i965: Document conventions for counting layers in 2D multisample buffers.

The "layer" parameters used in blorp, and the
intel_renderbuffer::mt_layer field, represent a physical layer rather
than a logical layer.  This is important for 2D multisample arrays on
Gen7+ because the UMS and CMS multisample layouts use N physical
layers to represent each logical layer, where N is the number of
samples.

Also add an assertion to blorp to help catch bugs if we fail to follow
these conventions.

Reviewed-by: Chad Versace <[email protected]>
Reviewed-by: Anuj Phogat <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/intel_fbo.h b/src/mesa/drivers/dri/i965/intel_fbo.h
index 02c357d0072..fb50f862aac 100644
--- a/src/mesa/drivers/dri/i965/intel_fbo.h
+++ b/src/mesa/drivers/dri/i965/intel_fbo.h
@@ -63,6 +63,11 @@ struct intel_renderbuffer
     *
     * For renderbuffers not created with glFramebufferTexture*(), mt_level and
     * mt_layer are 0.
+    *
+    * Note: for a 2D multisample array texture on Gen7+ using
+    * INTEL_MSAA_LAYOUT_UMS or INTEL_MSAA_LAYOUT_CMS, mt_layer is the physical
+    * layer holding sample 0.  So, for example, if mt->num_samples == 4, then
+    * logical layer n corresponds to mt_layer == 4*n.
     */
    unsigned int mt_level;
    unsigned int mt_layer;