[Mesa-dev] [PATCH 08/15] i965: Reserve space for "Vertex Count" in GS outputs.

[Kenneth Graunke]

On 11/30/2013 10:29 AM, Paul Berry wrote:
> On 12 November 2013 17:51, Kenneth Graunke <kenneth at whitecape.org
> <mailto:kenneth at whitecape.org>> wrote:
> 
>     Signed-off-by: Kenneth Graunke <kenneth at whitecape.org
>     <mailto:kenneth at whitecape.org>>
>     ---
>      src/mesa/drivers/dri/i965/brw_vec4_gs.c | 6 ++++++
>      1 file changed, 6 insertions(+)
> 
>     diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs.c
>     b/src/mesa/drivers/dri/i965/brw_vec4_gs.c
>     index b52d646..e802c1e 100644
>     --- a/src/mesa/drivers/dri/i965/brw_vec4_gs.c
>     +++ b/src/mesa/drivers/dri/i965/brw_vec4_gs.c
>     @@ -194,6 +194,12 @@ do_gs_prog(struct brw_context *brw,
>            c.prog_data.output_vertex_size_hwords * 32 *
>     gp->program.VerticesOut;
>         output_size_bytes += 32 *
>     c.prog_data.control_data_header_size_hwords;
> 
>     +   /* Broadwell stores "Vertex Count" as a full 8 DWord (32 byte)
>     URB output,
>     +    * which comes before the conttrol header.
>     +    */
>     +   if (brw->gen >= 8)
>     +      output_size_bytes += 32;
>     +
>         assert(output_size_bytes >= 1);
>         if (output_size_bytes > GEN7_MAX_GS_URB_ENTRY_SIZE_BYTES)
>            return false;
>     --
>     1.8.3.2
> 
> 
> It looks like changes also need to be made to
> vec4_gs_visitor::emit_urb_write_opcode() and
> vec4_gs_visitor::emit_control_data_bits() to offset the vertex data and
> control data by 32 bytes, and to vec4_gs_visitor::emit_thread_end() to
> cause the appropriate data to be written to the vertex count.  Are those
> changes elsewhere in the series?

I handled that in the gen8_vec4_generator class (introduced in patch 12):

The generate_urb_write() function reserves space:

void
gen8_vec4_generator::generate_urb_write(vec4_instruction *ir, bool vs)
{
   ...

   int global_offset = ir->offset;
   /* The GS needs to increment Global Offset by 1 to make room for the extra
    * "Vertex Count" payload at the beginning.
    */
   if (!vs)
      ++global_offset;

   ...
}

and generate_gs_thread_end() does a SEND with EOT that writes Vertex Count
at offset 0.

It seems to work pretty well.  Sorry for the awkward split in the patch series.

--Ken