[Mesa-stable] [PATCH] i965: Fix GPU hang related to multiple render targets and alpha testing

[Anuj Phogat]

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>
---
 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 921cc00..afb1057 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);
-- 
2.5.5