upstream-submissions/vb2-dma-resv: linux-media submission notes
Submission notes for the kernel/vb2-dma-resv-rfc series. Captures: - Pre-flight gates (full-tree compile-test, boot-test on ohm, runtime validation by uninstalling kwin-fourier and confirming chrome still plays through, syscall-count delta). - The git send-email recipe with the full CC list (linux-media, V4L2 maintainers, hantro maintainer Tomasz Figa, dma-buf reviewers Sumit Semwal / Christian König / Jason Ekstrand, dri-devel, linux-rockchip, Heiko / Jeffy). - Expected reviewer feedback (opt-in vs. auto-on, selftest request, signal-point semantics for async producers, Pick-to-LTS decision). - Withdraw conditions if upstream is already doing this work or the design has a fundamental issue we didn't see. This is the third upstream-submission draft in the campaign: upstream-submissions/qt6-base-fourier/ bugreports.qt.io draft upstream-submissions/kwin-fourier/ invent.kde.org MR draft upstream-submissions/vb2-dma-resv/ linux-media draft (this)
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# linux-media submission notes — vb2 dma_resv release-fence series
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The 3-patch RFC series at `marfrit-packages/kernel/vb2-dma-resv-rfc/`
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is ready to send. Cover letter is filled in; patches apply cleanly
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via `git am` against Linux 6.12 mainline; all kernel APIs verified.
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## Pre-flight gates (re-stated)
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1. **Full-tree compile-test** — apply the 3 patches to a complete
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kernel checkout (not a sparse one), `make defconfig` + enable
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`CONFIG_VIDEO_HANTRO=y` and `CONFIG_VIDEO_ROCKCHIP_RGA=y`,
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then build. A focused
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`make drivers/media/common/videobuf2/videobuf2-core.o
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drivers/media/platform/verisilicon/hantro_v4l2.o
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drivers/media/platform/rockchip/rga/rga-buf.o` is the minimal
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safe-to-send threshold; full `make` is the higher confidence
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bar.
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2. **Boot-test on ohm** — install the patched kernel, verify
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`lsmod` shows hantro_vpu and rockchip_rga loaded, run
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`mpv --vo=drm --hwdec=v4l2request bbb_1080p30_h264.mp4` and
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confirm playback still works (smoke test that we haven't
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broken the existing buffer-state machine).
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3. **Runtime validation** — install patched kernel,
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`pacman -R kwin-fourier` so KWin's stock `watchDmaBuf` is
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re-enabled, play `bbb_1080p30_h264.mp4` in chromium-fourier
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under KDE Plasma. Should plays through end-to-end because the
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fence the compositor waits on is now real (signals when hantro
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completes). Compare CPU profile against the kwin-fourier-bypassed
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baseline — should be equivalent or marginally lower.
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4. **Capture syscall numbers** — `strace -c -p $(pgrep
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kwin_wayland)` during a 60-second playback, before and after
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the kernel patch (with stock KWin, kwin-fourier uninstalled).
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Reports the ioctl reduction the patch enables (since
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`EXPORT_SYNC_FILE` will now signal real fences instead of stub
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ones, but the call itself remains). The reduction is greater
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if the KWin upstream-shape patch is also in, but the kernel
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patch alone is sufficient to make the existing KWin path
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correct.
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## Send command
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After all four gates pass, format-patch and send:
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```sh
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cd <linux-tree-with-patches-applied>
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git format-patch --cover-letter --base=auto -3 \
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-o /tmp/vb2-dma-resv-out HEAD
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# Verify cover letter has the right Subject and blurb (we already
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# filled them in, but git format-patch will overwrite — so paste
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# the canonical ones from kernel/vb2-dma-resv-rfc/0000-cover-letter.patch)
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git send-email \
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--to=linux-media@vger.kernel.org \
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--cc='Mauro Carvalho Chehab <mchehab@kernel.org>' \
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--cc='Hans Verkuil <hverkuil-cisco@xs4all.nl>' \
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--cc='Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>' \
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--cc='Tomasz Figa <tfiga@chromium.org>' \
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--cc='Sumit Semwal <sumit.semwal@linaro.org>' \
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--cc='Christian König <christian.koenig@amd.com>' \
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--cc='Jason Ekstrand <jason@jlekstrand.net>' \
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--cc='dri-devel@lists.freedesktop.org' \
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--cc='linux-rockchip@lists.infradead.org' \
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--cc='Heiko Stuebner <heiko@sntech.de>' \
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--cc='Jeffy Chen <jeffy.chen@rock-chips.com>' \
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/tmp/vb2-dma-resv-out/*.patch
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```
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Rationale for the CC list:
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- `linux-media@vger.kernel.org` — primary list (V4L2 / videobuf2)
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- Mauro / Hans / Ezequiel — V4L2 maintainers
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- Tomasz Figa — hantro maintainer (Collabora / Chromium overlap)
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- Sumit Semwal / Christian König / Jason Ekstrand — dma-buf
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reviewers (this patch touches `dma_resv` semantics, so the
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dma-buf folks need a look)
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- `dri-devel` — graphics list, the consumer side of the fence
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- `linux-rockchip@lists.infradead.org` — RK3xxx-specific feedback
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- Heiko Stübner — Rockchip / arm64 maintainer
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- Jeffy Chen — Rock-Chips media stack lead
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## Expected feedback
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- "Why opt-in vs auto-on for all CAPTURE queues?" — answered in
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cover letter; willing to switch to auto-on if reviewers prefer
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the audit burden.
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- "Does this need a vb2 selftest?" — videobuf2 doesn't have
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great in-tree dmabuf coverage; offer to add a minimal kselftest
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if reviewers ask.
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- "Is `vb2_buffer_done` the right signal point for asynchronous
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drivers?" — for hantro and rga it is (they're synchronous m2m).
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For drivers with async post-processing, may need a separate
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hook; out of scope for this RFC.
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- "Pick-to: stable trees?" — Maintainers will decide. The patch
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is invasive enough that LTS backport may not be wanted; the
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contract gap has been there since vb2's introduction so it's
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not a regression fix.
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## Withdraw conditions
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If reviewers point out that:
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- A driver in the wider tree already does this and we missed it
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(the auto-on path would just naturally pick that up; switch
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approach).
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- vb2's `dma_resv` integration is being done in a different
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upstream effort already (search lkml for `videobuf2 dma_resv`
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before sending — should already be in the cover letter).
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- The fence semantics are wrong for some driver class we didn't
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consider (most likely image-sensor → CSI → ISP → output, where
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the producer fence at the camera side and the user-visible
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fence at the ISP output may be different).
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…drop the series, write up the lessons, retry next cycle.
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