[Mesa-dev] [PATCH] i965: Prefer Meta over the BLT for BlitFramebuffer.

[Pohjolainen, Topi]

On Tue, Feb 17, 2015 at 05:39:33PM -0800, Kenneth Graunke wrote:
> There's some debate about whether we should use Meta or BLORP,
> but either should run circles around the BLT engine.
> 
> In particular, this means that Gen8+ will use the 3D engine for blits,
> like we do on Gen6-7.
> 
> Improves performance in "copypixrate -blit -back" (from Mesa demos)
> by 232.037% +/- 3.15795% (n=10) on Broadwell GT3e.

I've been also experimenting with the same test by disabling blorp for
fb-blits on IVB. I'm a little confused since even without your patch the blit
goes through meta instead of the BLT engine. It is the test for scissor
that prevents intel_blit_framebuffer_with_blitter() from doing the blit:

...
 /* If the source and destination are the same size with no mirroring,
       * the rectangles are within the size of the texture and there is no
       * scissor, then we can probably use the blit engine.
       */
      if (!(srcX0 - srcX1 == dstX0 - dstX1 &&
            srcY0 - srcY1 == dstY0 - dstY1 &&
            srcX1 >= srcX0 &&
            srcY1 >= srcY0 &&
            srcX0 >= 0 && srcX1 <= readFb->Width &&
            srcY0 >= 0 && srcY1 <= readFb->Height &&
            dstX0 >= 0 && dstX1 <= drawFb->Width &&
            dstY0 >= 0 && dstY1 <= drawFb->Height &&
            !(ctx->Scissor.EnableFlags))) {
         perf_debug("glBlitFramebuffer(): non-1:1 blit.  "
                    "Falling back to software rendering.\n");
...

I wonder where the performance difference actually comes from, could it
be the intel_prepare_render() that intel_blit_framebuffer_with_blitter()
calls before bailing out. With your patch that won't get called anymore.