[Mesa-dev] [PATCH] i965: Allocate at least some URB space even when max_vertices = 0.

[Anuj Phogat]

On Mon, Dec 5, 2016 at 1:39 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> Allocating zero URB space is a really bad idea.  The hardware has to
> give threads a handle to their URB space, and threads have to use that
> to terminate the thread.  Having it be an empty region just breaks a
> lot of assumptions.  Hence, why we asserted that it isn't possible.
>
> Unfortunately, it /is/ possible prior to Gen8, if max_vertices = 0.
> In theory a geometry shader could do SSBO/image access and maybe
> still accomplish something.  In reality, this is tripped up by
> conformance tests.
>
> Gen8+ already avoids this problem by placing the vertex count DWord
> in the URB entry header.  This fixes things on earlier generations.
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
> index 900d9d3..3894a63 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
> @@ -781,7 +781,13 @@ brw_compile_gs(const struct brw_compiler *compiler, void *log_data,
>     if (compiler->devinfo->gen >= 8)
>        output_size_bytes += 32;
>
> -   assert(output_size_bytes >= 1);
> +   /* Shaders can technically set max_vertices = 0, at which point we
> +    * may have a URB size of 0 bytes.  Nothing good can come from that,
> +    * so enforce a minimum size.
> +    */
> +   if (output_size_bytes == 0)
> +      output_size_bytes = 1;
> +
>     unsigned max_output_size_bytes = GEN7_MAX_GS_URB_ENTRY_SIZE_BYTES;
>     if (compiler->devinfo->gen == 6)
>        max_output_size_bytes = GEN6_MAX_GS_URB_ENTRY_SIZE_BYTES;
> --
> 2.10.2
>
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Sounds reasonable to me.
Reviewed-by: Anuj Phogat <anuj.phogat at gmail.com>