[Mesa-dev] [PATCH 7/8] i965/msaa: Add CMS support to blorp.

[Chad Versace]

On 07/06/2012 03:29 PM, Paul Berry wrote:
> This patch updates the blorp engine to properly handle the case where
> the surface being textured from uses Gen7's CMS MSAA layout.  The
> following changes were necessary:
> 
> - Before reading color values from the surface, we need to read from
>   the MCS buffer using the ld_mcs sampler message.  This is done by
>   the mcs_fetch() function, and the result is stored in the mcs_data
>   register.  This only needs to be done once per pixel, since the MCS
>   value is shared between all samples belonging to a pixel.
> 
> - When reading color values from the surface, we need to use the
>   ld2dms sampler message instead of the ld2dss message, and we need to
>   provide the value read from the MCS buffer as an argument.
> ---
>  src/mesa/drivers/dri/i965/brw_blorp_blit.cpp |   45 ++++++++++++++++++++++++-
>  1 files changed, 43 insertions(+), 2 deletions(-)

I see an easy optimization that's suitable for a small follow-up patch. But this
code works and passes tests, so let's commit it as-is first.

This patch is
Reviewed-by: Chad Versace <chad.versace at linux.intel.com>

> @@ -1126,8 +1148,13 @@ brw_blorp_blit_program::texel_fetch(struct brw_reg dst)
>        break;
>     case 7:
>        if (key->tex_samples > 0) {
> -         texture_lookup(dst, GEN7_SAMPLER_MESSAGE_SAMPLE_LD2DSS,
> -                        gen7_ld2dss_args, ARRAY_SIZE(gen7_ld2dss_args));
> +         if (key->tex_layout == INTEL_MSAA_LAYOUT_CMS) {
> +            texture_lookup(dst, GEN7_SAMPLER_MESSAGE_SAMPLE_LD2DMS,
> +                           gen7_ld2dms_args, ARRAY_SIZE(gen7_ld2dms_args));
> +         } else {
> +            texture_lookup(dst, GEN7_SAMPLER_MESSAGE_SAMPLE_LD2DSS,
> +                           gen7_ld2dss_args, ARRAY_SIZE(gen7_ld2dss_args));
> +         }

The hw docs suggest here "a simple optimization with probable large return in
performance". If mcs_data == 0, which should be the common case for most
fragments, then only sample array slice 0 needs to be accessed. That is, when
tex_layout is CMS then we should emit code like this here:

  if (mcs_data == 0) {
    ld2dss(S, X, Y);
  } else {
    ld2dms(S, mcs_data, X, Y);
  }

----
Chad Versace
chad.versace at linux.intel.com