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v3d/compiler: fix V3D double-rounding of .8 fixed-point XY coordinates

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Pre-V3D 4.3 hardware has a quirk where it expects XY coordinates in
.8 fixed-point format, but then it will internally round it to .6 fixed-point,
introducing a double rounding. The double rounding can cause very slight
differences in triangle raterization coverage that can actually be noticed by
some CTS tests.

The correct fix for this as recommended by Broadcom is to convert to
.8 fixed-point with ffloor().

Fixes:
dEQP-VK.renderpass.suballocation.subpass_dependencies.late_fragment_tests.*

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6677>
This commit is contained in:
Iago Toral Quiroga 2020-09-10 10:36:27 +02:00
parent 367ac07efc
commit 3182209673
1 changed files with 12 additions and 1 deletions
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13
src/broadcom/compiler/v3d_nir_lower_io.c
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@ -524,7 +524,18 @@ v3d_nir_emit_ff_vpm_outputs(struct v3d_compile *c, nir_builder *b,
scale = nir_load_viewport_y_scale(b);
pos = nir_fmul(b, pos, scale);
pos = nir_fmul(b, pos, rcp_wc);
pos = nir_f2i32(b, nir_fround_even(b, pos));
/* Pre-V3D 4.3 hardware has a quirk where it expects XY
* coordinates in .8 fixed-point format, but then it
* will internally round it to .6 fixed-point,
* introducing a double rounding. The double rounding
* can cause very slight differences in triangle
* raterization coverage that can actually be noticed by
* some CTS tests.
*
* The correct fix for this as recommended by Broadcom
* is to convert to .8 fixed-point with ffloor().
*/
pos = nir_f2i32(b, nir_ffloor(b, pos));
v3d_nir_store_output(b, state->vp_vpm_offset + i,
offset_reg, pos);
}