WP1+WP2 of design/desktop-presentation-rebuild.md. The shared Linux/Windows
session client presented arrival-paced with no pacing layer at all: two depth-2
newest-wins hops into a drain-to-newest and an immediate present. That IS the
lowest-latency intent, but it was unnamed, unselectable, and had no alternative
— and on a surface without MAILBOX (AMD's Windows driver offers none, and any
compositor holding images does the same) the swapchain's own FIFO becomes a
standing queue worth a measured 11-13 ms at 60 Hz.
WP1 — the settings cluster, under the keys the Apple client already writes into
the shared profile catalog (present_priority / smooth_buffer / vsync /
allow_vrr): mismatched names would ride SettingsOverlay::extra, carried but
never applied. PresentPriority::resolve mirrors the Android reference exactly
(anything but an explicit "smooth" is latency; a buffer outside 1..=3 becomes
2), so a profile authored on any client means the same thing on all of them.
Only the first two are consumed here; vsync/allow_vrr land in WP3.
WP2 — the engine (present_pace.rs, pure state + arithmetic, 6 tests):
- FrameStore: newest-wins slot, or the smoothing FIFO with preroll-to-capacity,
drop-oldest overflow, and an underflow that re-arms the preroll (repeat by
omission) — the Apple/Android semantics, with qDrop/qDry counters.
- LatchClock: the panel grid learned from VK_KHR_present_wait glass stamps,
min positive spacing capped by the mode refresh (measured, never queried —
VRR and Android's per-uid refresh lie both punish trusting a reported rate).
It now also publishes the host-facing LatchGrid, so the phase-lock report and
the local scheduler cannot disagree about the grid.
- PresentGate: one undisplayed present in flight on FIFO surfaces, with the
100 ms stale force-open. This is the standing-queue killer, and it is inert
on MAILBOX/IMMEDIATE and without present timing — where behaviour stays
byte-for-byte the shipped arrival pacing.
Wiring: glass samples drain every pass (a 1 Hz batch would starve clock and
gate) and the waiter pushes an SDL wake, so a gate reopen never waits out the
event timeout; smoothness serves one frame per latch slot and tightens the
loop's wait to that deadline; the adaptive slot margin starts at 0 and widens
+500 us per missed window toward 2.5 ms (a fixed lead was measured to be pure
display tax). PUNKTFUNK_PRESENTER=arrival disables the whole engine for field
A/B without a rebuild.
PyroWave collapses smoothness to latency for the stream: its plane-ring
retirement accounting assumes the depth-2 newest-wins hand-off, and all-intra
frames make buffering moot anyway.
Gates (punktfunk-rust-ci, linux/amd64, sources touched first so a warm target
cannot print a vacuous Finished): clippy -D warnings across pf-client-core,
pf-presenter and punktfunk-client-session; 80 + 32 tests pass; rustfmt clean.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
enricobuehlerandClaude Opus 5
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