[Mesa-dev] [PATCH] i965: Use software primitive restart when transform feedback active.

[Kenneth Graunke]

On 04/06/2013 08:25 PM, Paul Berry wrote:
> When transform feedback is active, the driver manually counts the
> number of primitives that run through the pipeline, so that if a batch
> buffer flush happens, the next batch buffer can pick up transform
> feedback where the last batch buffer left off.  Hardware-accelerated
> primitive restart interferes with this process (because it makes the
> primitive count depend not just on the number of vertices entering the
> pipeline, but also on the contents of the index buffer).  So, when
> transform feedback is active, we need to fall back to the software
> implementation of primitive restart.
>
> Fixes piglit test "spec/!OpenGL 3.1/primitive-restart-xfb flush".
>
> NOTE: This is a candidate for stable release branches.
> ---
>   src/mesa/drivers/dri/i965/brw_primitive_restart.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_primitive_restart.c b/src/mesa/drivers/dri/i965/brw_primitive_restart.c
> index e6902b4..d0f0038 100644
> --- a/src/mesa/drivers/dri/i965/brw_primitive_restart.c
> +++ b/src/mesa/drivers/dri/i965/brw_primitive_restart.c
> @@ -27,6 +27,7 @@
>
>   #include "main/imports.h"
>   #include "main/bufferobj.h"
> +#include "main/transformfeedback.h"
>
>   #include "brw_context.h"
>   #include "brw_defines.h"
> @@ -81,11 +82,18 @@ can_cut_index_handle_prims(struct gl_context *ctx,
>      struct brw_context *brw = brw_context(ctx);
>
>      if (brw->sol.counting_primitives_generated ||
> -       brw->sol.counting_primitives_written) {
> +       brw->sol.counting_primitives_written ||
> +       _mesa_is_xfb_active_and_unpaused(ctx)) {
>         /* Counting primitives generated in hardware is not currently
>          * supported, so take the software path. We need to investigate
>          * the *_PRIMITIVES_COUNT registers to allow this to be handled
>          * entirely in hardware.
> +       *
> +       * Note that when transform feedback is active, we also count primitives
> +       * (even if the client hasn't requested it), since that is the only way
> +       * we can start at the proper place in the transform feedback buffer
> +       * after a flush.  So we also have to fall back to software when
> +       * transform feedback is active and unpaused.
>          */
>         return false;
>      }

Gah.  This is unfortunate, but necessary.

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>