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

[Paul Berry]

On 11 July 2012 11:01, Chad Versace <chad.versace at linux.intel.com> wrote:

> 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.
>

Cool.  I'm curious to hear what your optimization idea is.


>
> 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);
>   }
>

I actually believe this will be slightly faster:

ld2dms(0, mcs_data, X, Y);
if (mcs_data != 0) {
  for (S = 1; S < num_samples; ++S)
    ld2dms(S, mcs_data, X, Y);
    average samples together;
  }
}

Because it will avoid issuing an extra sample instruction in the case where
some pixels have a zero value for mcs_data and others have a nonzero value
for mcs_data.  I've got that queued up for a patch series after this one is
reviewed :)


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