fourier — vb2-dma-resv kernel RFC

3-patch kernel RFC. Sent to linux-media + dri-devel + linaro-mm-sig + linux-rockchip + linux-arm-kernel + linux-kernel on 2026-04-29 19:53 UTC. Index: fourier.

• v1 sent: Cover Msg-Id 20260429195306.239666-1-mfritsche@reauktion.de. Thread on lore: https://lore.kernel.org/linux-media/20260429195306.239666-1-mfritsche@reauktion.de/
• Two reviewer replies received the same day. Both substantive, neither a NACK.
• v2 sketch backlogged. Drafted but not yet rewritten in user voice; awaiting review before patches get touched.

vb2 currently does not propagate V4L2 producer state into the dmabuf dma_resv exclusive fence. A downstream consumer (e.g. the KWin compositor polling a dmabuf for completion) cannot rely on the fence to indicate “decode finished, frame ready”. The compositor instead either spins on stub fences (the bug kwin-fourier 0001 works around) or has to do the wait the hard way at compositor level.

1. vb2 helper API vb2_buffer_attach_release_fence()
2. hantro opt-in (called from buf_queue — see review notes for why this is wrong)
3. rockchip-rga opt-in (same shape, same problem)

3d8deeb15581b754e4c061d4c4a13657aa08bc3c.camel@ndufresne.ca, 2026-04-29 21:22 UTC.

• Why implicit fencing instead of explicit?
• Critical contract violation: attaching the fence at buf_queue (CAPTURE side) publishes it before OUTPUT bitstream + parameters + request are guaranteed populated. A user that starves the OUTPUT queue leaves the fence un-armed forever. Violates the dma_fence finite-time-signal contract. Same wall Padovan's earlier explicit-sync series hit.
• Right attach point for hantro: right before the DEC bit on the control register, after bitstream + params + request are queued.
• Suggests basing the design on DMA_BUF_IOCTL_EXPORT_SYNC_FILE as the kick IOCTL, or attaching an out-fence to media_request. Cites DW100's m2m-to-request conversion as a precedent for that path being approachable.
• Mentions Freedreno + Etnaviv “back flash” issue with implicit fences on imported V4L2 buffers as higher-priority work — the GL drivers attach their submit fence into the V4L2 dmabuf's resv on the consumer side, causing the next compositor frame to wait on the GL job rather than the camera.

4c63c81b-14f7-4e6b-a733-2f98e6055ca8@amd.com, 2026-04-30 06:51 UTC, in reply to Nicolas.

• “+1” on Nicolas's contract critique.
• “Implicit fencing is basically just a workaround how the GPU HW used to work ~20 years ago and only rarely makes sense today.”
• But: “putting it behind a flag might be acceptable when you want to interact with implicit synced HW, e.g. a GPU.” — exactly the fourier compositor case.
• Maintainer-hat ask: annotate fence-publish path with Sima Vetter's dma_fence_begin_signalling() / dma_fence_end_signalling() primitives, run with lockdep enabled.

Net of the two reviews: not a NACK. A directed redirect with a permission slip (“behind a flag”) for the fourier use case. v2 sketch + prior-art reference saved in the gitea repo at:

• upstream-submissions/vb2-dma-resv/v2-cover-letter-sketch.md
• upstream-submissions/vb2-dma-resv/v2-prior-art-references.md

• Attach point moves from buf_queue to m2m device_run. Hantro: insert at drivers/media/platform/verisilicon/hantro_drv.c line 187 (between v4l2_m2m_buf_copy_metadata and ctx→codec_ops→run). RGA: lift v4l2_m2m_next_*_buf + the fence-attach above the ctrl_lock spinlock at drivers/media/platform/rockchip/rga/rga.c line 41 — the helper's dma_resv_lock is sleepable.
• Per-driver opt-in: new vb2_queue::supports_release_fences bool, set in queue init.
• Kconfig: CONFIG_VIDEOBUF2_RELEASE_FENCES, default n. Bool, not tristate.
• Lockdep: dma_fence_begin/end_signalling pairs around publish + signal paths. Validated by full bbb 1080p30 chromium playback under CONFIG_PROVE_LOCKING=y CONFIG_LOCKDEP=y.
• Tag: keep [PATCH RFC v2]. Attach-point move is non-trivial; one more design round at RFC is cheap.
• CC: Daniel/Sima Vetter (daniel.vetter@ffwll.ch) added; –in-reply-to the v1 cover so v2 threads under v1.

• media_request out-fence path (Nicolas's preferred long-term direction). Bigger redesign; runs in parallel with this opt-in producer-fence series and does not conflict.
• Auto-on per vb2-CAPTURE queue. Probably not — opt-in is the safer story given the unfixed freedreno/etnaviv back-flash on the consumer side.

• kwin-fourier 0001-skip-wait (currently shipped, kwin-fourier 1:6.6.4-3) skips the watchDmaBuf wait entirely. Pragmatic fix; works because no real fence ever lands today.
• kwin-fourier 0002-upstream-shape was an attempt to write the dmabuf-fd poll dance the way upstream would accept it. Reviewer on KDE MR 9157 (now closed) showed the technical claims were wrong — sync_file is a fence snapshot at export, dmabuf-fd polling tracks later-added fences, Mutter explicitly moved off the polling path on amdgpu. Not for re-submission.
• vb2-dma-resv RFC v2 fixes the gap at the producer level so kwin-fourier 0001's bypass is no longer needed: KWin's stock watchDmaBuf wait completes correctly because the producer fence now signals when hantro completes.
• Both are shippable independently. The kwin patch helps on kernels that never get the RFC; the RFC helps on compositors that never take the kwin patch. With both in tree the kwin patch becomes a no-op and can be retired.

lore-watch.sh only matches messages where mfritsche@reauktion.de appears in headers. Mid-thread replies that drop the address (the common case for review threads) are NOT caught. Both 2026-04-30 replies were missed. Manual walk:

ssh noether
git clone --bare --depth 500 \
  https://lore.kernel.org/linux-media/1 \
  ~/.local/state/lore-watch/linux-media-pi-1.git
cd ~/.local/state/lore-watch/linux-media-pi-1.git
git fetch --depth 1000 origin
git log --all --format='%H %ci %s' | \
  grep -i 'videobuf2.*dma_resv\|dma_resv.*videobuf2'
git show <sha>:m

linux-media's live public-inbox epoch is 1 as of 2026-04-30; probe higher numbers if no hits. Anubis blocks lore HTTP; git pack-protocol bypasses it cleanly.

• Patches (v1): kernel/vb2-dma-resv-rfc/ in gitea/marfrit-packages
• v1 submission notes (CC list, send recipe): upstream-submissions/vb2-dma-resv/lkml-submission-notes.md
• v2 sketch + prior art: upstream-submissions/vb2-dma-resv/v2-cover-letter-sketch.md, v2-prior-art-references.md