marfrit-packages/arch/kwin-fourier

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_resv exclusive fences (see kernel layer in the top-level README) and dma_buf_export_sync_file substitutes dma_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:

1. Build kwin-fourier with 0002 instead of 0001 (one PKGBUILD line change).
2. Install on ohm; restart Plasma session so the new kwin_wayland is mapped.
3. 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.
4. Capture before/after dma_buf_export_sync_file syscall counts via strace -c on kwin_wayland (the per-frame syscall savings are the patch's claimed benefit).
5. 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.