[Mesa-dev] [PATCH 10/18] i965: Speculatively flush the batch after transform feedback

[Chris Wilson]

On Tue, Jul 07, 2015 at 10:12:20AM +0100, Chris Wilson wrote:
> On Mon, Jul 06, 2015 at 09:05:18PM -0700, Kristian Høgsberg wrote:
> > On Mon, Jul 6, 2015 at 12:36 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> > > On Monday, July 06, 2015 11:33:15 AM Chris Wilson wrote:
> > >> Since the purpose of transform feedback tends to be for the client to
> > >> act upon the results to change the geometry in the scene, it is likely
> > >> that the client will soon be waiting upon the results. Flush the batch
> > >> early so that we don't build up a long queue of commands afterwards that
> > >> could delay the readback.
> > >> ---
> > >>  src/mesa/drivers/dri/i965/gen7_sol_state.c | 6 ++++++
> > >>  1 file changed, 6 insertions(+)
> > >>
> > >> diff --git a/src/mesa/drivers/dri/i965/gen7_sol_state.c b/src/mesa/drivers/dri/i965/gen7_sol_state.c
> > >> index 857ebe5..13dbe5b 100644
> > >> --- a/src/mesa/drivers/dri/i965/gen7_sol_state.c
> > >> +++ b/src/mesa/drivers/dri/i965/gen7_sol_state.c
> > >> @@ -494,6 +494,12 @@ gen7_end_transform_feedback(struct gl_context *ctx,
> > >>
> > >>     brw_batch_end(&brw->batch);
> > >>
> > >> +   /* We will likely want to read the results in the very near future, so
> > >> +    * push this primitive to hardware if it is currently idle.
> > >> +    */
> > >> +   if (!brw_batch_busy(&brw->batch))
> > >> +      brw_batch_flush(&brw->batch);
> > >> +
> > >>     /* EndTransformFeedback() means that we need to update the number of
> > >>      * vertices written.  Since it's only necessary if DrawTransformFeedback()
> > >>      * is called and it means mapping a buffer object, we delay computing it
> > >>
> > >
> > > We need some data to justify this change.
> > 
> > I think even the theory is not correct - transform feedback is
> > typically fed back into the GPU (as new geometry, eg) rather than
> > consumed by the CPU, and in that case the flush is not helpful. But at
> > the end of the day, data will tell.
> 
> How are they fed back? Can the xfb buffer be bound to the vertex buffer?
> (Genuine question! The only examples I've seen were for testing by the
> CPU.)

I've reviewed the code again, and gen7_end_transform_feedback() is always
followed by brw_compute_xfb_vertices_written (and a read of the sol
buffer) afaict, maybe not immediately but always before the next
transform feedback.

Also afaict it is not possible to map the sol buffer directly into the
application.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre