i965/gs: Set the REORDER bit in 3DSTATE_GS.

Ivy Bridge's "reorder enable" bit gives us a binary choice for the
order in which vertices from triangle strips are delivered to the
geometry shader.  Neither choice follows the OpenGL spec, but setting
the bit is better, because it gets triangle orientation correct.

Haswell replaces the "reorder enable" bit with a new "reorder mode"
bit (which occupies the same location in the command packet).  This
bit gives us a different binary choice, which affects both triangle
strips and triangle strips with adjacency.  Setting the bit ("reorder
trailing") gives the proper order according to the OpenGL spec.

So in either case we want to set the bit.

On Ivy Bridge, fixes piglit test "triangle-strip-orientation".

On Haswell, fixes piglit tests "glsl-1.50-geometry-primitive-types
{GL_TRIANGLE_STRIP,GL_TRIANGLE_STRIP_ADJACENCY}" and
"glsl-1.50-geometry-tri-strip-ordering-with-prim-restart *".

v2: Rename the bit to "REORDER_TRAILING" for consistency with Haswell
docs.

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

2 files changed, 29 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_defines.h b/src/mesa/drivers/dri/i965/brw_defines.h
index c1e7f31fe8b..5ba9d45c9e8 100644
--- a/src/mesa/drivers/dri/i965/brw_defines.h
+++ b/src/mesa/drivers/dri/i965/brw_defines.h
@@ -1381,6 +1381,7 @@ enum brw_message_target {
 # define GEN6_GS_SO_STATISTICS_ENABLE			(1 << 9)
 # define GEN6_GS_RENDERING_ENABLE			(1 << 8)
 # define GEN7_GS_INCLUDE_PRIMITIVE_ID			(1 << 4)
+# define GEN7_GS_REORDER_TRAILING			(1 << 2)
 # define GEN7_GS_ENABLE					(1 << 0)
 /* DW6 */
 # define HSW_GS_CONTROL_DATA_FORMAT_SHIFT		31
diff --git a/src/mesa/drivers/dri/i965/gen7_gs_state.c b/src/mesa/drivers/dri/i965/gen7_gs_state.c
index 3dd5896eee8..c272b7d8b57 100644
--- a/src/mesa/drivers/dri/i965/gen7_gs_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_gs_state.c
@@ -105,6 +105,33 @@ upload_gs_state(struct brw_context *brw)
          (0 << GEN6_GS_URB_ENTRY_READ_OFFSET_SHIFT) |
          (prog_data->dispatch_grf_start_reg <<
           GEN6_GS_DISPATCH_START_GRF_SHIFT);
+
+      /* Note: the meaning of the GEN7_GS_REORDER_TRAILING bit changes between
+       * Ivy Bridge and Haswell.
+       *
+       * On Ivy Bridge, setting this bit causes the vertices of a triangle
+       * strip to be delivered to the geometry shader in an order that does
+       * not strictly follow the OpenGL spec, but preserves triangle
+       * orientation.  For example, if the vertices are (1, 2, 3, 4, 5), then
+       * the geometry shader sees triangles:
+       *
+       * (1, 2, 3), (2, 4, 3), (3, 4, 5)
+       *
+       * (Clearing the bit is even worse, because it fails to preserve
+       * orientation).
+       *
+       * Triangle strips with adjacency always ordered in a way that preserves
+       * triangle orientation but does not strictly follow the OpenGL spec,
+       * regardless of the setting of this bit.
+       *
+       * On Haswell, both triangle strips and triangle strips with adjacency
+       * are always ordered in a way that preserves triangle orientation.
+       * Setting this bit causes the ordering to strictly follow the OpenGL
+       * spec.
+       *
+       * So in either case we want to set the bit.  Unfortunately on Ivy
+       * Bridge this will get the order close to correct but not perfect.
+       */
       uint32_t dw5 =
          ((brw->max_gs_threads - 1) << max_threads_shift) |
          (brw->gs.prog_data->control_data_header_size_hwords <<
@@ -113,6 +140,7 @@ upload_gs_state(struct brw_context *brw)
          GEN6_GS_STATISTICS_ENABLE |
          (brw->gs.prog_data->include_primitive_id ?
           GEN7_GS_INCLUDE_PRIMITIVE_ID : 0) |
+         GEN7_GS_REORDER_TRAILING |
          GEN7_GS_ENABLE;
       uint32_t dw6 = 0;