[Mesa-dev] [PATCH 12/18] mesa: Skip ES 3.0/3.1 transform feedback primitive counting error.

[Ian Romanick]

On 08/01/2016 10:29 AM, Kenneth Graunke wrote:
> This error condition is not implementable when using tessellation or
> geometry shaders.  The text was also removed from the ES 3.2 spec.
> I believe the intended behavior is to remove the error condition
> when either OES_geometry_shader or OES_tessellation_shader are
> exposed.
> 
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/main/api_validate.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/src/mesa/main/api_validate.c b/src/mesa/main/api_validate.c
> index 2ee2cd8..68f55b6 100644
> --- a/src/mesa/main/api_validate.c
> +++ b/src/mesa/main/api_validate.c
> @@ -546,8 +546,20 @@ validate_draw_arrays(struct gl_context *ctx, const char *func,
>      *
>      * This is in contrast to the behaviour of desktop GL, where the extra
>      * primitives are silently dropped from the transform feedback buffer.
> +    *
> +    * This text is removed in ES 3.2, presumably because it's not really
> +    * implementable with geometry and tessellation shaders.  In fact,
> +    * issue 13 of the OES_geometry_shader spec says:
> +    *
> +    * "Since we no longer require being able to predict how much geometry will
> +    *  be generated [...]"
> +    *
> +    * It isn't speaking about this condition specifically, but speaks of
> +    * lifting similar transform feedback drawing restrictions.
>      */

This is the spec text that I quoted in my similar patch:

+    *
+    * The OES_geometry_shader spec says:
+    *
+    *    Issues:
+    *
+    *    (13) Does this extension change how transform feedback operates
+    *    compared to unextended OpenGL ES 3.0 or 3.1?
+    *
+    *    RESOLVED: Yes. Because dynamic geometry amplification in a geometry
+    *    shader can make it difficult if not impossible to predict the amount
+    *    of geometry that may be generated in advance of executing the shader,
+    *    the draw-time error for transform feedback buffer overflow conditions
+    *    is removed and replaced with the GL behavior (primitives are not
+    *    written and the corresponding counter is not updated)...

> -   if (_mesa_is_gles3(ctx) && _mesa_is_xfb_active_and_unpaused(ctx)) {
> +   if (_mesa_is_gles3(ctx) && _mesa_is_xfb_active_and_unpaused(ctx) &&
> +       !_mesa_has_OES_geometry_shader(ctx) &&
> +       !_mesa_has_OES_tessellation_shader(ctx)) {
>        size_t prim_count = vbo_count_tessellated_primitives(mode, count, 1);
>        if (xfb_obj->GlesRemainingPrims < prim_count) {
>           _mesa_error(ctx, GL_INVALID_OPERATION,
>