[Mesa-dev] [PATCH 02/15] i965: Configure how to store *64*PASSTHRU vertex components

[Ian Romanick]

On 04/28/2016 01:40 PM, Antia Puentes wrote:
> From the Broadwell specification, structure VERTEX_ELEMENT_STATE
> description:
> 
>    "When SourceElementFormat is set to one of the *64*_PASSTHRU
>     formats,  64-bit components are stored in the URB without any
>     conversion. In this case, vertex elements must be written as 128
>     or 256 bits, with VFCOMP_STORE_0 being used to pad the output
>     as required. E.g., if R64_PASSTHRU is used to copy a 64-bit Red component into
>     the URB, Component 1 must be specified as VFCOMP_STORE_0 (with
>     Components 2,3 set to VFCOMP_NOSTORE) in order to output a 128-bit
>     vertex element, or Components 1-3 must be specified as VFCOMP_STORE_0
>     in order to output a 256-bit vertex element. Likewise, use of
>     R64G64B64_PASSTHRU requires Component 3 to be specified as VFCOMP_STORE_0
>     in order to output a 256-bit vertex element."
> 
> Uses 128-bits to write double and dvec2 vertex elements, and 256-bits for
> dvec3 and dvec4 vertex elements.
> 
> Signed-off-by: Juan A. Suarez Romero <jasuarez at igalia.com>
> Signed-off-by: Antia Puentes <apuentes at igalia.com>
> ---
>  src/mesa/drivers/dri/i965/gen8_draw_upload.c | 34 ++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/src/mesa/drivers/dri/i965/gen8_draw_upload.c b/src/mesa/drivers/dri/i965/gen8_draw_upload.c
> index fe5ed35..14f7091 100644
> --- a/src/mesa/drivers/dri/i965/gen8_draw_upload.c
> +++ b/src/mesa/drivers/dri/i965/gen8_draw_upload.c
> @@ -217,6 +217,40 @@ gen8_emit_vertices(struct brw_context *brw)
>           break;
>        }
>  
> +      /* From the BDW PRM, Volume 2d, page 586 (VERTEX_ELEMENT_STATE):
> +       *
> +       *     "When SourceElementFormat is set to one of the *64*_PASSTHRU
> +       *     formats, 64-bit components are stored in the URB without any
> +       *     conversion. In this case, vertex elements must be written as 128
> +       *     or 256 bits, with VFCOMP_STORE_0 being used to pad the output
> +       *     as required. E.g., if R64_PASSTHRU is used to copy a 64-bit Red
> +       *     component into the URB, Component 1 must be specified as
> +       *     VFCOMP_STORE_0 (with Components 2,3 set to VFCOMP_NOSTORE)
> +       *     in order to output a 128-bit vertex element, or Components 1-3 must
> +       *     be specified as VFCOMP_STORE_0 in order to output a 256-bit vertex
> +       *     element. Likewise, use of R64G64B64_PASSTHRU requires Component 3
> +       *     to be specified as VFCOMP_STORE_0 in order to output a 256-bit vertex
> +       *     element."
> +       */
> +      if (input->glarray->Doubles) {
> +         switch (input->glarray->Size) {
> +         case 0:
> +         case 1:
> +         case 2:
> +            /*  Use 128-bits instead of 256-bits to write double and dvec2
> +             *  vertex elements.
> +             */
> +            comp2 = BRW_VE1_COMPONENT_NOSTORE;
> +            comp3 = BRW_VE1_COMPONENT_NOSTORE;
> +            break;
> +         case 3:
> +            /* Pad the output using VFCOMP_STORE_0 as suggested
> +             * by the BDW PRM.
> +             */
> +            comp3 = BRW_VE1_COMPONENT_STORE_0;

I think this would be better as

      if (input->glarray->Size < 3) {
         ...
      } else {
         ...
      }

At the very least... there should be a "break" at the end of the 3 case.
 It's not strictly necessary, but my eye keeps noticing that it's not there.

> +         }
> +      }
> +
>        OUT_BATCH((input->buffer << GEN6_VE0_INDEX_SHIFT) |
>                  GEN6_VE0_VALID |
>                  (format << BRW_VE0_FORMAT_SHIFT) |
>