On Tuesday, July 07, 2015 04:46:22 PM Chris Wilson wrote: > 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 <kenn...@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.)
Yes, it can. Just glBindBuffer() some buffers around. Or, I suspect one could bind it as a texture buffer object or SSBO and then use a compute shader on the results. With GL 4.x, the "avoid synchronizing with the CPU" mentality is a lot more prevalent, due to the advent of compute shaders. > > 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. Sadly, yes. We have a primitive count and we need a vertex count - so, a tiny bit of math. Ideally, we would use the Gen7.5 MI_MATH+ feature to do this, eliminating the CPU-GPU synchronization point. > Also afaict it is not possible to map the sol buffer directly into the > application. > -Chris It definitely is - the application creates GL buffer objects and binds them for use with transform feedback. They can certainly glMapBufferRange() those buffers. --Ken
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