forked from unom/punktfunk
enricobuehlerandClaude Opus 5
6b3c582eb1
`VK_PRESENT_MODE_FIFO_LATEST_READY_EXT` is FIFO's tear-free vblank pacing that
presents the LATEST READY image at each refresh and retires the older ones,
instead of draining a queue. That is precisely what the software glass gate
emulates — so where the driver offers it, the driver does the job, and it does
it exactly where the gate matters most: a surface with no MAILBOX gets
newest-wins behaviour back without the app holding frames.
Found by asking the surface what it actually offers rather than trusting a
comment: the previous commit's `surface present modes` line read back
`[MAILBOX, 1000361000, FIFO]` on NVIDIA/Wayland, and 1000361000 is this mode.
The extension postdates the Vulkan headers ash 0.38 is generated from (1.3.281),
so there is no binding — hence the bare number in the log. It is hand-declared
here: mode value, extension name, and
`VkPhysicalDevicePresentModeFifoLatestReadyFeaturesEXT` spliced into the device
pNext chain. One trap worth naming: the SURFACE advertises the mode even with
the extension disabled, and using it on that basis is undefined — so the ladder
only offers it when the device feature actually came back true and we enabled it.
The gate/probe predicate had to split in two, and the distinction is the point:
* `needs_glass_gate()` — FIFO and FIFO_RELAXED only. NOT this mode: gating on
top of a driver that already retires stale images would hold frames back to
emulate something the presentation engine is doing, paying the serialisation
twice, which is the ~27 ms the last commit measured.
* `vblank_locked()` — the whole FIFO family INCLUDING this mode, because it
still presents on the refresh boundary, so the VRR cadence probe's premise
("with VRR off, a present waits for vblank") still holds.
Ranking: MAILBOX first (measured good at 1.4 ms), then LATEST_READY, then plain
FIFO — so a MAILBOX-less surface reaches newest-wins in the driver rather than
in our gate.
MEASURED ON GLASS (.21, NVIDIA 610.43.03, GNOME/Wayland): the extension probe,
feature enable and swapchain creation all succeed with a mode ash has no binding
for. Default ladder selects MAILBOX with `fifo_latest_ready=true`; the VRR ladder
selects `present_mode=1000361000` and measures `display 2.6 ms (pace 0.6 + latch
2.0)` — against 13-28 ms for plain FIFO + gate on the same box. The vblank-locked
path is now MAILBOX-class.
That changes the previous commit's reversal. The VRR ladder was reverted to
opt-in because it led with plain FIFO and cost ~27 ms; led with LATEST_READY it
costs 0.6 ms over MAILBOX. So `allow_vrr` is automatic again WHERE THE DEVICE
OFFERS THE MODE, and stays behind `PUNKTFUNK_VRR_FIFO=1` where it does not — on
those drivers the ladder would fall back to plain FIFO and the regression
returns. Both branches are pinned by tests. This also retires a dead switch: the
"Follow variable refresh rate" row did nothing at all after the reversal, and now
does something real on any driver with the extension.
⚠ Still unverified off this box: whether Windows and Intel drivers expose the
mode at all. Nothing measured here carries over — Windows Vulkan WSI goes through
DXGI, so exposing the enum and mapping it usefully onto flip-model semantics are
separate questions, and Intel is a different vendor stack again. Both facts are
logged unconditionally now (`surface present modes` + `fifo_latest_ready=`), so
one run on any box settles it. The code is safe either way: the mode is only
requested where the device feature enabled, and `allow_vrr` only goes automatic
there — everywhere else the shipped MAILBOX-first behaviour is unchanged.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>