test0r
bc2c97d16e
0002's upstream-shape patch turned out to be semantically equivalent to stock (sync_file snapshots fences at export, polling the dmabuf fd tracks later-added fences — the patch swaps one ioctl for one F_DUPFD with a behaviour change Mutter explicitly moved away from on amdgpu). Hosts running 0002 are effectively stock and stall during V4L2 stateless playback because the producer doesn't populate dma_resv exclusive fences, so the sync_file is a stub and KWin waits forever. 0001 (bypass the watchDmaBuf wait entirely) is the working downstream carry. Trade-off is that frames can present before the producer signals, but on V4L2 stateless hardware where no fence is populated at all, there's nothing to wait for in practice. Upstream fix lives in the linux-media RFC (vb2 dma_resv producer fences, sent 2026-04-29). Once that lands and producer drivers opt in, the 0001 carry can be retired and the upstream-shape 0002 reconsidered. Both patches stay on disk; 0002 is not being upstreamed (MR 9157 closed on reviewer feedback).
kwin-fourier
A diagnostic patch that no-ops Transaction::watchDmaBuf in KWin
6.6.4. Test fixture, not the upstream-bound shape.
Background
KWin's Transaction::watchDmaBuf
(src/wayland/transaction.cpp:265) calls
DMA_BUF_IOCTL_EXPORT_SYNC_FILE on every plane of every imported
dmabuf and parks the transaction on a QSocketNotifier(POLLIN)
waiting for the resulting sync_file fd to become readable.
For V4L2-produced dmabufs (hantro / rockchip-rga and any other vb2 driver), that fence either:
- Stub-signals immediately, because vb2 doesn't populate
dma_resvexclusive fences (see kernel layer in the top-level README) anddma_buf_export_sync_filesubstitutesdma_fence_get_stub(). Pure latency cost: a synchronous ioctl + socket-notifier setup per frame, for a fence that signals in microseconds and represents nothing real. - Signals very late or not at all, on edge cases that we hit during the chromium-fourier validation campaign. KWin's transaction parks indefinitely; the previous wl_buffer never gets released to the client; the client's V4L2 capture pool starves; hard stall.
Patches
This directory carries two patches — the PKGBUILD applies only
0001 for now (validated on ohm), while 0002 is the
upstream-bound shape staged here for later validation and
submission.
0001-transaction-bypass-watchDmaBuf-fence-wait.patch (currently shipped)
No-ops Transaction::watchDmaBuf entirely. Every transaction
commits without waiting on implicit-sync fences for the dmabufs
it imports. Test fixture, validated end-to-end on ohm: the
patch unblocks chromium-fourier 1080p30 H.264 playback under KDE
Plasma 6.6.4 Wayland on RK3566 + panfrost + mainline 6.19.
0002-transaction-poll-dmabuf-fd-directly-upstream-shape.patch (unvalidated, upstream-bound)
Rewrites Transaction::watchDmaBuf to call poll(POLLIN) on the
dmabuf fd directly via a duplicated fd in a QSocketNotifier,
instead of going through DMA_BUF_IOCTL_EXPORT_SYNC_FILE plus a
sync_file fd. The dma-buf core has supported polling the dmabuf fd
for implicit-sync write fences since the introduction of the
feature; the export-then-poll round-trip is per-frame syscall
overhead with no semantic difference.
This shape preserves KWin's defense — the wait still actually waits on the producer's fence — while shedding the per-frame overhead. It is not validated yet and is offered here as the shape upstream review will likely converge on. Validation gates before swapping the PKGBUILD to apply 0002 instead of 0001:
- Build kwin-fourier with 0002 instead of 0001 (one PKGBUILD line change).
- Install on ohm; restart Plasma session so the new
kwin_waylandis mapped. - Run chromium-fourier + bbb sample as before. Expectation: plays through end-to-end at the same ~81 % combined CPU. Equivalence with 0001 confirms the upstream shape works without weakening defenses.
- Capture before/after
dma_buf_export_sync_filesyscall counts viastrace -conkwin_wayland(the per-frame syscall savings are the patch's claimed benefit). - Submit to invent.kde.org/plasma/kwin against
master.
Why patch 0001 is not the upstream-bound shape
Wayland's security model is "compositor trusts no client" —
watchDmaBuf is a defense against a misbehaving client that attaches
a buffer the GPU is still writing. The blanket no-op makes a
correctness-equivalent assumption (every Wayland client honors the spec) that KWin maintainers are reasonably unwilling to take
unconditionally.
The upstream-correct fix lives in the kernel (vb2 / hantro /
rga don't populate dma_resv — fix that, KWin's wait now works
correctly because the fence is real). Once the kernel side lands,
KWin can either keep its current wait (now correct) or migrate to
poll(POLLIN) directly on the dmabuf fd, skipping the
EXPORT_SYNC_FILE ioctl.
The kwin-fourier patch in this repo is the diagnostic that identified the kernel bug and lets the chromium-fourier validation proceed today on stock kernel + KWin. It will be rewritten or removed once the kernel side is upstream.
Building / installing
makepkg -si
The PKGBUILD inherits from upstream Arch's kwin 6.6.4-1, applies
the single watchDmaBuf bypass, and bumps epoch=1 to dominate
upstream pkgrel.
Side effect
Across the test session, every wp_linux_dmabuf client on the compositor (chrome, brave, mpv, VLC, …) feels markedly snappier on Mali-class hardware because the per-frame sync_file roundtrip is gone. A pleasant accident; the cleaner, kernel-side fix will preserve the speedup without weakening the defense.