[Mesa-dev] [PATCH] i965: Don't try to use the hardware blitter for multisampled miptrees.

[Paul Berry]

On 21 February 2014 19:15, Kenneth Graunke <kenneth at whitecape.org> wrote:

> The blitter is completely ignorant of MSAA buffer layouts, so any
> attempt to use BLT paths with MSAA buffers is likely to break
> spectacularly.
>
> In most cases, BLORP handles MSAA blits, so we never hit this bug.
> Until recently, it also wasn't worth fixing, since Meta couldn't handle
> MSAA either, so there was nothing to fall back to.  But now there is.
>
> +143 piglit tests on Broadwell (which doesn't have BLORP support).
> Surprisingly, three also start failing.  No changes on Ivybridge.
>

That actually doesn't surprise me too much.  Since color buffers use an
array-like sample layout (sample N in the Nth layer), trying to use the
blitter on a multisampled buffer just gets you sample 0 from each pixel.
 Which for some tests is good enough to pass (even though the blit is
actually doing the wrong thing).  So it's possible that a few of these
tests that were passing by luck with the blitter don't yet have proper Meta
support :)


>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/intel_blit.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/intel_blit.c
> b/src/mesa/drivers/dri/i965/intel_blit.c
> index df85dc9..cedb230 100644
> --- a/src/mesa/drivers/dri/i965/intel_blit.c
> +++ b/src/mesa/drivers/dri/i965/intel_blit.c
> @@ -165,6 +165,10 @@ intel_miptree_blit(struct brw_context *brw,
>     mesa_format src_format = _mesa_get_srgb_format_linear(src_mt->format);
>     mesa_format dst_format = _mesa_get_srgb_format_linear(dst_mt->format);
>
> +   /* The blitter doesn't understand multisampling at all. */
> +   if (src_mt->num_samples > 0 || dst_mt->num_samples > 0)
> +      return false;
> +
>

You might consider moving this check up to the top of the function, since
it doesn't depend on src_format or dst_format.  But it's not a big deal.
 Either way, the patch is:

Reviewed-by: Paul Berry <stereotype441 at gmail.com>
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