gallium: Add new PIPE_CAP_SURFACE_SAMPLE_COUNT

This new pipe cap and the new nr_samples field in pipe_surface lets a
state tracker bind a render target with a different sample count than
the resource. This allows for implementing
EXT_multisampled_render_to_texture and
EXT_multisampled_render_to_texture2.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>

src/gallium/auxiliary/util/u_framebuffer.c

@@ -229,13 +229,19 @@ util_framebuffer_get_num_samples(const struct pipe_framebuffer_state *fb)
    if (!(fb->nr_cbufs || fb->zsbuf))
       return MAX2(fb->samples, 1);
 
+   /**
+    * If a driver doesn't advertise PIPE_CAP_SURFACE_SAMPLE_COUNT,
+    * pipe_surface::nr_samples will always be 0.
+    */
    for (i = 0; i < fb->nr_cbufs; i++) {
       if (fb->cbufs[i]) {
-         return MAX2(1, fb->cbufs[i]->texture->nr_samples);
+         return MAX3(1, fb->cbufs[i]->texture->nr_samples,
+                     fb->cbufs[i]->nr_samples);
       }
    }
    if (fb->zsbuf) {
-      return MAX2(1, fb->zsbuf->texture->nr_samples);
+      return MAX3(1, fb->zsbuf->texture->nr_samples,
+                  fb->zsbuf->nr_samples);
    }
 
    return 1;

src/gallium/docs/source/screen.rst

@@ -477,6 +477,9 @@ subpixel precision bias in bits during conservative rasterization.
   0 means no limit.
 * ``PIPE_CAP_MAX_VERTEX_ELEMENT_SRC_OFFSET``: The maximum supported value for
   of pipe_vertex_element::src_offset.
+* ``PIPE_CAP_SURFACE_SAMPLE_COUNT_TEXTURE``: Whether the driver
+  supports pipe_surface overrides of resource nr_samples. If set, will
+  enable EXT_multisampled_render_to_texture.
 
 .. _pipe_capf:
 

src/gallium/include/pipe/p_defines.h

@@ -832,6 +832,7 @@ enum pipe_cap
    PIPE_CAP_MAX_COMBINED_HW_ATOMIC_COUNTER_BUFFERS,
    PIPE_CAP_MAX_TEXTURE_UPLOAD_MEMORY_BUDGET,
    PIPE_CAP_MAX_VERTEX_ELEMENT_SRC_OFFSET,
+   PIPE_CAP_SURFACE_SAMPLE_COUNT,
 };
 
 /**

src/gallium/include/pipe/p_state.h

@@ -443,6 +443,13 @@ struct pipe_surface
    uint16_t width;               /**< logical width in pixels */
    uint16_t height;              /**< logical height in pixels */
 
+   /**
+    * Number of samples for the surface.  This will be 0 if rendering
+    * should use the resource's nr_samples, or another value if the resource
+    * is bound using FramebufferTexture2DMultisampleEXT.
+    */
+   unsigned nr_samples:8;
+
    union pipe_surface_desc u;
 };