intel/fs/gen12: Fix hangs with per-sample SIMD32 fragment shader dispatch.

The Gen12 docs are rather contradictory regarding the dispatch
configurations supported by the fragment shader -- The same table
present in previous generations seems to imply that only one dispatch
mode can be enabled when doing per-sample shading, but a restriction
documented in the 3DSTATE_PS_BODY page implies the opposite: That
SIMD32 can only be used in combination with some other dispatch mode.

The latter seems to match the behavior of real hardware as I could
tell from my testing: A bunch of multisample test-cases that do
per-sample shading hang if we only provide a SIMD32 shader.

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

diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 0fa0be683f043c21464daa78344e496f6e4ee2c5..0c6fe0bbf3b60c14550f1b5403129e5da212a32b 100644 (file)
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -8793,13 +8793,20 @@ brw_compile_fs(const struct brw_compiler *compiler, void *log_data,
    if (prog_data->persample_dispatch) {
       /* Starting with SandyBridge (where we first get MSAA), the different
        * pixel dispatch combinations are grouped into classifications A
-       * through F (SNB PRM Vol. 2 Part 1 Section 7.7.1).  On all hardware
+       * through F (SNB PRM Vol. 2 Part 1 Section 7.7.1).  On most hardware
        * generations, the only configurations supporting persample dispatch
-       * are are this in which only one dispatch width is enabled.
+       * are those in which only one dispatch width is enabled.
+       *
+       * The Gen12 hardware spec has a similar dispatch grouping table, but
+       * the following conflicting restriction applies (from the page on
+       * "Structure_3DSTATE_PS_BODY"), so we need to keep the SIMD16 shader:
+       *
+       *  "SIMD32 may only be enabled if SIMD16 or (dual)SIMD8 is also
+       *   enabled."
        */
       if (simd32_cfg || simd16_cfg)
          simd8_cfg = NULL;
-      if (simd32_cfg)
+      if (simd32_cfg && devinfo->gen < 12)
          simd16_cfg = NULL;
    }