Module pass_timing 

Per-pass GPU timing on Vulkan via TIMESTAMP queries. The query pool holds one per-frame block of SLOTS_PER_FRAME slots; the whole-frame timer lives in slots [0, 1] of each block and one (start, end) pair per PassId follows (start at slot 2 + 2i, end at slot 3 + 2i). Mirrors directx/pass_timing.rs.

The start buffer resets the whole block and writes the whole-frame start; each per-pass command buffer writes its own (start, end) pair around its encode; the end buffer writes the whole-frame end. The CPU reads the previous trip’s block at the top of draw_frame (after the matching fence wait gates the GPU writes) and publishes the per-pass microseconds into RenderStats.pass_times_us.

Vulkan note. Unlike D3D12 (which can pre-write every slot so a pass that did not run still reads a value), Vulkan forbids writing a timestamp to a query that is already written without an intervening reset. So a pass absent from this frame’s graph leaves its (reset-but-unwritten) slots unavailable; the readback uses WITH_AVAILABILITY and reports 0 for any slot whose pair is not both available. The shared StatHud.passes_text then filters the zero slots.

Layout reasoning. Keeping the whole-frame pair at the front of each block lets the existing gpu_frame_us readback stay the first pair of the frame’s block; only the per-frame stride changes (from 2 to SLOTS_PER_FRAME).

This is GPU-free slot-index arithmetic (no ash/vk types), so it lives in concinnity-render and its layout tests count toward coverage; the Vulkan backend re-exports it under crate::vulkan::pass_timing.

Constants

SLOTS_PER_FRAME

Per-frame block: [whole_frame_start, whole_frame_end, pass0_start, pass0_end, …, pass(PASS_COUNT-1)_start, pass(PASS_COUNT-1)_end]. 2 * (PASS_COUNT + 1) u64 query slots.

Functions

frame_block_base

First query slot of frame frame’s block.

pass_pair

(start, end) query slots for pass within frame’s block.

whole_frame_pair

(start, end) query slots for the whole-frame pair of frame. Matches the legacy layout (whole-frame at the first pair of each block).