[Mesa-dev] [PATCH] i915: Always emit W on gen3

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Unlike the older gen2 hardware, gen3 performs perspective
correct interpolation even for the primary/secondary colors.
To do that it naturally needs us to emit W for the vertices.

Currently we emit W only when at least one texture coordinate
set gets emitted. This means the interpolation of color will
change depending on whether texcoords/varyings are used or not.
That's probably not what anyone would expect, so let's just
always emit W to get consistent behaviour. Trying to avoid
emitting W seems like more hassle than it's worth, especially
as bspec seems to suggest that the hardware will perform the
perspective division anyway.

This used to be broken until it was accidentally fixed it in
commit c349031c27b7 ("i915: Fix texcoord vs. varying collision
in fragment programs") by introducing a bug that made the driver
always emit W. After fixing that bug in commit c1eedb43f32f
("i915: Fix wpos_tex vs. -1 comparison") we went back to the
old behaviour and caused an apparent regression.

Fixes: c1eedb43f32f ("i915: Fix wpos_tex vs. -1 comparison")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101451
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 src/mesa/drivers/dri/i915/i915_fragprog.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/src/mesa/drivers/dri/i915/i915_fragprog.c b/src/mesa/drivers/dri/i915/i915_fragprog.c
index e19ca60bd334..3657b2d82565 100644
--- a/src/mesa/drivers/dri/i915/i915_fragprog.c
+++ b/src/mesa/drivers/dri/i915/i915_fragprog.c
@@ -1252,12 +1252,10 @@ i915ValidateFragmentProgram(struct i915_context *i915)
    intel->coloroffset = 0;
    intel->specoffset = 0;
 
-   if (inputsRead & VARYING_BITS_TEX_ANY || p->wpos_tex != I915_WPOS_TEX_INVALID) {
-      EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_4F_VIEWPORT, S4_VFMT_XYZW, 16);
-   }
-   else {
-      EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_3F_VIEWPORT, S4_VFMT_XYZ, 12);
-   }
+   /* Always emit W to get consistent perspective
+    * correct interpolation of primary/secondary colors.
+    */
+   EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_4F_VIEWPORT, S4_VFMT_XYZW, 16);
 
    /* Handle gl_PointSize builtin var here */
    if (ctx->Point._Attenuated || ctx->VertexProgram.PointSizeEnabled)
-- 
2.13.0