense to use higher level functions anyway.

I'll respin with just the 2nd hunk and try to explain a bit better why
it's ok, like Chris requested.
-Daniel

>
> > }
> >
> > if (needs_clflush_after)
> > @@ -3825,12 +3822,8 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
> > vma->node.color = cache_level;
> > obj->cache_level = cache_level;
> >
> > - if (obj->cache_dirty &&
> > - obj->base.write_domain != I915_GEM_DOMAIN_CPU &&
> > - cpu_write_needs_clflush(obj)) {
> > - if (i915_gem_clflush_object(obj, true))
> > - i915_gem_chipset_flush(obj->base.dev);
> > - }
> > + if (obj->cache_dirty)
> > + i915_gem_object_flush_cpu_write_domain(obj);
> >
> > return 0;
> > }
> > --
> > 2.1.4
>
> --
> Ville Syrjälä
> Intel OTC
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch