[Mesa-dev] [PATCH] i965: Implement proper workaround for Gen4 GPU CONSTANT_BUFFER hangs.

[Matt Turner]

On Mon, Apr 13, 2015 at 1:26 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> I finally managed to dig up some information on our mysterious GPU hangs.
> A wiki page from the Crestline validation team mentions that they found
> a GPU hang in "Serious Sam 2" (on Windows) with remarkably similar
> conditions to the ones we've seen in Google Chrome and glmark2.
>
> Apparently, if WM_STATE has "PS Use Source Depth" enabled, CC_STATE has
> most depth state disabled, and you issue a CONSTANT_BUFFER command and
> immediately draw, the depth interpolator makes a small mistake that
> leads to hangs.
>
> Most of the traces I looked at contained a CONSTANT_BUFFER packet
> immediately followed by 3DPRIMITIVE, or at least very few packets.
> It appears they also have "PS Use Source Depth" enabled - either at the
> hang, or a little before it.  So I think this is our bug.
>
> The workaround is to emit a non-pipelined state packet after issuing a
> CONSTANT_BUFFER packet.  This is really similar to the workaround I
> developed in commit c4fd0c9052dd391d6f2e9bb8e6da209dfc7ef35b.
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_curbe.c | 39 +++++++++++++++++++++++------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_curbe.c b/src/mesa/drivers/dri/i965/brw_curbe.c
> index e45e2ab..a773a89 100644
> --- a/src/mesa/drivers/dri/i965/brw_curbe.c
> +++ b/src/mesa/drivers/dri/i965/brw_curbe.c
> @@ -285,19 +285,6 @@ brw_upload_constant_buffer(struct brw_context *brw)
>      */
>
>  emit:
> -   /* Work around mysterious 965 hangs that appear to happen if you do
> -    * two 3DPRIMITIVEs with only a CONSTANT_BUFFER inbetween.  If we
> -    * haven't already flushed for some other reason, explicitly do so.
> -    *
> -    * We've found no documented reason why this should be necessary.
> -    */
> -   if (brw->gen == 4 && !brw->is_g4x &&
> -       (brw->ctx.NewDriverState & (BRW_NEW_BATCH | BRW_NEW_PSP)) == 0) {
> -      BEGIN_BATCH(1);
> -      OUT_BATCH(MI_FLUSH);
> -      ADVANCE_BATCH();
> -   }
> -
>     /* BRW_NEW_URB_FENCE: From the gen4 PRM, volume 1, section 3.9.8
>      * (CONSTANT_BUFFER (CURBE Load)):
>      *
> @@ -317,6 +304,31 @@ emit:
>                 (brw->curbe.total_size - 1) + brw->curbe.curbe_offset);
>     }
>     ADVANCE_BATCH();
> +
> +   /* Work around a Broadwater/Crestline depth interpolator bug.  The following
> +    * sequence will cause GPU hangs:
> +    *
> +    * 1. Change state so that all depth related fields in CC_STATE are disabled,
> +    *    and in WM_STATE, only "PS Use Source Depth" is enabled.
> +    * 2. Emit a CONSTANT_BUFFER packet.
> +    * 3. Draw via 3DPRIMITIVE.
> +    *
> +    * The recommended workaround is to emit a non-pipelined state change after
> +    * emitting CONSTANT_BUFFER, in order to drain the windowizer pipeline.
> +    *
> +    * We arbitrarily choose 3DSTATE_GLOBAL_DEPTH_CLAMP_OFFSET (as it's small),
> +    * and always emit it when "PS Use Source Depth" is set.  We could be more
> +    * precise, but the additional complexity is probably not worth it.
> +    *
> +    * BRW_NEW_FRAGMENT_PROGRAM
> +    */
> +   const struct gl_program *fp = &brw->fragment_program->Base;
> +   if (brw->gen == 4 && !brw->is_g4x && (fp->InputsRead & (1 << VARYING_SLOT_POS))) {

Probably want to line wrap here after the second &&.

Reviewed-by: Matt Turner <mattst88 at gmail.com>