The user's ask: 'put in our own frame rate timer that doesn't drop
frames so we are not depending on the OS.' The OS can still own the
display latch (SF) — render-ahead presents ~1 ms after the edge with
~7 ms margin, so owning the RENDER clock is sufficient.
WHILE ANIMATING, the UI thread no longer waits on any OS wake:
* poll is non-blocking (timeout 0); the loop spins at ~3-4k iters/s
(250 us hrtimer park after each no-render iteration — an hrtimer
park cannot be swallowed the way ALooper timeouts were, measured
4-6 ms late from inside kernel/epoll)
* the render decision is grid-driven: render when in a not-yet-
rendered slot AND >=0.3 ms past its edge. No saw_wake.
* the Choreographer + eventfd remain only as the GRID SENSOR.
Two grid bugs found and fixed while landing this (both made the first
driver build DOUBLES 8-10% of frames):
* ANCHOR POLICY (wgpu_30_surface::note_vsync_edge): VSYNC_EDGE_NS is
a SMOOTH grid anchor, not 'last observed edge'. A callback that
fits the extrapolation (<1.5 ms deviation) does not move the
anchor — a LATE callback re-anchoring shifts the anchor by
(interval - P) and makes an already-rendered slot look NEW to a
value-space guard. Period updates now require single-slot deltas.
* BOOKKEEPING in SLOT-INDEX space (last_rendered_slot), not edge
values, and ALL render paths mark it: locked accept, unlocked
dirty render, and the render-ahead FLUSH (the flush not marking
caused the +1.1/+1.2 true doubles after animating flickers).
* POINTER window 200 -> 400 ms: bursty input (adb drags; finger
pauses) expired 'animating' mid-drag, firing the locked->unlocked
flush on top of the dirty render (double per flicker, ~5%).
* flush now also skips when the iteration already rendered.
Phone, sltest (3s+1s+3s adb drags, jank_eval referee):
buildC (pre-wake-fd-fix): 127 drops + 12 stalls
slt14-spin (this driver): 2 drops + 1 stall + 8 dbl + 4 late
in 978 frames; phase p10-p90 0.86-0.90 ms
gallery (real textures, same driver): drag windows 6+2 drops,
7+11 doubles in 820 frames; phase p10-p90 0.73-0.95 ms
Remaining (visible next): drag-START warmup (first frame 20-40 ms:
CPU clock ramp after idle — the 'fling start jank' the user reports)
and isolated 2-slot drops (gallery: FullCache insert/eviction cost on
the UI thread).
Cost: ~60-70% of one core during active scroll; the perpetual
runnability also forces EAS big-core placement (the thread was
measured at 820 MHz on little cores during scroll — a large part of
the gallery's finger jank).
Greg Pomerantz
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