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Mirror github.com/marfrit/fourier kwin-fourier/0002 + README update to the local Arch package source. PKGBUILD unchanged — still applies 0001 (the diagnostic bypass). The 0002 patch (poll dmabuf fd directly, drop the EXPORT_SYNC_FILE + sync_file roundtrip) is staged for validation; when validated, swap the source array entry in PKGBUILD from 0001 to 0002 and rebuild.
111 lines
4.6 KiB
Markdown
111 lines
4.6 KiB
Markdown
# kwin-fourier
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A diagnostic patch that no-ops `Transaction::watchDmaBuf` in KWin
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6.6.4. **Test fixture, not the upstream-bound shape.**
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## Background
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KWin's `Transaction::watchDmaBuf`
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(`src/wayland/transaction.cpp:265`) calls
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`DMA_BUF_IOCTL_EXPORT_SYNC_FILE` on every plane of every imported
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dmabuf and parks the transaction on a `QSocketNotifier(POLLIN)`
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waiting for the resulting sync_file fd to become readable.
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For V4L2-produced dmabufs (hantro / rockchip-rga and any other vb2
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driver), that fence either:
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- **Stub-signals immediately**, because vb2 doesn't populate
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`dma_resv` exclusive fences (see kernel layer in the top-level
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README) and `dma_buf_export_sync_file` substitutes
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`dma_fence_get_stub()`. Pure latency cost: a synchronous ioctl +
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socket-notifier setup per frame, for a fence that signals in
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microseconds and represents nothing real.
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- **Signals very late or not at all**, on edge cases that we hit
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during the chromium-fourier validation campaign. KWin's
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transaction parks indefinitely; the previous wl_buffer never gets
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released to the client; the client's V4L2 capture pool starves;
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hard stall.
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## Patches
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This directory carries **two** patches — the PKGBUILD applies only
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`0001` for now (validated on ohm), while `0002` is the
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upstream-bound shape staged here for later validation and
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submission.
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### `0001-transaction-bypass-watchDmaBuf-fence-wait.patch` *(currently shipped)*
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No-ops `Transaction::watchDmaBuf` entirely. Every transaction
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commits without waiting on implicit-sync fences for the dmabufs
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it imports. **Test fixture, validated end-to-end on ohm**: the
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patch unblocks chromium-fourier 1080p30 H.264 playback under KDE
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Plasma 6.6.4 Wayland on RK3566 + panfrost + mainline 6.19.
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### `0002-transaction-poll-dmabuf-fd-directly-upstream-shape.patch` *(unvalidated, upstream-bound)*
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Rewrites `Transaction::watchDmaBuf` to call `poll(POLLIN)` on the
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dmabuf fd directly via a duplicated fd in a `QSocketNotifier`,
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instead of going through `DMA_BUF_IOCTL_EXPORT_SYNC_FILE` plus a
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sync_file fd. The dma-buf core has supported polling the dmabuf fd
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for implicit-sync write fences since the introduction of the
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feature; the export-then-poll round-trip is per-frame syscall
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overhead with no semantic difference.
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This shape preserves KWin's defense — the wait still actually
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*waits* on the producer's fence — while shedding the per-frame
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overhead. It is **not validated yet** and is offered here as the
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shape upstream review will likely converge on. Validation gates
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before swapping the PKGBUILD to apply 0002 instead of 0001:
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1. Build kwin-fourier with 0002 instead of 0001 (one PKGBUILD line
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change).
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2. Install on ohm; restart Plasma session so the new
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`kwin_wayland` is mapped.
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3. Run chromium-fourier + bbb sample as before. Expectation:
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plays through end-to-end at the same ~81 % combined CPU.
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Equivalence with 0001 confirms the upstream shape works
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without weakening defenses.
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4. Capture before/after `dma_buf_export_sync_file` syscall counts
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via `strace -c` on `kwin_wayland` (the per-frame syscall savings
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are the patch's claimed benefit).
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5. Submit to invent.kde.org/plasma/kwin against `master`.
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## Why patch 0001 is *not* the upstream-bound shape
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Wayland's security model is "compositor trusts no client" —
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watchDmaBuf is a defense against a misbehaving client that attaches
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a buffer the GPU is still writing. The blanket no-op makes a
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correctness-equivalent assumption (`every Wayland client honors the
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spec`) that KWin maintainers are reasonably unwilling to take
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unconditionally.
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**The upstream-correct fix lives in the kernel** (vb2 / hantro /
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rga don't populate `dma_resv` — fix that, KWin's wait now works
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correctly because the fence is real). Once the kernel side lands,
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KWin can either keep its current wait (now correct) or migrate to
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`poll(POLLIN)` directly on the dmabuf fd, skipping the
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`EXPORT_SYNC_FILE` ioctl.
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The kwin-fourier patch in this repo is the **diagnostic** that
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identified the kernel bug and lets the chromium-fourier validation
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proceed today on stock kernel + KWin. It will be rewritten or
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removed once the kernel side is upstream.
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## Building / installing
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```sh
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makepkg -si
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```
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The PKGBUILD inherits from upstream Arch's kwin 6.6.4-1, applies
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the single watchDmaBuf bypass, and bumps `epoch=1` to dominate
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upstream pkgrel.
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## Side effect
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Across the test session, every wp_linux_dmabuf client on the
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compositor (chrome, brave, mpv, VLC, …) feels markedly snappier on
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Mali-class hardware because the per-frame sync_file roundtrip is
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gone. A pleasant accident; the cleaner, kernel-side fix will
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preserve the speedup without weakening the defense.
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