pvr: Explain why dynamic descriptors are allocated separately.

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20008>

src/imagination/vulkan/pvr_descriptor_set.c

@@ -585,9 +585,10 @@ VkResult pvr_CreateDescriptorSetLayout(
 
          internal_binding->shader_stage_mask |= (1U << stage);
 
-         /* TODO: Do we have to allocate them at the end? We could speed it
-          * by allocating them here if not. */
-         /* We allocate dynamics primary and secondaries at the end. */
+         /* We allocate dynamics primary and secondaries separately so that we
+          * can do a partial update of USC shared registers by just DMAing the
+          * dynamic section and not having to re-DMA everything again.
+          */
          if (descriptor_type != VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER_DYNAMIC &&
              descriptor_type != VK_DESCRIPTOR_TYPE_STORAGE_BUFFER_DYNAMIC) {
             struct pvr_descriptor_size_info size_info;