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
0eef03a i915: Always emit W on gen3
src/mesa/drivers/dri/i915/i915_fragprog.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)