feat(moq-video): add the V4L2 stateful M2M hardware encoder and decoder by Frando · Pull Request #3332 · moq-dev/moq

Frando · 2026-09-02T11:17:52Z

Adds H.264 encode and decode backends for the kernel's stateful V4L2 memory-to-memory codec interface, behind a new off-by-default v4l2 feature. This is the hardware codec path on a Raspberry Pi and on most other ARM SoCs, so a board with neither an NVIDIA GPU nor a VAAPI stack can encode what it captures instead of republishing what rpicam-vid already encoded or spending its CPU on openh264.

This PR is part of a series to update iroh-live to latest moq, see n0-computer/iroh-live#45. The code and below description was written by Claude Code

quest/m3/teleop/v4l2-encode.md on dev already scopes the encoder half of this, down to the same bcm2835-codec target and the same reasoning about which boards still have an encoder worth using. The decoder came along because it is the same device layer driven the other way.

Now run on a Raspberry Pi 4

Updated 2026-09-03: both backends have been exercised on hardware. The description below was written when neither had, and the parts that said so have been corrected rather than deleted, so the history of the claim stays readable.

The board is a Raspberry Pi 4 on Raspberry Pi OS Bookworm, kernel 6.6.31, aarch64, driving bcm2835-codec. Both were reached through a downstream CLI that selects a backend by name.

Encoder. Opens /dev/video11, negotiates NV12 at 640x360, and publishes an H.264 stream that decodes back to a correct picture: fourteen seconds recorded to fMP4 came to 490 KB of Constrained Baseline, and a frame extracted from it is the room the camera was pointed at. Raw pictures came from rpicam-vid --codec yuv420, since a Pi's CSI camera is not reachable through V4L2 capture at all.
Decoder. It is what automatic selection picks on that board, ahead of the openh264 fallback, and it played a 640x360 stream for a minute with no decode failures. Three screenshots of the player ten seconds apart differ, so the picture was moving rather than one decoded frame left on screen.

The feature stays off by default. One SoC is not the several this drives, and the v4l crate's bindgen still wants libclang at build time.

Summary

M2M is one device node with two queues: OUTPUT for what userspace feeds in, CAPTURE for what the hardware hands back. An encoder takes raw frames on OUTPUT and returns an elementary stream on CAPTURE, and a decoder runs the same node the other way around, so both backends share src/v4l2.rs for the ioctl sequence, the mmap buffer pool and the plane arithmetic. On a Raspberry Pi the two are sibling nodes of one driver: /dev/video10 decodes and /dev/video11 encodes, both bcm2835-codec.
Node numbering is per SoC, so a node is found by what it converts rather than by a path table. VIDIOC_QUERYCAP proves multi-planar M2M and streaming, VIDIOC_ENUM_FMT over every V4L2 node proves the format pair, and Rockchip, Amlogic and Exynos numbering therefore works too. MOQ_V4L2_ENCODER and MOQ_V4L2_DECODER override the search.
No new dependency. The backends use the raw layer of v4l, which the camera capture path already pulls in: v4l::v4l_sys is bindgen'd videodev2.h and supplies every ioctl struct, v4l::v4l2::vidioc supplies the request codes. Three requests are missing from that table (VIDIOC_SUBSCRIBE_EVENT, VIDIOC_DQEVENT, VIDIOC_G_SELECTION) and their codes are built the way linux/ioctl.h builds them, in about a dozen lines. The alternative was v4l2r, a second V4L2 crate for the same ioctls.
Stateful decoding only. A Raspberry Pi 4's separate HEVC block and Rockchip's rkvdec are stateless V4L2 decoders driven through the media request API with per-slice parameters, which is a different interface rather than another format to add to this one.

The driver decides the raw layout

VIDIOC_S_FMT is a negotiation, not an instruction. The driver answers with its own fourcc, its own bytesperline, and a row count that can exceed the height that was asked for. All three are read back, and one Planes type is the only thing either backend consults about where chroma sits.

The row count is what bites, because a driver reports it nowhere except through sizeimage. Chroma goes at stride * padded_rows, with padded_rows recovered from sizeimage. Writing it at stride * height leaves the driver reading zeroes for chroma, which comes out as a green picture. The decoder reads chroma back from the same place, and takes the compose rectangle from VIDIOC_G_SELECTION, since coding rounds the height up to whole macroblocks and 1080p codes as 1088 rows.

Draining the encoder is a command, not a wait

The kernel's stateful encoder contract is that an encoder holding a frame releases it for one reason, which is being told the stream stopped: V4L2_ENC_CMD_STOP, dequeue CAPTURE to the buffer flagged V4L2_BUF_FLAG_LAST, then V4L2_ENC_CMD_START to resume with the state from before the drain. A flush falls on every group boundary here, so an encoder with a lookahead or two-pass rate control would fail the first boundary and kill the broadcast if flush could only wait for outstanding buffers. A driver that refuses the command keeps a best-effort wait, which is right for exactly the encoders that would have been fine without it.

bcm2835-codec happens to be one-in-one-out, so a Pi would have passed the version that only waited. Hardware validation would not have caught this one.

Public API changes

None. The v4l2 feature is new and off by default; both backends are pub(crate) and reached through the existing automatic selection or by name, as encode::Kind::Named("v4l2") and decode::Kind::Named("v4l2"). No pub item in rs/moq-* is added, renamed, or changed.

Test plan

cargo fmt --all --check.
cargo clippy -p moq-video --all-targets --features v4l2 -- -D warnings, and the same with default features to catch a gating mistake.
cargo test -p moq-video --features v4l2: 105 pass.
cargo check -p moq-video --features v4l2 --lib --target aarch64-unknown-linux-gnu.
Each of the two commits builds and tests on its own.

The unit tests cover what a device is not needed for: the plane arithmetic in both directions, the timestamp matching that pairs a coded buffer with the frame it came from, parameter sets that arrive on a coded buffer of their own, and a buffer the driver flagged V4L2_BUF_FLAG_ERROR.

A Pi 4 has since proved the first two of these: the driver accepts the format and control sequence, and the emitted Annex-B plays back. What is still unproven, and would be worth a second pass from anyone with the hardware:

set_bitrate on a running encoder. Congestion control retunes through it and nothing in the run above changed the rate.
Whether the flush deadline is generous enough under load. bcm2835-codec is one-in-one-out, so the drain command path this PR adds is not what it exercised.
Resolutions past 640x360, and 1080p in particular, where coding rounds the height to 1088 and the compose rectangle from VIDIOC_G_SELECTION is what crops it back.
Any SoC that is not a Pi: Rockchip, Amlogic and Exynos numbering is handled by search rather than a path table, and that search has now been exercised on exactly one driver.
32-bit ARM, still unchecked for want of a cross compiler, and still where the plane arithmetic is most likely to differ.

(Written by Claude Code)

Conflicts with #3331

Both register a Linux hardware decode candidate in the same list in decode/backend/mod.rs and both extend the same sentence of its module doc, so whichever merges second needs a three-line resolution: keep both candidates, VAAPI before V4L2. Nothing else in the two overlaps.

Review round

A later pass found two things against the kernel documentation, both fixed here.

The encoder failed its first frame of every session. Backend::encode collects finished buffers before it queues one, and vb2 checks q->streaming before it looks for a buffer, so a dequeue on a queue that has not been started answers EINVAL rather than the EAGAIN an empty one gives. A queue that is not streaming now reports itself empty.

The coded and raw formats were negotiated in the wrong order. dev-encoder.rst sets the coded CAPTURE format first and warns that doing so derives a new OUTPUT format, so the stride and padded row count were being read from exactly the two numbers the driver is then free to replace, and those are what place the chroma plane. VIDIOC_S_FMT also carried bytesperline and sizeimage over from the preceding G_FMT; both are zeroed now, which is how V4L2 asks the driver to size a plane and what GStreamer, Chromium and libcamera all send.

That review was against dev-encoder.rst, dev-decoder.rst, videodev2.h and the vb2 core, with no V4L2 M2M device on the machine it was written on. The Pi 4 run described at the top came later and did not contradict any of it: the first-frame fix and the format ordering are both on the path that ran.

Takeover (`c1362eb`)

Taken over to land. Three things changed on top of the commits above, and the two sentences above saying the feature "stays off by default" no longer hold.

The H.264 level is chosen from every Table A-1 limit, not frame size alone: macroblocks per second from config.framerate and the configured bitrate too. 1080p60 was labelled level 4.0 (245,760 MB/s) when it needs 4.2 (522,240), which a driver may clamp the rate to. The whole menu from 1.0 to 5.1 is walked and the first level all three limits fit is set, with tests for the framerate and bitrate columns.
A coded buffer with no VCL NAL answers no frame, whatever its timestamp. Parameter sets emitted on their own under HEADER_MODE_SEPARATE are stamped zero, and zero is also the timestamp of a capture's first frame and of the one frame Config::probe encodes. Matched by timestamp alone, the header was published as that frame and the IDR answering it was carried into the next access unit. The regression test stamps both at zero.
v4l2 is forwarded by moq-cli and moq-transcode, and on by default in all three crates. The documented --features "capture v4l2" build was rejected as an unknown feature. On by default per the policy that landed in feat(audio,video): compile the device, render, and VAAPI code by default #3353: it costs a build nothing capture does not already pay (the same v4l crate and its bindgen), and an off-by-default feature is one just check never compiles. Automatic selection still tries it last on Linux and falls through when no M2M node opens; Kind::Named("openh264") sidesteps it on a board whose driver misbehaves.

The two quests that scoped this work (quest/m3/teleop/v4l2-encode.md, quest/m3/video-embedded.md) are trimmed to what remains: shipping the backend in a released moq-cli, which is blocked on CLI packaging, the hardware note, and the EGL import.

A second commit (f32c5b3) answers the review of the first. Both backends rebuilt the timestamp from the buffer's microsecond timeval, which changes a Timestamp's scale; the microseconds are now only the key a frame rides the driver under, and the original is answered back. The decoder dropped the compose rectangle's origin, so a stream cropped on its left or top would have been read from the coded corner; Planes now folds the origin into every offset. The level table runs through 6.2 and refuses a config past it instead of understating it. The one finding left open is that no decoder can be drained at a group or track end: decode::Backend has no flush or finish, NVDEC pipelines the same way, so it is a trait-level gap scoped in quest/m3/decode-drain.md rather than one this PR adds. A third commit (0dbb352) adds the decoder-side tests the second was missing, and a fourth (5b6e931) reads each plane's payload from its reported data_offset rather than from zero. A fifth (e0da61c) makes a dropped raw buffer forget only its own frame, and drops rather than carries a picture that answers no frame.

Checked with just check here and, for the Linux-only code, cargo fmt --check, cargo clippy --all-targets -D warnings, and the v4l2 unit tests under default features in a Debian container; 29 pass. No hardware was reachable for a re-run on a Pi.

(Written by Claude Fable 5.1)

…er (moq-dev#3332) Adds the V4L2 memory-to-memory backends, which is what gives a Raspberry Pi and similar SoCs hardware H.264 without a vendor SDK. Follows the stateful encoder and decoder interfaces: VIDIOC_ENCODER_CMD to drain, V4L2_BUF_FLAG_LAST to end it, and SOURCE_CHANGE to pick up the decoder's format. Squashed from `pr/v4l2-m2m` at 44b3292, open as moq-dev#3332.

…er (moq-dev#3332) Adds the V4L2 memory-to-memory backends, which is what gives a Raspberry Pi and similar SoCs hardware H.264 without a vendor SDK. Follows the stateful encoder and decoder interfaces: VIDIOC_ENCODER_CMD to drain, V4L2_BUF_FLAG_LAST to end it, and SOURCE_CHANGE to pick up the decoder's format. Squashed from `pr/v4l2-m2m` at 2da408a, open as moq-dev#3332.

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chatgpt-codex-connector · 2026-09-03T07:57:16Z

 | `capture` | yes | Native device capture (`v4l` and `zune-jpeg` on Linux) |
 | `nvidia` | yes | NVENC encode and NVDEC decode on Linux (`cudarc`, `moq-nvenc`) |
 | `vaapi` | no | Intel/AMD encode on Linux (`moq-vaapi`), unvalidated on hardware |
+| `v4l2` | no | V4L2 M2M encode and decode on ARM SoCs (Raspberry Pi and friends), unvalidated on hardware |


Correct the stale V4L2 validation status

The same patch's rs/moq-video/Cargo.toml documentation records successful Raspberry Pi 4 runs for both backends, but this table still tells users that V4L2 is unvalidated; the CLI page and both backend module docs repeat the stale claim. Update these descriptions to reflect the hardware validation performed in this change. (Written by GPT-5.6 Sol)

AGENTS.md reference: AGENTS.md:L128-L128

Useful? React with 👍 / 👎.

Already corrected by the author in 3c84abd and 78ecbf2, which this review predates. c1362eb goes one step further and drops the opt-in wording, since the feature is on by default now. (written by Claude Fable 5.1)

chatgpt-codex-connector · 2026-09-03T07:57:17Z

 H.264 it picks a hardware encoder (VideoToolbox on macOS, NVENC on Linux NVIDIA,
-or VAAPI on Linux Intel/AMD when built with the `vaapi` feature) when one is
-present, falling back to the built-in software encoder (openh264); force either
-with `--hardware` / `--software`. A hardware encoder that was compiled in but
+VAAPI on Linux Intel/AMD when built with the `vaapi` feature, or an ARM SoC's
+V4L2 M2M encoder when built with `v4l2`) when one is present, falling back to


Forward the V4L2 feature through the CLI

In a moq-cli build, v4l2 is not a declared feature in rs/moq-cli/Cargo.toml, unlike the forwarded nvidia, vaapi, and pipewire features. Consequently the natural documented build command using --features "capture v4l2" is rejected as an unknown feature, and normal CLI builds cannot enable this backend by the name used here. Add forwarding through moq-cli and, for the transcode path, moq-transcode. (Written by GPT-5.6 Sol)

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Fixed in c1362eb. moq-cli forwards v4l2 to moq-video?/v4l2 and moq-transcode?/v4l2, and moq-transcode forwards it to moq-video/v4l2, the same shape as nvidia and vaapi. It is also on by default in all three crates per the default-features policy that landed in #3353: it costs a build nothing capture does not already pay, and an off-by-default feature is one just check never compiles. (written by Claude Fable 5.1)

chatgpt-codex-connector · 2026-09-03T07:57:17Z

+	fn matched(&mut self, timestamp: Duration, payload: Bytes) -> Option<Bytes> {
+		let Some(at) = self.frames.iter().position(|frame| *frame == timestamp) else {
+			self.carry(payload);
+			return None;


Distinguish an unstamped header from timestamp zero

When a driver emits SPS/PPS in a separate CAPTURE buffer stamped with zero and the first submitted frame also has timestamp zero, which the public encoder API and Config::probe both permit, this lookup incorrectly matches the header to that frame. The header is emitted as the completed frame, while the actual IDR is later treated as unmatched and carried into the next access unit, corrupting the first group. Detect header-only buffers independently of timestamp or use a timestamp representation that cannot collide with a real frame. (Written by GPT-5.6 Sol)

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Fixed in c1362eb. A coded buffer is now judged on its bytes before its timestamp: one with no VCL NAL (types 1 to 5) answers no frame whatever it is stamped with, and is carried ahead of the next buffer that does. The regression test queues a frame at zero, hands in SPS/PPS stamped zero, and checks the header is held back and joined to the IDR that follows. (written by Claude Fable 5.1)

chatgpt-codex-connector · 2026-09-03T07:57:17Z

+fn h264_level(size: Size) -> i32 {
+	let macroblocks = size.width.div_ceil(16) * size.height.div_ceil(16);


Account for frame rate when selecting the H.264 level

This selects the SPS level only from macroblocks per frame, but H.264 levels also limit macroblocks per second. For example, 1920x1080 at 60 fps is labeled Level 4.0 here even though that rate requires a higher level, so a driver may reject or clamp the requested rate or emit a stream whose declared level understates its requirements. Include config.framerate and the other relevant level constraints when selecting the level. (Written by GPT-5.6 Sol)

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Fixed in c1362eb. h264_level now walks Table A-1 for every level the control has offered since it was introduced (1.0 through 5.1) and picks the first one that fits the frame size, macroblocks per second from config.framerate, and the configured bitrate. 1080p60 lands on 4.2, 720p60 on 3.2, and a 720p30 stream moves from 3.1 to 3.2 only past 14 Mbit/s; each of those is a test. (written by Claude Fable 5.1)

coderabbitai · 2026-09-03T07:59:51Z

Walkthrough

The change enables default-on Linux v4l2 support with v4l and libc. Shared V4L2 plumbing now handles compose-rectangle origins during plane layout. New H.264 decoder and encoder backends support stateful M2M devices, source changes, timestamp restoration, queue draining, controls, bitrate updates, and fallback paths. Crate features, backend selection, documentation, and quest plans now include the updated V4L2 behavior.

Merge Risk: 🟡 Moderate · up to 0dbb3

V4L2 encoding and decoding can associate frames with incorrect timestamps or lose valid output after a later input-buffer failure. Resolve these backend correctness issues before merging; the outstanding package-version policy concern also remains open.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 84.13% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 126 functions across 8 files. (10 skipped: …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding V4L2 stateful M2M hardware encoder and decoder backends.
Description check	✅ Passed	The description directly explains the V4L2 encode and decode implementation, feature forwarding, hardware validation, tests, and scope. It is related to the changeset despite containing outdated state…

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Inline comments:
In `@doc/lib/rs/crate/moq-video.md`:
- Around line 59-60: Update the validation statements for the VAAPI and V4L2
backends: in doc/lib/rs/crate/moq-video.md lines 59-60, distinguish VAAPI as
unvalidated and V4L2 as validated; in doc/bin/cli.md line 377, remove the claim
that neither backend has real-hardware validation; and in
doc/lib/rs/crate/moq-video.md line 72, document Raspberry Pi 4 validation for
V4L2 while retaining the off-by-default policy where broader coverage is
required.

In `@rs/moq-video/src/encode/backend/v4l2.rs`:
- Around line 30-33: The V4L2 documentation is stale: update the encoder
paragraph near the v4l2 encoder module docs to state that Raspberry Pi 4
hardware validation with bcm2835-codec produced a valid 640x360 H.264 stream,
retaining only genuinely unvalidated details; also update the decoder paragraph
near the v4l2 decoder module docs to state that a 640x360 stream played without
decode failures on the same board. Apply the corresponding documentation changes
in rs/moq-video/src/encode/backend/v4l2.rs lines 30-33 and
rs/moq-video/src/decode/backend/v4l2.rs lines 27-30.

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doc/bin/cli.md
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rs/moq-video/Cargo.toml
rs/moq-video/src/decode/backend/mod.rs
rs/moq-video/src/decode/backend/v4l2.rs
rs/moq-video/src/decode/decoder.rs
rs/moq-video/src/encode/backend/mod.rs
rs/moq-video/src/encode/backend/v4l2.rs
rs/moq-video/src/encode/encoder.rs
rs/moq-video/src/lib.rs
rs/moq-video/src/v4l2.rs

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…er (moq-dev#3332) Adds the V4L2 memory-to-memory backends, which is what gives a Raspberry Pi and similar SoCs hardware H.264 without a vendor SDK. Follows the stateful encoder and decoder interfaces: VIDIOC_ENCODER_CMD to drain, V4L2_BUF_FLAG_LAST to end it, and SOURCE_CHANGE to pick up the decoder's format. Both backends have since run on a Raspberry Pi 4 (bcm2835-codec, Raspberry Pi OS Bookworm): the encoder opened /dev/video11 and published Constrained Baseline 640x360 that decodes back to a correct picture, and the decoder is what automatic selection picks on that board and played a stream for a minute with no failures. The feature comment said neither had been validated on hardware and no longer does. Squashed from `pr/v4l2-m2m` at 9517082, open as moq-dev#3332.

…er (moq-dev#3332) Squashed from `pr/v4l2-m2m` at 9517082, open as moq-dev#3332.

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rs/moq-video/Cargo.toml (1)
8-8: 📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Revert the package version bump.

The PR adds an opt-in backend, but it does not request a release. Restore version = "0.0.21" unless this change is explicitly part of a release.

As per coding guidelines, rs/moq-video/Cargo.toml: Do not bump package versions unless the user explicitly asks for a version bump or release.
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In `@rs/moq-video/Cargo.toml` at line 8, Restore the package version in the
Cargo.toml package metadata from 0.0.22 to 0.0.21; do not introduce or retain a
version bump for this backend change.
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In `@rs/moq-video/Cargo.toml`:
- Line 8: Restore the package version in the Cargo.toml package metadata from
0.0.22 to 0.0.21; do not introduce or retain a version bump for this backend
change.

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rs/moq-video/Cargo.toml

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Adds an H.264 encoder backend driving the kernel's stateful memory-to-memory codec interface, which is how most ARM SoCs expose their video encoder: a Raspberry Pi's VideoCore (`bcm2835-codec`), and the equivalent block on Rockchip, Amlogic, Allwinner and Exynos parts. Until now a Pi could only republish what `rpicam-vid` had already encoded, or spend its CPU on openh264. - New non-default, Linux-gated `v4l2` feature, alongside `nvidia` and `vaapi`. It adds no runtime dependency: the interface is ioctls on a device node. - `src/v4l2.rs` holds the device, the mmap buffer pool, and the plane arithmetic, so the decode half can be added on top of the same layer. - No new crate. The backend uses the raw layer of `v4l`, which the camera capture path already depends on: `v4l::v4l_sys` supplies the `videodev2.h` structs and `v4l::v4l2::vidioc` the request codes, so no ioctl struct is hand-rolled and the workspace grows no dependency. The alternative (`v4l2r`) would have added one for the same ioctls. - The node is found by what it converts (`VIDIOC_QUERYCAP` plus `VIDIOC_ENUM_FMT` over every V4L2 node) rather than by a path table, since node numbering is per SoC. `MOQ_V4L2_ENCODER` overrides it. Two driver behaviors this gets right, both learned on Pi hardware. The H.264 level is set from the resolution before `VIDIOC_S_FMT`, because bcm2835-codec defaults to level 1.0 (128x96) and refuses anything larger otherwise; and both `REPEAT_SEQ_HEADER` and `PREPEND_SPSPPS_TO_IDR` are asked for, because drivers implement one each and a subscriber joining at a later keyframe needs SPS/PPS in band ahead of it. Stride and height alignment is the third, and it is the one that shows. `VIDIOC_S_FMT` is a negotiation: the driver answers with its own fourcc, its own `bytesperline`, and a padded row count it reports only through `sizeimage`. `Planes` reads all three back and places chroma at `stride * padded_rows`. Writing it at `stride * height` instead leaves the driver reading zeroes for chroma, which comes out as a green picture. Draining is a command, not a wait. The kernel's stateful encoder contract is that an encoder holding a frame releases it for one reason, which is being told the stream stopped: `V4L2_ENC_CMD_STOP`, dequeue CAPTURE to the buffer flagged `V4L2_BUF_FLAG_LAST`, then `V4L2_ENC_CMD_START` to resume with the state from before the drain. See dev-encoder.rst, "Drain". A flush falls on every group boundary, so an encoder deeper than one-in-one-out (a lookahead, two-pass rate control) would fail the first boundary and kill the broadcast if flush could only wait. A driver that refuses the command keeps the old best-effort wait, which is right for exactly the encoders that would have been fine anyway. Coded buffers are matched to source frames by timestamp rather than counted. The driver copies an OUTPUT buffer's timestamp onto the CAPTURE buffer its work came out on, and a coded buffer matching no submitted timestamp is held and goes in front of the next access unit. That is what a driver defaulting to `V4L2_MPEG_VIDEO_HEADER_MODE_SEPARATE` produces, since it emits SPS/PPS on a coded buffer of their own answering no source frame. The header mode is asked for as `JOINED_WITH_1ST_FRAME` where the driver has the control, but the accounting does not depend on it. Both that control and `V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME` are best-effort, because a driver answering `EINVAL` to either should not fail a keyframe request: `V4L2_CID_MPEG_VIDEO_GOP_SIZE` still produces keyframes, on the driver's own GOP boundary rather than where the caller asked. Unit tests cover the timestamp matching, the held parameter sets, a buffer the driver flagged `V4L2_BUF_FLAG_ERROR`, and the plane arithmetic. Compile-verified only, on x86 with no M2M codec device. The ioctl sequence comes from an implementation that ran on Pi Zero 2 W, Pi 3 and Pi 4, but this port has not been run on a Pi and the emitted bitstream needs one to confirm at playback.

The mirror of the V4L2 encoder, on the same device layer and usually on a sibling node of the same driver: a Raspberry Pi decodes on `/dev/video10` and encodes on `/dev/video11`, both `bcm2835-codec`. Behind the same `v4l2` feature, since it is one dependency and one code path. Access units go in on the OUTPUT queue in decode order and pictures come back on CAPTURE as CPU I420. The driver copies each input buffer's timestamp onto the picture it produced, so presentation times survive decoder delay with no bookkeeping in the backend. A stateful decoder parses the stream itself and announces the picture size with a `V4L2_EVENT_SOURCE_CHANGE`, so the CAPTURE queue does not exist until the first parameter sets have been fed. `decode` returns no frames until the event arrives, which is the buffering the backend trait already allows for, and the same event later means the stream changed size. A size change is an implicit drain that ends with a buffer flagged `V4L2_BUF_FLAG_LAST`, so the queue is drained to that buffer before it is torn down and renegotiated: releasing it on the event alone would discard every picture the driver had already decoded from before the change. See dev-decoder.rst, "Dynamic Resolution Change". The drain is bounded, so a driver that never sends the flag costs the tail rather than the stream. A stream the driver can never size, an unsupported profile or a bytestream the parser never syncs to, would otherwise cap the caller at four calls a second indefinitely with nothing in the log to say why. The per-call wait is 250 ms, since a subscriber can join anywhere in a group and the first access units may well be undecodable, but five seconds without a source change fails with the time it waited. The CAPTURE format is selected rather than accepted. `Device::check` proves the node offers a format we can read at open, but that set narrows once the stream is parsed and the driver's default need not be in it: amphion answers `NV12_8L128` and mtk-vcodec `MM21`, both tiled. `VIDIOC_ENUM_FMT` re-reads what the driver supports for the stream it has parsed and `VIDIOC_S_FMT` picks an 8-bit 4:2:0 format out of it, which is the step dev-decoder.rst puts there. Alignment is read back on this side too. `VIDIOC_G_FMT` reports the coded size, which is rounded up to whole macroblocks (1080p codes as 1088 rows), and `VIDIOC_G_SELECTION` reports the compose rectangle inside it that is the actual picture. Chroma is read from `bytesperline * padded_rows`, mirroring where the encoder writes it. The shared layer grows what the decoder needs: `VIDIOC_G_FMT` read-back, source change subscription and dequeue, the compose rectangle, `VIDIOC_G_CTRL` for the driver's minimum buffer count, releasing a queue for renegotiation, and the read direction of the plane arithmetic. `VIDIOC_SUBSCRIBE_EVENT`, `DQEVENT` and `G_SELECTION` are not in the `v4l` crate's request table, so their codes are built here the way `linux/ioctl.h` builds them. H.264 only, and only stateful. A Raspberry Pi 4's separate HEVC block and Rockchip's rkvdec are stateless V4L2 decoders driven through the media request API with per-slice parameters, which is a different interface rather than another format here. Compile-verified only, on x86 with no M2M codec device. The sequence follows the kernel's stateful decoder documentation and an implementation that ran on Pi Zero 2 W, Pi 3 and Pi 4, but it needs a Pi to confirm.

Three defects found reading the backends against the kernel's stateful encoder and decoder documentation. The encoder failed on its first frame. `Backend::encode` collects finished input buffers before it takes a free one, and the first call reaches that collection before either queue has been started. `vb2_core_dqbuf` checks `q->streaming` ahead of everything else and answers `EINVAL`, not the `EAGAIN` an empty queue answers, so the very first frame returned a `DQBUF` error. A queue that has not been started now reports itself empty instead of asking the driver. The encoder negotiated its formats in the wrong order. dev-encoder.rst puts the coded CAPTURE format first and warns that setting it derives a new OUTPUT format, so the raw stride and padded row count read back from an OUTPUT format negotiated first are the ones the driver may since have replaced. Those two numbers are what `Planes` places chroma from. `VIDIOC_S_FMT` carried `bytesperline` and `sizeimage` over from the `VIDIOC_G_FMT` that precedes it, which describe whatever format the queue held before the call. Zero is how V4L2 asks the driver to size a plane itself, and it is what every other userspace V4L2 client sends. Also replaces eleven `std::mem::zeroed` calls with one `unsafe trait Arg` whose contract states the invariant they all relied on, leaving each ioctl site with a `// SAFETY:` comment that justifies the ioctl rather than a comment about the ioctl sitting above the zeroing. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

…ffers A coded buffer whose timestamp matches no queued frame is held to go in front of the next access unit, which is what `HEADER_MODE_SEPARATE` parameter sets need. A driver that does not copy the OUTPUT timestamp onto the CAPTURE buffer it answered with matches nothing at all, and every access unit of the stream was being held instead. Held bytes now stop at 64 KiB, well past what parameter sets cost and well short of a picture, and reaching that limit discards them: bytes that answer no frame are only parameter sets while there are few of them, and putting a whole picture in front of a later access unit would corrupt the track rather than repair it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

`SOURCE_CHANGE_BUDGET` and two comments used words the writing guide bans. The renamed constant also had the wrong reason attached to it: the decoder layer above holds every access unit back until the first keyframe, so the backend's first submission always carries parameter sets and the wait is not covering a join far from one. The comment on `set_bitrate` promised that a driver which refuses is not asked again, which nothing implements. What actually happens is that the caller's control loop stops adapting on `BitrateUnsupported`. `Dequeued::flags` is read only through `failed` and `last`, as its own comment says, so it no longer needs to be visible past them. The remaining edits cut sentences that restate their neighbours, and note on `wait` that its blocking is deliberate because every backend here owns an OS thread. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

…crate `VIDIOC_SUBSCRIBE_EVENT`, `VIDIOC_DQEVENT` and `VIDIOC_G_SELECTION` are built here because the `v4l` crate's table omits them, and a code built wrong is `ENOTTY` at runtime on whichever board reaches it first, with nothing in the tree to catch it before then. The test builds one code per direction for requests the crate does export, so the shifts, the direction bits, and the struct size have to agree on whatever target this compiles for. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

The explanation of why the enumeration stops on any error belongs with the loop, not stranded between the request struct and the call it justifies. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

The feature comment said neither backend had been validated on real hardware, which was true when it was written and is not any more. Both ran on a Raspberry Pi 4 (bcm2835-codec, Raspberry Pi OS Bookworm, kernel 6.6.x) driven through a downstream CLI: - The encoder opened `/dev/video11`, negotiated NV12 at 640x360, and published a stream that decodes back to a correct picture. Fourteen seconds recorded to fMP4 came to 490 KB of Constrained Baseline. - The decoder is what automatic selection picks on that board over the openh264 fallback, and it played a 640x360 stream for a minute with no decode failures and a picture that moved. Still off by default: one SoC is not the several this drives, and the `v4l` crate's bindgen still wants libclang at build time.

Both module docs still opened with NOT YET VALIDATED ON HARDWARE, which the review picked up against the pull request's own description. Both halves have run on a Pi 4 since: the encoder produces Constrained Baseline with the parameter sets ahead of every keyframe and a correct picture at playback, and the decoder plays 720p at a steady 30fps while holding several hundred milliseconds of pictures in its own queue, measured at about 690ms end to end against a software decoder on the same Pi. The docs say that instead.

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Inline comments:
In `@rs/moq-video/Cargo.toml`:
- Line 61: Update the v4l2 feature in Cargo.toml so the v4l dependency is
enabled only for Linux targets, either by moving it into the Linux-specific
dependency configuration or by adding an equivalent non-Linux restriction;
retain libc as appropriate and ensure non-Linux builds do not compile v4l.

In `@rs/moq-video/src/encode/backend/v4l2.rs`:
- Line 135: Update h264_level and its call at the V4L2_CID_MPEG_VIDEO_H264_LEVEL
control to select the lowest H.264 level satisfying frame size,
macroblocks-per-second from configured frame rate, and configured bitrate before
VIDIOC_S_FMT. Add coverage for 1920x1080 at 60 fps asserting Level 4.2 is
selected.

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doc/bin/cli.md
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rs/moq-video/src/encode/backend/v4l2.rs

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doc/lib/rs/crate/moq-video.md

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…eaders off timestamp zero Two defects in the V4L2 encoder, found in review, plus the feature plumbing that made the backend unreachable from a `moq-cli` build. The level was chosen from frame size alone. Table A-1 also caps macroblocks per second and bitrate, so 1080p60 was labelled level 4.0 (245,760 MB/s) when it needs 4.2 (522,240), and a driver may clamp the rate to the level it was given. `h264_level` now walks the whole menu the control has offered since it was introduced (1.0 through 5.1) and takes the first level all three limits fit, with tests for the framerate and bitrate columns. Parameter sets emitted on a coded buffer of their own were told apart from a frame by timestamp alone. A driver in `HEADER_MODE_SEPARATE` stamps them with whatever the buffer held, which is zero, and zero is also a timestamp a real frame carries: the first frame of a capture, and the one frame `Config::probe` encodes. The header then matched that frame and was published as it, while the IDR answering it matched nothing and was carried into the next access unit. A coded buffer is now judged on its bytes: one with no VCL NAL answers no frame whatever its timestamp says, and is carried ahead of the next one that does. The regression test stamps both at zero. `moq-cli` and `moq-transcode` did not forward `v4l2`, so the documented build command was rejected as an unknown feature. Both forward it now, and it is on by default in all three crates, per the default-features policy landed in moq-dev#3353: it costs a build nothing `capture` does not already pay (the same `v4l` crate and bindgen), and an off-by-default feature is one `just check` never compiles. The docs no longer describe it as opt-in. The two quests that scoped this work are trimmed to what remains: `teleop/v4l2-encode.md` now covers shipping the backend in a released `moq-cli` (blocked on CLI packaging) and the hardware note, and `video-embedded.md` covers only the EGL import. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

chatgpt-codex-connector · 2026-09-04T17:23:11Z

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chatgpt-codex-connector · 2026-09-04T17:29:01Z

+			}
+		}
+
+		let timestamp = Duration::from_micros(frame.timestamp.as_micros() as u64);


Preserve the encoder's original timestamp scale

When callers provide timestamps in a scale other than microseconds, this conversion discards the original Timestamp: as_micros() truncates fractional microseconds, and the completed frame is reconstructed with Timestamp::from_micros. For example, 90 kHz or nanosecond input timestamps are changed even though Encoder::encode promises to return the timestamp of the source frame, and structurally equal instants at different scales are intentionally distinct. Keep the original timestamp in Pending and use only a generated V4L2 token for matching. (Written by GPT-5.6 Sol) rs/CLAUDE.mdL109-L109

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Fixed in f32c5b3. The microseconds are now only the key a frame rides the driver under: Pending keeps the original Timestamp alongside it and answers the access unit with that, so a 90 kHz instant comes back as the same 90 kHz instant. There is a test that queues one and checks the answer is structurally equal to the input. (written by Claude Fable 5.1)

chatgpt-codex-connector · 2026-09-04T17:29:01Z

+		self.coded.plane_mut(index, 0)[..access_unit.len()].copy_from_slice(access_unit);
+
+		let bytesused = [access_unit.len() as u32];
+		let timestamp = Duration::from_micros(timestamp.as_micros() as u64);


Preserve decoder input timestamps across V4L2

For streams using a non-microsecond timescale, such as the common 90 kHz video clock, this floors the timestamp to whole microseconds and drain later creates a new microsecond-scale Timestamp. Decoded and transcoded frames therefore no longer carry the exact timestamp supplied to decode, including its structurally significant scale. Associate each V4L2 timestamp token with the original Timestamp and restore that value when the picture is dequeued. (Written by GPT-5.6 Sol) rs/CLAUDE.mdL109-L109

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Fixed in f32c5b3 and tested in 0dbb352. The decoder remembers each submitted access unit's Timestamp under its microsecond key in a bounded queue (64 entries), restores it when the picture dequeues (searched, not popped, since pictures may come back in presentation order), and falls back to the buffer's own timestamp for a picture the driver stamped with nothing submitted. (written by Claude Fable 5.1)

chatgpt-codex-connector · 2026-09-04T17:29:01Z

+		// SAFETY: `VIDIOC_G_SELECTION` takes a `v4l2_selection`.
+		unsafe { self.ioctl(request::G_SELECTION, &mut selection) }.ok()?;
+
+		let size = Size::new(selection.r.width, selection.r.height);


Honor the compose rectangle's origin

When an H.264 stream has a visible crop with a nonzero left or top offset, this retains only the compose rectangle's dimensions. Planes::read consequently starts at buffer offset zero and returns the coded picture's upper-left region instead of the rectangle selected by the driver, producing a shifted or padded image. Carry selection.r.left and .top into the plane offsets as well as using its width and height. (Written by GPT-5.6 Sol)

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Fixed in f32c5b3. Device::visible now returns a Rect with left and top, and Planes::new folds the origin into every component offset: luma by the full amount, chroma by half a row and half a column (whole bytes for interleaved NV12, half for planar). The bounds check includes the origin, and a 1920x1088 test with a 16,8 origin pins the offsets for both chroma layouts. (written by Claude Fable 5.1)

…e compose origin Three findings from the review of the last commit, all in the new V4L2 code. Both backends rebuilt the timestamp from the buffer's `struct timeval`, which carries whole microseconds. A `Timestamp` is an instant at its own scale, so a 90 kHz tick went in and a different, microsecond-scale instant came out, against the crate's rule that scale is structural. The microseconds are now only the key a frame rides the driver under: the encoder's `Pending` and a bounded queue in the decoder remember the original alongside it and answer the picture with that. A driver that stamps a picture with nothing that was submitted still gets the buffer's own. The decoder read the compose rectangle's size and dropped its origin, so a stream cropped on its left or top edge would have been read from the coded picture's corner. `Device::visible` returns a `Rect`, and `Planes::new` folds its origin into every component's offset, chroma by half. The encoder passes the whole picture, as before. The H.264 level table stopped at 5.1 and used it for anything larger, so 4K60 would have been labelled with a level it does not fit. The table now runs through 6.2, with the four entries newer than some build hosts' headers spelled as the constants the UAPI fixes them to, and a config past 6.2 is refused rather than understated. The one finding not fixed here is that no decoder can be drained: the decode `Backend` trait has no flush or finish, which NVDEC shares, so it is a trait-level gap rather than one this code adds. `quest/m3/decode-drain.md` scopes it. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

The encoder half of the timestamp fix had a test and the decoder half did not, so reverting the decoder's bookkeeping would have left every test green. `remember` and `restore` are now free functions over the queue, and three tests pin what they promise: a 90 kHz timestamp comes back unchanged and in presentation rather than submission order, a picture the driver stamped with nothing submitted keeps the buffer's own, and the queue forgets its oldest entry past the bound. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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chatgpt-codex-connector · 2026-09-04T20:01:25Z

+
+		let mut bytesused = [0; MAX_PLANES];
+		for (used, plane) in bytesused.iter_mut().zip(&planes) {
+			*used = plane.bytesused;


Honor per-plane data offsets

On a V4L2 driver that dequeues a multi-planar buffer with a nonzero v4l2_plane.data_offset, which the UAPI permits when a plane has a header, this loop preserves only bytesused and discards where the payload begins. access_unit and Planes::read consequently consume from mapping offset zero, so encoding can publish header bytes and decoding can shift every component. Preserve each dequeued plane's offset and apply it when slicing the reported payload. (Written by GPT-5.6 Sol)

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Fixed in 5b6e931. Dequeued now carries each plane's data_offset, Queue::payload hands back the plane from there, and written is bytesused less the offset, which is what the encoder's access-unit copy and the decoder's empty-picture check and Planes::read use. (written by Claude Fable 5.1)

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Inline comments:
In `@rs/moq-video/src/encode/backend/v4l2.rs`:
- Line 528: The V4L2 encode and decode paths must use unique monotonic
microsecond keys for outstanding frames instead of truncating timestamps
directly. In rs/moq-video/src/encode/backend/v4l2.rs lines 528-528, update the
queued-frame key allocation and retain the original Timestamp in Pending; apply
the same change in rs/moq-video/src/decode/backend/v4l2.rs lines 395-395 for
submitted access units and submitted storage.
- Line 634: Update Pending::dropped to remove only the failed frame at index at
instead of draining all frames through it. Preserve later pending frames and add
a test that fails the second queued frame, then successfully matches the first
frame’s coded buffer.

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doc/bin/cli.md
doc/lib/rs/crate/moq-video.md
quest/m3/README.md
quest/m3/decode-drain.md
quest/m3/teleop/README.md
quest/m3/teleop/v4l2-encode.md
quest/m3/video-embedded.md
rs/moq-cli/Cargo.toml
rs/moq-transcode/Cargo.toml
rs/moq-video/Cargo.toml
rs/moq-video/src/decode/backend/v4l2.rs
rs/moq-video/src/encode/backend/v4l2.rs
rs/moq-video/src/v4l2.rs

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A driver may put a header ahead of a plane's payload and report where the payload starts through `v4l2_plane.data_offset`, with `bytesused` counted from the start of the plane. Both backends read from offset zero: the encoder would have published the header as part of the access unit, and the decoder would have read every component shifted. `Dequeued` keeps each plane's offset, `Queue::payload` slices from it, and `written` is the payload length past it, which is what the empty-picture check and the access unit copy now use. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

`Pending::dropped` drained every frame up to the one the driver flagged, on the reasoning `matched` uses for answers: an encoder does not go back. A dropped raw buffer says nothing of the kind. Raw buffers come back in their own order, so a later frame can fail while an earlier one still has its access unit on the way, and draining took the earlier frame with it. Its access unit then matched nothing, and an unmatched buffer was carried in front of the next access unit, which put a whole picture where a header goes. Only the flagged frame is removed now. And a picture that answers no frame is dropped with a warning rather than carried: it has no timestamp to be published under, and the only driver that produces one is a driver that is not copying timestamps at all. Carrying stays for what holds no picture, which is the parameter sets it was for. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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chatgpt-codex-connector · 2026-09-04T20:24:16Z

+		let mut event = libc::pollfd {
+			fd: self.file.as_raw_fd(),
+			events: libc::POLLIN | libc::POLLOUT | libc::POLLPRI,
+			revents: 0,


Poll only for readiness the caller will consume

When the decoder is waiting for its initial source-change event, this mask also wakes on POLLOUT. Once the driver returns an OUTPUT buffer for an undecodable access unit, POLLOUT remains level-triggered until that buffer is dequeued, but the loop in decode checks only take_source_change() and never drains OUTPUT. A stream that cannot announce its size therefore spins continuously for each 250 ms wait, potentially occupying a full CPU core until the five-second limit. Use an event-specific poll mask or reclaim completed coded buffers in that loop.

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Frando mentioned this pull request Sep 2, 2026

feat(moq-video): add the VAAPI H.264 decoder and import its pictures zero-copy #3331

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Frando marked this pull request as ready for review September 3, 2026 07:52

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Comment thread doc/lib/rs/crate/moq-video.md Outdated

Comment thread rs/moq-video/src/encode/backend/v4l2.rs Outdated

Frando added a commit to Frando/moq that referenced this pull request Sep 3, 2026

feat(moq-video): add the V4L2 stateful M2M hardware encoder and decod…

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…er (moq-dev#3332) Squashed from `pr/v4l2-m2m` at 9517082, open as moq-dev#3332.

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Frando mentioned this pull request Sep 4, 2026

refactor!: align to moq n0-computer/iroh-live#45

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Frando and others added 9 commits September 4, 2026 09:37

docs(moq-video): put the ENUM_FMT safety comment next to its ioctl

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The explanation of why the enumeration stops on any error belongs with the loop, not stranded between the request struct and the call it justifies. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Frando force-pushed the pr/v4l2-m2m branch from 8e5bff4 to 78ecbf2 Compare September 4, 2026 07:39

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kixelated and others added 2 commits September 4, 2026 12:06

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Comment thread rs/moq-video/src/encode/backend/v4l2.rs

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kixelated and others added 2 commits September 4, 2026 13:05

kixelated merged commit a30e2ae into moq-dev:main Sep 4, 2026
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chore: release #3352

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kixelated mentioned this pull request Sep 5, 2026

fix(moq-video): make moq-video compile on dev again #3423

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		fn h264_level(size: Size) -> i32 {
		let macroblocks = size.width.div_ceil(16) * size.height.div_ceil(16);

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Frando commented Sep 2, 2026 • edited by kixelated Loading Uh oh! There was an error while loading. Please reload this page.

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Now run on a Raspberry Pi 4

Summary

The driver decides the raw layout

Draining the encoder is a command, not a wait

Public API changes

Test plan

Conflicts with #3331

Review round

Takeover (c1362eb)

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Walkthrough

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Frando commented Sep 2, 2026 •

edited by kixelated

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Takeover (`c1362eb`)

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