# linux-media submission notes — vb2 dma_resv release-fence series

The 3-patch RFC series at `marfrit-packages/kernel/vb2-dma-resv-rfc/`
is ready to send. Cover letter is filled in; patches apply cleanly
via `git am` against Linux 6.12 mainline; all kernel APIs verified.

## Pre-flight gates (re-stated)

1. **Full-tree compile-test** — apply the 3 patches to a complete
   kernel checkout (not a sparse one), `make defconfig` + enable
   `CONFIG_VIDEO_HANTRO=y` and `CONFIG_VIDEO_ROCKCHIP_RGA=y`,
   then build. A focused
   `make drivers/media/common/videobuf2/videobuf2-core.o
   drivers/media/platform/verisilicon/hantro_v4l2.o
   drivers/media/platform/rockchip/rga/rga-buf.o` is the minimal
   safe-to-send threshold; full `make` is the higher confidence
   bar.
2. **Boot-test on ohm** — install the patched kernel, verify
   `lsmod` shows hantro_vpu and rockchip_rga loaded, run
   `mpv --vo=drm --hwdec=v4l2request bbb_1080p30_h264.mp4` and
   confirm playback still works (smoke test that we haven't
   broken the existing buffer-state machine).
3. **Runtime validation** — install patched kernel,
   `pacman -R kwin-fourier` so KWin's stock `watchDmaBuf` is
   re-enabled, play `bbb_1080p30_h264.mp4` in chromium-fourier
   under KDE Plasma. Should plays through end-to-end because the
   fence the compositor waits on is now real (signals when hantro
   completes). Compare CPU profile against the kwin-fourier-bypassed
   baseline — should be equivalent or marginally lower.
4. **Capture syscall numbers** — `strace -c -p $(pgrep
   kwin_wayland)` during a 60-second playback, before and after
   the kernel patch (with stock KWin, kwin-fourier uninstalled).
   Reports the ioctl reduction the patch enables (since
   `EXPORT_SYNC_FILE` will now signal real fences instead of stub
   ones, but the call itself remains). The reduction is greater
   if the KWin upstream-shape patch is also in, but the kernel
   patch alone is sufficient to make the existing KWin path
   correct.

## Send command

After all four gates pass, format-patch and send:

```sh
cd <linux-tree-with-patches-applied>
git format-patch --cover-letter --base=auto -3 \
                 -o /tmp/vb2-dma-resv-out HEAD
# Verify cover letter has the right Subject and blurb (we already
# filled them in, but git format-patch will overwrite — so paste
# the canonical ones from kernel/vb2-dma-resv-rfc/0000-cover-letter.patch)
git send-email \
    --to=linux-media@vger.kernel.org \
    --cc='Mauro Carvalho Chehab <mchehab@kernel.org>' \
    --cc='Hans Verkuil <hverkuil-cisco@xs4all.nl>' \
    --cc='Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>' \
    --cc='Tomasz Figa <tfiga@chromium.org>' \
    --cc='Sumit Semwal <sumit.semwal@linaro.org>' \
    --cc='Christian König <christian.koenig@amd.com>' \
    --cc='Jason Ekstrand <jason@jlekstrand.net>' \
    --cc='dri-devel@lists.freedesktop.org' \
    --cc='linux-rockchip@lists.infradead.org' \
    --cc='Heiko Stuebner <heiko@sntech.de>' \
    --cc='Jeffy Chen <jeffy.chen@rock-chips.com>' \
    /tmp/vb2-dma-resv-out/*.patch
```

Rationale for the CC list:

- `linux-media@vger.kernel.org` — primary list (V4L2 / videobuf2)
- Mauro / Hans / Ezequiel — V4L2 maintainers
- Tomasz Figa — hantro maintainer (Collabora / Chromium overlap)
- Sumit Semwal / Christian König / Jason Ekstrand — dma-buf
  reviewers (this patch touches `dma_resv` semantics, so the
  dma-buf folks need a look)
- `dri-devel` — graphics list, the consumer side of the fence
- `linux-rockchip@lists.infradead.org` — RK3xxx-specific feedback
- Heiko Stübner — Rockchip / arm64 maintainer
- Jeffy Chen — Rock-Chips media stack lead

## Expected feedback

- "Why opt-in vs auto-on for all CAPTURE queues?" — answered in
  cover letter; willing to switch to auto-on if reviewers prefer
  the audit burden.
- "Does this need a vb2 selftest?" — videobuf2 doesn't have
  great in-tree dmabuf coverage; offer to add a minimal kselftest
  if reviewers ask.
- "Is `vb2_buffer_done` the right signal point for asynchronous
  drivers?" — for hantro and rga it is (they're synchronous m2m).
  For drivers with async post-processing, may need a separate
  hook; out of scope for this RFC.
- "Pick-to: stable trees?" — Maintainers will decide. The patch
  is invasive enough that LTS backport may not be wanted; the
  contract gap has been there since vb2's introduction so it's
  not a regression fix.

## Withdraw conditions

If reviewers point out that:

- A driver in the wider tree already does this and we missed it
  (the auto-on path would just naturally pick that up; switch
  approach).
- vb2's `dma_resv` integration is being done in a different
  upstream effort already (search lkml for `videobuf2 dma_resv`
  before sending — should already be in the cover letter).
- The fence semantics are wrong for some driver class we didn't
  consider (most likely image-sensor → CSI → ISP → output, where
  the producer fence at the camera side and the user-visible
  fence at the ISP output may be different).

…drop the series, write up the lessons, retry next cycle.