gallium: add PIPE_CAP_PREFER_REAL_BUFFER_IN_CONSTBUF0

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6946>

docs/gallium/screen.rst

@@ -594,6 +594,7 @@ The integer capabilities:
 * ``PIPE_CAP_NO_CLIP_ON_COPY_TEX``: Driver doesn't want x/y/width/height clipped based on src size when doing a copy texture operation (eg: may want out-of-bounds reads that produce 0 instead of leaving the texture content undefined)
 * ``PIPE_CAP_MAX_TEXTURE_MB``: Maximum texture size in MB (default is 1024)
 * ``PIPE_CAP_DEVICE_PROTECTED_CONTENT``: Whether the device support protected / encrypted content.
+* ``PIPE_CAP_PREFER_REAL_BUFFER_IN_CONSTBUF0``: The state tracker is encouraged to upload constants into a real buffer and bind it into constant buffer 0 instead of binding a user pointer. This may enable a faster codepath in a gallium frontend for drivers that really prefer a real buffer.
 
 .. _pipe_capf:
 

src/gallium/auxiliary/util/u_screen.c

@@ -440,6 +440,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_ALPHA_TO_COVERAGE_DITHER_CONTROL:
    case PIPE_CAP_NO_CLIP_ON_COPY_TEX:
    case PIPE_CAP_MAX_TEXTURE_MB:
+   case PIPE_CAP_PREFER_REAL_BUFFER_IN_CONSTBUF0:
       return 0;
 
    case PIPE_CAP_SHADER_ATOMIC_INT64:

src/gallium/drivers/radeonsi/si_get.c

@@ -165,6 +165,7 @@ static int si_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_FRONTEND_NOOP:
    case PIPE_CAP_DEMOTE_TO_HELPER_INVOCATION:
    case PIPE_CAP_MULTI_DRAW:
+   case PIPE_CAP_PREFER_REAL_BUFFER_IN_CONSTBUF0:
       return 1;
 
    case PIPE_CAP_GLSL_ZERO_INIT:

src/gallium/include/pipe/p_defines.h

@@ -977,6 +977,7 @@ enum pipe_cap
    PIPE_CAP_MAX_TEXTURE_MB,
    PIPE_CAP_SHADER_ATOMIC_INT64,
    PIPE_CAP_DEVICE_PROTECTED_CONTENT,
+   PIPE_CAP_PREFER_REAL_BUFFER_IN_CONSTBUF0,
 };
 
 /**