[Nouveau] [RFC] mesa/st: Avoid passing a NULL buffer to the drivers

[Tobias Klausmann]

If we capture transform feedback from n stream in (n-1) buffers we face a
NULL buffer, use the buffer (n-1) to capture the output of stream n.

This fixes one piglit test with nvc0:
   arb_gpu_shader5-xfb-streams-without-invocations

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann at mni.thm.de>
---
 src/mesa/state_tracker/st_cb_xformfb.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/mesa/state_tracker/st_cb_xformfb.c b/src/mesa/state_tracker/st_cb_xformfb.c
index 8f75eda..5a12da4 100644
--- a/src/mesa/state_tracker/st_cb_xformfb.c
+++ b/src/mesa/state_tracker/st_cb_xformfb.c
@@ -123,6 +123,11 @@ st_begin_transform_feedback(struct gl_context *ctx, GLenum mode,
       struct st_buffer_object *bo = st_buffer_object(sobj->base.Buffers[i]);
 
       if (bo) {
+         if (!bo->buffer)
+            /* If we capture transform feedback from n streams into (n-1)
+             * buffers we have to write to buffer (n-1) for stream n.
+             */
+            bo = st_buffer_object(sobj->base.Buffers[i-1]);
          /* Check whether we need to recreate the target. */
          if (!sobj->targets[i] ||
              sobj->targets[i] == sobj->draw_count ||
-- 
2.2.1