panvk-bifrost/mesa-panvk-bifrost/phase0_findings_iter5.md

Phase 0 — substrate for iter5

Opened 2026-05-19 after iter4 close GREEN.

Locked research question — iter5

Render a non-fullscreen triangle into a 64×64 R8G8B8A8_UNORM attachment via a vertex buffer + UBO. Vertex buffer: 3 vertices, each (pos vec2 + color vec3) = 20 bytes (with 8-byte align padding → 32-byte stride). UBO: single mat4 transform (scaling 0.8 in x/y, identity otherwise). Triangle in scaled-NDC: v0(-0.5,-0.5) red, v1(0.5,-0.5) green, v2(0,0.5) blue. Fragment shader outputs interpolated color (mixed RGB at centroid).

Verify:

1. Centroid pixel (32, 28) has all of R, G, B > 0x10 (interpolated, non-black).
2. Top-left pixel (0, 0) is exactly 0x00000000 (clear, outside triangle).
3. Top-right pixel (63, 0) is exactly 0x00000000 (clear, outside triangle).
4. Covered pixel count (non-clear pixels) ∈ [800, 1600] (triangle area ≈ 1310 pixels).

If GREEN: iter6 stress-tests with a multi-draw scene or a Zink-on-PanVk smoke. If RED: characterize vertex input / UBO descriptor / NIR varying interpolation.

Why this shape

iter4 closed the descriptor model for fragment-stage texture binding. iter5 adds vertex-stage descriptor binding + vertex input (the vertex-side counterpart). What's new:

• Vertex buffer (VK_BUFFER_USAGE_VERTEX_BUFFER_BIT) + vkCmdBindVertexBuffers
• VkPipelineVertexInputStateCreateInfo with non-empty bindings + attributes (pos location 0, color location 1)
• UBO (VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER) bound to vertex stage
• Vertex shader reads attribute layouts + UBO via descriptor
• Interpolated varying (color) from vertex → fragment

Hypothesis space

1. Vertex input bindings on Bifrost. Bifrost's attribute descriptor model has been a divergence point per panvk_vX_cmd_draw.c's PANVK_BIFROST_DESC references. First time we exercise non-zero VkPipelineVertexInputStateCreateInfo.
2. UBO descriptor binding for vertex stage. Different from iter4's fragment-stage COMBINED_IMAGE_SAMPLER.
3. Vertex-stage descriptor lowering in NIR (Bifrost-specific code paths).
4. Varying interpolation (color) from vertex output → fragment input.
5. UBO data fetch — does the GPU actually read the matrix from the bound buffer correctly?
6. Non-fullscreen rasterization — partial coverage, edge pixels, anti-aliased-or-not boundaries on Bifrost's tile binner.

In-scope (LOCKED 2026-05-19 for iter5)

• 1 vertex buffer (3 verts), interleaved pos+color, 32-byte stride.
• 1 UBO (64 bytes, mat4).
• 1 descriptor set with 1 UBO binding bound to vertex stage.
• Triangle in NDC, scaled by 0.8 via UBO matrix.
• 4-point pixel-level verification + range-bound coverage count.

Out-of-scope (LOCKED 2026-05-19 for iter5)

• Index buffer / vkCmdDrawIndexed.
• Multiple draws.
• Push constants.
• Texture sampling (iter4 already covered).
• Depth / stencil.
• Blending (clear=opaque-black, triangle has α=1).
• MSAA.
• Compressed formats.
• Mipmaps.
• Real workloads (vkcube/vkmark/Zink) — that's iter6+.

Reference

• Prior closes: iter1, iter2, iter3, iter4.