gallium: add texture gather support to gallium (v3)

This adds support to gallium for a TG4 instruction,
and two CAPs. The first CAP is required for GL_ARB_texture_gather.

The second CAP is required to expose GL_ARB_gpu_shader5.

However so far we haven't found any hardware that natively
exposes the textureGatherOffsets feature from GL, so just
lower it for now. If hardware appears for this we can add
another CAP to allow TG4 to take 4 offsets.

v2: add component selection src and a cap to say
hw can do it. (st can use to help control
GL_ARB_gpu_shader5/GLSL 4.00). Add docs.

v3: rename to SM5, add docs.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/gallium/docs/source/screen.rst

@@ -182,6 +182,12 @@ The integer capabilities:
   vertex components output by a single invocation of a geometry shader.
   This is the product of the number of attribute components per vertex and
   the number of output vertices.
+* ``PIPE_CAP_MAX_TEXTURE_GATHER_COMPONENTS``: Max number of components
+  in format that texture gather can operate on. 1 == RED, ALPHA etc,
+  4 == All formats.
+* ``PIPE_CAP_TEXTURE_GATHER_SM5``: Whether the texture gather
+  hardware implements the SM5 features, component selection,
+  shadow comparison, and run-time offsets.
 
 
 .. _pipe_capf:

src/gallium/docs/source/tgsi.rst

@@ -986,6 +986,42 @@ XXX doesn't look like most of the opcodes really belong here.
 
   dst.z = texture_depth(unit, lod)
 
+.. opcode:: TG4 - Texture Gather (as per ARB_texture_gather)
+               Gathers the four texels to be used in a bi-linear
+               filtering operation and packs them into a single register.
+               Only works with 2D, 2D array, cubemaps, and cubemaps arrays.
+               For 2D textures, only the addressing modes of the sampler and
+               the top level of any mip pyramid are used. Set W to zero.
+               It behaves like the TEX instruction, but a filtered
+               sample is not generated. The four samples that contribute
+               to filtering are placed into xyzw in clockwise order,
+               starting with the (u,v) texture coordinate delta at the
+               following locations (-, +), (+, +), (+, -), (-, -), where
+               the magnitude of the deltas are half a texel.
+
+               PIPE_CAP_TEXTURE_SM5 enhances this instruction to support
+               shadow per-sample depth compares, single component selection,
+               and a non-constant offset. It doesn't allow support for the
+               GL independent offset to get i0,j0. This would require another
+               CAP is hw can do it natively. For now we lower that before
+               TGSI.
+
+.. math::
+
+   coord = src0
+
+   component = src1
+
+   dst = texture_gather4 (unit, coord, component)
+
+(with SM5 - cube array shadow)
+
+   coord = src0
+   
+   compare = src1
+
+   dst = texture_gather (uint, coord, compare)
+
 
 Integer ISA
 ^^^^^^^^^^^^^^^^^^^^^^^^