[Mesa-dev] [PATCH] i965: allocate at least 1 BLEND_STATE element

[Kenneth Graunke]

On Wednesday, July 01, 2015 10:16:28 AM Mike Stroyan wrote:
> When there are no color buffer render targets, gen6 and gen7 still
> use the first BLEND_STATE element to determine alpha test.
> gen6_upload_blend_state was allocating zero elements when
> ctx->Color.AlphaEnabled was false.
> That left _3DSTATE_CC_STATE_POINTERS or _3DSTATE_BLEND_STATE_POINTERS
> pointing to random data from some previous brw_state_batch().
> That sometimes suppressed depth rendering when those bits
> happened to mean COMPAREFUNC_NEVER.
> This produced flickering shadows for dota2 reborn.
> ---
>  src/mesa/drivers/dri/i965/gen6_cc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/gen6_cc.c b/src/mesa/drivers/dri/i965/gen6_cc.c
> index 2bfa271..2b76e24 100644
> --- a/src/mesa/drivers/dri/i965/gen6_cc.c
> +++ b/src/mesa/drivers/dri/i965/gen6_cc.c
> @@ -51,7 +51,7 @@ gen6_upload_blend_state(struct brw_context *brw)
>      * with render target 0, which will reference BLEND_STATE[0] for
>      * alpha test enable.
>      */
> -   if (nr_draw_buffers == 0 && ctx->Color.AlphaEnabled)
> +   if (nr_draw_buffers == 0)
>        nr_draw_buffers = 1;
>  
>     size = sizeof(*blend) * nr_draw_buffers;
> 

Great catch!

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

And pushed:
   9d408a4..fe2b748  master -> master

I think we ought to change gen8_blend_state.c as well, but I'm not quite
sure what change to make.  Either we should make the same change you did
here, or delete the whole "We need at least 1 BLEND_STATE written"
block.

On Gen8+, it looks like the alpha test and other functions that might
discard pixels are all in the shared/common DWord, and the per-color
target DWord pairs look relatively harmless.  I suppose the null RT
would still refer to BLEND_STATE[0]...so it might still be worth
emitting one.  Any thoughts?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: This is a digitally signed message part.
URL: <http://lists.freedesktop.org/archives/mesa-dev/attachments/20150702/81d12c7c/attachment.sig>