panfrost: fix earlyzs settings for alpha_to_coverage

When alpha_to_coverage is enabled, the zs_update_operation field
must be set to force_late, according to (some of) the documentation.
Actually the docs are ambiguous; the main thrust is that late coverage
updates are only required when Z or S is written, or when occlusion
queries are enabled. But there is a side note in a table that indicates
force_late should be used for coverage updates even if Z or S is not
written.

Logically this shouldn't be necessary and the note is probably
just lazily written. But it turns out that we do seem to need the
force_late setting on valhall. It's currently unclear whether
there's a hardware issue on valhall, or some other issue.

Fixes piglit ext_framebuffer_multisample-*alpha-to-coverage* tests
on valhall.

Reviewed-by: Lars-Ivar Hesselberg Simonsen <lars-ivar.simonsen@arm.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31181>

src/panfrost/lib/pan_earlyzs.c

@@ -51,9 +51,16 @@ analyze(const struct pan_shader_info *s, bool writes_zs_or_oq,
     * be deferred until the value is known after the ZS_EMIT instruction,
     * if present. ZS_EMIT must precede ATEST, so the value is known when
     * ATEST executes, justifying the late test/update.
+    * Also, if alpha_to_coverage is set that also forces a late update.
+    * NOTE: it's not at all clear why alpha_to_coverage always requires
+    * a late update; the late update should only really be required if
+    * we're writing z or stencil, or testing for occlusion queries.
+    * The docs are somewhat contradictory on this point.
+    * But empirically we observe this requirement on Valhall, and doing
+    * the update later is never wrong (just potentially a bit slower).
     */
    bool shader_writes_zs = (s->fs.writes_depth || s->fs.writes_stencil);
-   bool late_update = shader_writes_zs;
+   bool late_update = shader_writes_zs || alpha_to_coverage;
    bool late_kill = shader_writes_zs;
 
    /* Late coverage updates are required if the coverage mask depends on

src/panfrost/lib/tests/test-earlyzs.cpp

@@ -116,7 +116,7 @@ TEST(EarlyZS, SideFXNoShaderZS)
    CASE(FORCE_EARLY, FORCE_LATE, SIDEFX);
    CASE(FORCE_EARLY, FORCE_LATE, SIDEFX | DISCARD);
    CASE(FORCE_EARLY, FORCE_LATE, SIDEFX | WRITES_COV);
-   CASE(FORCE_EARLY, FORCE_LATE, SIDEFX | ALPHA2COV);
+   CASE(FORCE_LATE, FORCE_LATE, SIDEFX | ALPHA2COV);
 }
 
 TEST(EarlyZS, SideFXNoShaderZSAlt)
@@ -124,20 +124,20 @@ TEST(EarlyZS, SideFXNoShaderZSAlt)
    CASE(WEAK_EARLY, FORCE_LATE, ZS_ALWAYS_PASSES | SIDEFX);
    CASE(WEAK_EARLY, FORCE_LATE, ZS_ALWAYS_PASSES | SIDEFX | DISCARD);
    CASE(WEAK_EARLY, FORCE_LATE, ZS_ALWAYS_PASSES | SIDEFX | WRITES_COV);
-   CASE(WEAK_EARLY, FORCE_LATE, ZS_ALWAYS_PASSES | SIDEFX | ALPHA2COV);
+   CASE(FORCE_LATE, FORCE_LATE, ZS_ALWAYS_PASSES | SIDEFX | ALPHA2COV);
 }
 
 TEST(EarlyZS, NoSideFXNoShaderZS)
 {
    CASE(FORCE_EARLY, FORCE_EARLY, 0);
-   CASE(FORCE_EARLY, FORCE_EARLY, ALPHA2COV | DISCARD | WRITES_COV);
+   CASE(FORCE_LATE, FORCE_EARLY, ALPHA2COV | DISCARD | WRITES_COV);
    CASE(FORCE_EARLY, FORCE_EARLY, ZS_WRITEMASK);
 }
 
 TEST(EarlyZS, NoSideFXNoShaderZSAlt)
 {
    CASE(WEAK_EARLY, WEAK_EARLY, ZS_ALWAYS_PASSES);
-   CASE(WEAK_EARLY, WEAK_EARLY,
+   CASE(FORCE_LATE, WEAK_EARLY,
         ZS_ALWAYS_PASSES | ALPHA2COV | DISCARD | WRITES_COV);
    CASE(WEAK_EARLY, WEAK_EARLY, ZS_ALWAYS_PASSES | ZS_WRITEMASK);
 }