i965: Flush pipeline on EndTransformFeedback.

A common use case for transform feedback is to perform one draw
operation that writes transform feedback output to a buffer, followed
by a second draw operation that consumes that buffer as vertex input.
Since vertex input is consumed at an earlier pipeline stage than
writing transform feedback output, we need to flush the pipeline to
ensure that the transform feedback output is completely written before
the data is consumed.

In an ideal world, we would do some dependency tracking, so that we
would only flush the pipeline if the next draw call was about to
consume data generated by a previous draw call in the same batch.
However, since we don't have that sort of dependency tracking
infrastructure right now, we just unconditionally flush the buffer
every time glEndTransformFeedback() is called.  This will cause a
performance hit compared to the ideal case (since we will sometimes
flush the pipeline unnecessarily), but fortunately the performance hit
will be confined to circumstances where transform feedback is in use.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
index d8cad54667a82e8323a6906010e6a73df13932e2..2096be9f86cb0e5228ec229f74782aa4a0b0d790 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -117,6 +117,7 @@ static void brwInitDriverFunctions( struct dd_function_table *functions )
    brw_init_queryobj_functions(functions);
 
    functions->PrepareExecBegin = brwPrepareExecBegin;
+   functions->EndTransformFeedback = brw_end_transform_feedback;
 }
 
 bool

diff --git a/src/mesa/drivers/dri/i965/brw_context.h b/src/mesa/drivers/dri/i965/brw_context.h
index febd4fe43659b636cf58a7abeb691ea2b94b8f2a..6fa71a3c14e940d4a78370e756ae9d5911e2eb6b 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_context.h
+++ b/src/mesa/drivers/dri/i965/brw_context.h
@@ -1076,6 +1076,11 @@ brw_update_sol_surface(struct brw_context *brw,
 bool
 brw_fprog_uses_noperspective(const struct gl_fragment_program *fprog);
 
+/* gen6_sol.c */
+void
+brw_end_transform_feedback(struct gl_context *ctx,
+                           struct gl_transform_feedback_object *obj);
+
 
 
 /*======================================================================

diff --git a/src/mesa/drivers/dri/i965/gen6_sol.c b/src/mesa/drivers/dri/i965/gen6_sol.c
index 491b39cce12424628cd3966e87678a8d217e0649..38aedcc4297899278d633040fdab90b867abb869 100644 (file)
--- a/src/mesa/drivers/dri/i965/gen6_sol.c
+++ b/src/mesa/drivers/dri/i965/gen6_sol.c
@@ -27,6 +27,7 @@
  */
 
 #include "brw_context.h"
+#include "intel_batchbuffer.h"
 #include "brw_defines.h"
 
 static void
@@ -69,3 +70,18 @@ const struct brw_tracked_state gen6_sol_surface = {
    },
    .emit = gen6_update_sol_surfaces,
 };
+
+void
+brw_end_transform_feedback(struct gl_context *ctx,
+                           struct gl_transform_feedback_object *obj)
+{
+   /* After EndTransformFeedback, it's likely that the client program will try
+    * to draw using the contents of the transform feedback buffer as vertex
+    * input.  In order for this to work, we need to flush the data through at
+    * least the GS stage of the pipeline, and flush out the render cache.  For
+    * simplicity, just do a full flush.
+    */
+   struct brw_context *brw = brw_context(ctx);
+   struct intel_context *intel = &brw->intel;
+   intel_batchbuffer_emit_mi_flush(intel);
+}