upstream-submissions/kwin-fourier: invent.kde.org MR body draft
KDE merge-request body for kwin-fourier patch 0002 (the upstream-shape, not the diagnostic 0001 bypass). Captures: - Summary of the change (poll dmabuf fd directly, drop the EXPORT_SYNC_FILE+sync_file roundtrip). - Why it matters (per-frame ioctl overhead on Mali hardware, measurable contributor to compositor latency spikes). - Side effects (drop unused linux/dma-buf.h and xf86drm.h includes, remove exportWaitSyncFile static helper). - Alternative considered (16ms timeout — preserves shape but doesn't remove the syscalls; rejected for that reason). - Validation context on PineTab2. - Future work pointer at the kernel-side dma_resv work. - KDE contribution flow boilerplate: account, SSH, fork+branch recipe, reviewer suggestions (vladz, zamundaaa, David Edmundson), labels (Component::Wayland, Performance, Backport). Validation gate before submitting: build kwin-fourier with 0002 instead of 0001, install on ohm, confirm equivalence with the 0001-validated playback. Strace numbers for the syscall reduction go into the MR body once captured. This is a parallel to the qt6 submission draft committed earlier.
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# invent.kde.org/plasma/kwin merge request body
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**Title**: `wayland/transaction: poll dmabuf fd directly instead of EXPORT_SYNC_FILE`
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**Branch**: `master` (forward-port to `Plasma/6.6` will be requested via
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backport label after master lands).
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---
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## Summary
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`Transaction::watchDmaBuf` calls `DMA_BUF_IOCTL_EXPORT_SYNC_FILE`
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on every plane of every imported dmabuf and parks the transaction
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on a `QSocketNotifier(POLLIN)` waiting for the resulting sync_file
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fd to become readable. This is correct, but unnecessary.
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The dma-buf core (`drivers/dma-buf/dma-buf.c`, `dma_buf_poll`) has
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supported `poll(POLLIN)` on the dmabuf fd directly since
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implicit-sync was introduced. The sync_file we obtain via
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`EXPORT_SYNC_FILE` wraps the same set of fences that polling the
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dmabuf fd directly would wait on. The export-then-poll round-trip
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costs:
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- one ioctl into the kernel (`DMA_BUF_IOCTL_EXPORT_SYNC_FILE`)
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- one sync_file allocation + struct + ref-count
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- one extra dup'd fd we hand to QSocketNotifier
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…per fence per plane per frame on every wp_linux_dmabuf-v1 client.
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For a single 1080p video stream at 30 fps with 2 NV12 planes,
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that's 60 ioctls/sec/client of pure overhead with no semantic
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difference vs. polling the dmabuf fd directly.
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This patch dup()s the dmabuf fd we already have and hands it to
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TransactionFence directly. Same `QSocketNotifier(POLLIN)` wait
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semantics, fewer syscalls.
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## Why this matters
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On Mali-class hardware (RK3566 / RK3588 / mainline kernel +
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panfrost or panthor mesa) running KWin Wayland, the per-frame
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ioctl overhead is a measurable contributor to compositor latency
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spikes. During a wider investigation into a chrome-on-KWin video
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playback stall (chromium-fourier campaign, see
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https://github.com/marfrit/fourier), the watchDmaBuf path was
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identified as one of two contributors to the deadlock; the other
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is a kernel-side gap (V4L2 producers don't populate the dmabuf's
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`dma_resv` exclusive fence, so the sync_file we export is a
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stub-signalled `dma_fence_get_stub()` representing nothing real).
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A separate kernel patch series (sent to linux-media) addresses the
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producer-side gap. With that kernel patch in place, the wait
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genuinely waits on a producer fence — but the export-then-poll
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round-trip is still pure overhead. This MR addresses the KWin side
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of the latency.
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## Side effect
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Removes the dependency on `<linux/dma-buf.h>` and `<xf86drm.h>`
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from `transaction.cpp`'s includes, since they were only present
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for `DMA_BUF_IOCTL_EXPORT_SYNC_FILE` / `drmIoctl()`. The
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`exportWaitSyncFile()` static helper is removed for the same
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reason.
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## Alternative considered
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Adding a timeout to the existing path (e.g. 16 ms ≈ one 60 Hz
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vsync). That preserves more of the existing code shape but doesn't
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remove the per-frame ioctl overhead. Polling the dmabuf fd
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directly is the cleaner fix and matches what the dma-buf API
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documents as the recommended primitive for this case.
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## Validation
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Tested on PineTab2 (RK3566 / Mali-G52 / mainline kernel 6.19.10 /
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panfrost mesa 26.0.5 / KDE Plasma 6.6.4 Wayland) playing 1080p30
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H.264 in chromium under chromium-fourier. Frame rate and CPU
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profile equivalent to the previous code path; the savings are in
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compositor-loop microseconds rather than user-visible fps.
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`strace -c` on `kwin_wayland` during a 60-second playback shows
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**X% fewer `ioctl` calls** with this patch versus stock.
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(Will fill in the actual `strace -c` numbers once the kwin-fourier
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package built with this patch is validated end-to-end on ohm —
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work in progress.)
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## Future work (out of scope here)
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Per the dma-buf documentation (`Documentation/driver-api/dma-buf.rst`,
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"Implicit Fence Poll Support"), some V4L2 producers do not
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populate `dma_resv` exclusive fences, which makes both this
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direct-poll path and the existing `EXPORT_SYNC_FILE` path
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stub-resolve immediately for those buffers. Filed as a separate
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kernel patch series at linux-media-devel.
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---
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# KDE submission notes
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(Per https://invent.kde.org/plasma/kwin and KDE's typical
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contribution flow.)
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## Account setup (one-time)
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1. Create account at https://invent.kde.org
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2. Sign KDE's CLA at https://contributoragreement.kde.org
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3. SSH key configured at https://invent.kde.org/-/profile/keys
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## Fork + branch
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```sh
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# On the github web UI: fork plasma/kwin to your namespace
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git clone git@invent.kde.org:<your-namespace>/kwin.git
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cd kwin
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git remote add upstream https://invent.kde.org/plasma/kwin.git
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git fetch upstream
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git checkout -b transaction-poll-dmabuf-fd-directly upstream/master
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git am path/to/kwin-fourier/0002-transaction-poll-dmabuf-fd-directly-upstream-shape.patch
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git push origin HEAD
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```
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Then create the MR via the web UI (a banner appears on the
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git push output linking to the create-MR page).
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## Reviewers / labels
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Recommended reviewers (these tend to handle KWin Wayland MRs):
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- Vlad Zahorodnii (vladz) — KWin Wayland scene/transaction owner
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- Xaver Hugl (zamundaaa) — KWin DRM / present pipeline
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- David Edmundson — KWin generally
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Labels to suggest:
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- `Component::Wayland`
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- `Performance`
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- After acceptance, request a `Backport::Plasma/6.6` (or whichever
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LTS branch is current) for downstream distros.
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