[Mesa-dev] [PATCH 0/6] Mesa/Gallium vertex array state optimizations

[Marek Olšák]

On Sun, Feb 13, 2011 at 8:45 PM, Keith Whitwell <keithw at vmware.com> wrote:

> Marek,
>
> These patches look good, but have you covered the case where the
> application is changing the contents of vertex arrays without
> rebinding/notifying GL in any way?
>
> eg. an app could do:
>
>   memcpy(varray, foo, ...);
>   glDrawArrays(...);
>   memcpy(varray, bar, ...);
>   glDrawArrays(...);
>
> with these changes will drivers still notice the difference?
>

Keith,

Yes, they will. If vertex buffers are not re-set in st_draw_vbo,
redefine_user_buffer is called for each user buffer which is set and that
tells a driver which buffer ranges need to be re-uploaded. This can be found
in the last hunk of the last patch, specifically:

@@ -646,6 +664,26 @@ st_draw_vbo(struct gl_context *ctx,
 #endif
   }

+   /* Notify the driver that the content of user buffers may have been
+    * changed. */
+   if (!new_array && st->num_user_vbs) {
+      for (i = 0; i < st->num_user_vbs; i++) {
+         if (st->user_vb[i]) {
+            unsigned stride = st->user_vb_stride[i];
+
+            if (stride) {
+               pipe->redefine_user_buffer(pipe, st->user_vb[i],
+                                          min_index * stride,
+                                          (max_index + 1 - min_index) *
stride);
+            } else {
+               /* stride == 0 */
+               pipe->redefine_user_buffer(pipe, st->user_vb[i],
+                                          0, st->user_vb[i]->width0);
+            }
+         }
+      }
+   }
+
   setup_index_buffer(ctx, ib, &ibuffer);
   pipe->set_index_buffer(pipe, &ibuffer);

What remains to implement is using this information in drivers to re-upload
the buffer ranges marked with redefine_user_buffer. r300g, r600g, some
nouveau drivers, and anything which uses Draw do not need this information,
so they are safe. I think the only driver which needs special handling is
svga, but I don't know that driver so well to be able to do it.

If svga used u_vbuf_mgr, which can determine the dirty buffer ranges by
other means and can re-upload vertices by itself, this redefine_user_buffer
hook wouldn't be needed (besides for updating width0). It would also clean
up svga quite a bit, because u_vbuf_mgr can transparently translate vertices
to formats and layouts compatible with D3D9.

Marek
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