Mesa (master): i965: Fix GPU hang related to multiple render targets and alpha testing

[Anuj Phogat]

Module: Mesa
Branch: master
Commit: b9df2251c17e3ce52fa55c81f492591e08c3ee04
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=b9df2251c17e3ce52fa55c81f492591e08c3ee04

Author: Anuj Phogat <anuj.phogat at gmail.com>
Date:   Mon Oct 24 16:03:00 2016 -0700

i965: Fix GPU hang related to multiple render targets and alpha testing

This patch should have been the part of commit e592f7df.
In a situation when there are multiple render targets with alpha testing
enabled, if fragment shader doesn't write to draw buffer zero, it causes
the GPU hang on SKL. No GPU hang is seen on HSW. Simulator gives a
warning for all gen6+ h/w:
"Illegal render target write message length 0xa expected 0xc"

This patch fixes the GPU hang as well as the simulator warning with
new piglit test fbo-mrt-alphatest-no-buffer-zero-write:
https://patchwork.freedesktop.org/patch/118212

No regressions in Jenkins CI system.

Cc: "12.0 13.0" <mesa-stable at lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat at gmail.com>
Reviewed-by: Ben Widawsky <ben at bwidawsk.net>

---

 src/mesa/drivers/dri/i965/brw_fs.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index f839d36..3cdf0bf 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -3673,6 +3673,12 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
        */
       setup_color_payload(bld, key, &sources[length], src0_alpha, 1);
       length++;
+   } else if (key->replicate_alpha && inst->target != 0) {
+      /* Handle the case when fragment shader doesn't write to draw buffer
+       * zero. No need to call setup_color_payload() for src0_alpha because
+       * alpha value will be undefined.
+       */
+      length++;
    }
 
    setup_color_payload(bld, key, &sources[length], color0, components);