[Mesa-dev] [PATCH 3/7] gallium: add FBFETCH opcode to retrieve the current sample value

[Ilia Mirkin]

On Wed, Jan 4, 2017 at 4:24 AM, Erik Faye-Lund <kusmabite at gmail.com> wrote:
> On Jan 2, 2017 06:03, "Ilia Mirkin" <imirkin at alum.mit.edu> wrote:
>
> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
> ---
>  src/gallium/auxiliary/tgsi/tgsi_info.c     |  2 +-
>  src/gallium/docs/source/tgsi.rst           | 11 +++++++++++
>  src/gallium/include/pipe/p_shader_tokens.h |  2 +-
>  3 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/src/gallium/auxiliary/tgsi/tgsi_info.c
> b/src/gallium/auxiliary/tgsi/tgsi_info.c
> index 37549aa..e34b8c7 100644
> --- a/src/gallium/auxiliary/tgsi/tgsi_info.c
> +++ b/src/gallium/auxiliary/tgsi/tgsi_info.c
> @@ -106,7 +106,7 @@ static const struct tgsi_opcode_info
> opcode_info[TGSI_OPCODE_LAST] =
>     { 1, 3, 0, 0, 0, 0, 0, COMP, "CMP", TGSI_OPCODE_CMP },
>     { 1, 1, 0, 0, 0, 0, 0, CHAN, "SCS", TGSI_OPCODE_SCS },
>     { 1, 2, 1, 0, 0, 0, 0, OTHR, "TXB", TGSI_OPCODE_TXB },
> -   { 0, 1, 0, 0, 0, 0, 1, NONE, "", 69 },      /* removed */
> +   { 1, 1, 0, 0, 0, 0, 0, OTHR, "FBFETCH", TGSI_OPCODE_FBFETCH },
>     { 1, 2, 0, 0, 0, 0, 0, COMP, "DIV", TGSI_OPCODE_DIV },
>     { 1, 2, 0, 0, 0, 0, 0, REPL, "DP2", TGSI_OPCODE_DP2 },
>     { 1, 2, 1, 0, 0, 0, 0, OTHR, "TXL", TGSI_OPCODE_TXL },
> diff --git a/src/gallium/docs/source/tgsi.rst
> b/src/gallium/docs/source/tgsi.rst
> index d2d30b4..accbe1d 100644
> --- a/src/gallium/docs/source/tgsi.rst
> +++ b/src/gallium/docs/source/tgsi.rst
> @@ -2561,6 +2561,17 @@ Resource Access Opcodes
>    image, while .w will contain the number of samples for multi-sampled
>    images.
>
> +.. opcode:: FBFETCH - Load data from framebuffer
> +
> +  Syntax: ``FBFETCH dst, output``
> +
> +  Example: ``FBFETCH TEMP[0], OUT[0]``
> +
> +  Returns the color of the current position in the framebuffer from
> +  before this fragment shader invocation. Always returns the same
> +  value from multiple calls for a particular output within a single
> +  invocation.
> +
>
>
> What does this instruction do under MRT? Tegra (AR20) has the ability to
> select the MRT index...

Fetches the relevant output's value. Just as with the new sysval-based
implementation,

DECL SV[0], COLOR[1]

would fetch the value in RT1. This could be used to implement
EXT_shader_framebuffer_fetch, although more cap's would be necessary
to differentiate the use-cases.

Cheers,

  -ilia