mesa-panvk-bifrost: r5 -> r6 — advertise VK_EXT_legacy_dithering on Bifrost

Backports Mesa main's unconditional flip of VK_EXT_legacy_dithering.
Pure-software composition; no new HW path. vk_render_pass already gates
on enabled_features.legacyDithering and panvk_vX_blend + pan_format
already plumb the dithered BLEND descriptor (BFMT2 table has MALI_BLEND_AU
encodings for RGB565/RGB5A1/RGBA4/RGB10A2 on PAN_ARCH 7). Our r5 base
just hadn't picked up the cherry-pick.

Phase 7 verify on ohm (PineTab2 / RK3566 / Mali-G52 r1 MC1) with a
locally-built r6 lib at /tmp/r6_test_lib/:

  Feature advertisement:
    r5: VK_EXT_legacy_dithering not in extension list, legacyDithering=false
    r6: VK_EXT_legacy_dithering rev 2 advertised, legacyDithering=true

  dEQP subsets (delta):
    dEQP-VK.api.*.legacy_dithering*    r5/r6 both:  2 P / 0 F (identical)
    dEQP-VK.renderpass.dithering.*     r5/r6 both:  0 P / 0 F / 94 NS (identical)
    dEQP-VK.renderpass2.dithering.*    r5/r6 both:  0 P / 0 F / 94 NS (identical)

  Net: zero regressions, advertisement-only delta as expected.

Second-model review (per bugfix-process step 4) traced the full code
path through vk_render_pass + panvk_vX_cmd_draw + panvk_vX_blend +
pan_format BFMT2. No interaction with our r1 nullDescriptor (disjoint
paths). Mesa upstream marks ext DONE for panvk in docs/features.txt.

ARM's own libmali r51p0 driver (BXODROIDN2PL, 2024-08) lists
VK_EXT_legacy_dithering in its Vulkan extension string table,
confirming the feature is shipped by ARM for Mali-G52-class hardware.

Follow-up out of scope: the 94 renderpass-dithering tests show as
NotSupported on both r5 and r6 — there's a separate panvk-side
prereq the dEQP harness checks (likely a specific format-feature
combination). Worth investigating in a future iteration.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

arch/mesa-panvk-bifrost/PKGBUILD

@@ -30,7 +30,7 @@
 
 pkgname=mesa-panvk-bifrost
 _mesaver=26.0.6
-pkgver=26.0.6.r5
+pkgver=26.0.6.r6
 pkgrel=1
 pkgdesc="Patched Mesa libvulkan_panfrost.so exposing Bifrost-gen Mali to Vulkan apps (panvk-bifrost campaign)"
 arch=('aarch64')
@@ -82,6 +82,7 @@ source=(
     "0003-panvk-bifrost-vk-ext-transform-feedback.patch"
     "0004-panvk-bifrost-xfb-primitive-decomposition.patch"
     "0005-panvk-bifrost-fragment-stores-atomics.patch"
+    "0006-panvk-bifrost-legacy-dithering.patch"
     "brave-vulkan"
     "icd.json"
 )
@@ -94,6 +95,7 @@ sha256sums=(
     'SKIP'
     'SKIP'
     'SKIP'
+    'SKIP'
 )
 
 prepare() {
@@ -140,6 +142,16 @@ prepare() {
     # dEQP-VK.image.store.* show no new Failed vs r4 baseline.
     patch -p1 < "${srcdir}/0005-panvk-bifrost-fragment-stores-atomics.patch"
 
+    # r6 (2026-05-25): advertise VK_EXT_legacy_dithering. Backports Mesa
+    # main's unconditional flip. Pure-software composition; vk_render_pass
+    # already gates on enabled_features.legacyDithering and panvk_vX_blend
+    # + pan_format already plumb the dithered BLEND descriptor (BFMT2 table
+    # has MALI_BLEND_AU encodings for RGB565/RGB5A1/RGBA4/RGB10A2 on
+    # PAN_ARCH 7). Closes the EXT_legacy_dithering gap surfaced by
+    # marfrit/panvk-bifrost research/r6_r7_*. ARM blob r51p0 confirms the
+    # extension as Mali-G52-architecture supported.
+    patch -p1 < "${srcdir}/0006-panvk-bifrost-legacy-dithering.patch"
+
     # Sanity-check the patches landed.
     grep -q "KHR_robustness2 = true," src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "EXT_robustness2 = true," src/panfrost/vulkan/panvk_vX_physical_device.c
@@ -153,6 +165,9 @@ prepare() {
     grep -q "panvk_per_arch(nir_lower_xfb)" src/panfrost/vulkan/panvk_vX_shader.c
     # r5 sanity: fragmentStoresAndAtomics = true patch landed
     grep -q "fragmentStoresAndAtomics = true ||" src/panfrost/vulkan/panvk_vX_physical_device.c
+    # r6 sanity: VK_EXT_legacy_dithering advertised
+    grep -q '\.EXT_legacy_dithering = true,' src/panfrost/vulkan/panvk_vX_physical_device.c
+    grep -q '\.legacyDithering = true,' src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "xfb_topology" src/panfrost/vulkan/panvk_shader.h
     grep -q "panvk_xfb_topology" src/panfrost/vulkan/panvk_shader.h
     test -f src/panfrost/vulkan/panvk_vX_xfb_lower.c