Re: [PATCH 2/2] drm/i915: use semaphores for the display plane

[Chris Wilson <chris@chris-wilson.co.uk>]

On Sat, 31 Mar 2012 01:38:24 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Mar 21, 2012 at 05:19:13PM -0700, Ben Widawsky wrote:
> > In theory this will have performance and power improvements. Performance
> > because we don't need to stall when the scanout BO is busy, and power
> > because we don't have to stall when the BO is busy ie. we can get the
> > work done sooner and put the CPU to sleep (and one less interrupt
> > required).
> > 
> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c |    8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index ce2fee5..96ad162 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -3041,9 +3041,7 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
> >   * any flushes to be pipelined (for pageflips).
> >   *
> >   * For the display plane, we want to be in the GTT but out of any write
> > - * domains. So in many ways this looks like set_to_gtt_domain() apart from the
> > - * ability to pipeline the waits, pinning and any additional subtleties
> > - * that may differentiate the display plane from ordinary buffers.
> > + * domains. So in many ways this looks like set_to_gtt_domain().
> >   */
> 
> For the comment bikeshed, what about:
> 
> "Prepare buffer for display plane (scanout, cursors, etc).
> Can be called from an uninterruptible phase (modesetting).
> 
> "For display planes we need to flush, synchronize with any outstanding
> rendering (pipelined using semaphores, if available, in case of a
> pageflip) and additionally need to ensure that the cache_level is coherent
> for the scanout engine."

It's an improvement over what we have certainly. Still doesn't explain
the why, but I've writers block as well.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre