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The decode half of the VAAPI backend, next to the encoder that has been behind the same feature since it landed. On Linux the hardware decode tier was NVIDIA-only; this gives Intel and AMD boxes a hardware H.264 path instead of falling through to openh264. moq-vaapi's new `decode` module does the work: one Annex-B access unit in, tightly-packed NV12 out, with the parameter sets read from the stream. This adapts it the way the encode backend does, deinterleaving NV12 to the CPU I420 the rest of the crate speaks, and registers it after NVDEC in the hardware candidate list so a machine with both keeps preferring NVDEC. Output is CPU I420 rather than a zero-copy DMA-BUF surface on purpose. VA-API decode targets on Intel come back Y-tiled (DRM modifier 0x100000000000002), which the Vulkan renderer cannot import, so exporting one here would only move the download somewhere less convenient. That changes once the re-tile lands. `I420::from_nv12` and `deinterleave_uv` were already written for the Media Foundation and PipeWire capture paths; their cfg now includes the vaapi feature rather than growing a second copy. Tested on Intel Meteor Lake (iHD 26.1.5): the added round trip encodes a gradient with openh264 and decodes it through the hardware, asserting the returned planes match the input and that each picture carries its own timestamp. It skips itself on a host with no usable VA stack, like the NVDEC tests do.
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| moq-vaapi = "0.0.3" | ||
| # The decoder this crate's VAAPI backend calls is not in 0.0.3 yet; see | ||
| # moq-dev/vaapi#2. Point this back at a release once one carries `decode`. | ||
| moq-vaapi = { git = "https://github.com/Frando/vaapi", branch = "pr/decode" } |
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When the Rust release workflow packages moq-video, Cargo rejects this inherited git dependency because it has no version requirement, even though it is optional in moq-video (cargo package reports that all dependencies must specify a version). Adding the old 0.0.3 version would not suffice because packaged manifests discard the git source and that release lacks the new decode API. This therefore blocks release-plz from publishing the crate until moq-vaapi has a crates.io release containing decode and the workspace dependency points to it.
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Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review. WalkthroughThe change adds a Linux VAAPI H.264 decoder behind the Merge Risk: 🟠 High · up to The new Linux VAAPI decoder can be transferred across threads even though its safety depends on thread confinement, creating a potential memory-safety issue. A cancelled or failed end-of-stream drain can also permanently drop buffered frames and report end-of-stream, while the unreleased dependency reduces build reproducibility and rollback confidence. These issues should be fixed or explicitly accepted before merging. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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In `@rs/moq-video/src/decode/backend/vaapi.rs`:
- Line 74: Update Consumer::read end-of-stream handling so a None input drains
the VAAPI decoder instead of returning immediately. Expose the required drain
operation through Backend and Sink, return all flushed frames before reporting
end of stream, and preserve normal decode behavior for available access units.
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Cargo.tomlrs/moq-video/Cargo.tomlrs/moq-video/src/decode/backend/mod.rsrs/moq-video/src/decode/backend/vaapi.rsrs/moq-video/src/frame.rs
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`Consumer::read` returned `None` the moment the track did, which for a backend that buffers means the last pictures of every stream are dropped on the floor. VAAPI is that backend: H.264 releases a picture from the DPB only once a later one needs its slot, and the slot count follows the sequence's reference and reorder limits rather than the reorder depth the stream actually used, so the tail is several pictures rather than one. `Backend::flush` is the drain, with a default returning no frames so the backends that decode one-in one-out need nothing. That is all of them but VAAPI: openh264 is one-in one-out, Media Foundation runs with its output-reorder buffer disabled, NVDEC pins `ulMaxDisplayDelay` to zero, and VideoToolbox decodes without temporal processing. VAAPI overrides it onto `moq_vaapi::decode::Decoder::flush`, which runs the C.4.5 bumping process to completion. It is exposed up through `decode::Decoder` and `decode::Sink`, where it lands in order on the codec thread behind the access units it follows, and `Consumer::read` calls it once when the track ends and hands out what comes back before reporting the end. Measured on Intel Meteor Lake (iHD 26.1.5). Feeding five openh264-encoded pictures through the hardware returned four; the sixth picture of an x264 stream at its defaults costs two, and at ref=3 with no B-frames it costs three. Both new tests were watched failing against the old code with exactly the last picture missing, and both assert that `decode` alone came up short before asserting that the flush makes up the difference, so neither can quietly go vacuous. Not addressed here: moq-transcode decodes group by group without a flush at the boundaries, so a VAAPI decoder there carries the tail of one group into the next, past the keyframe that opens it. The encode side already flushes at every boundary for that reason. It needs the same treatment and its own test.
`vaapi` enables `dmabuf` without `render`, so in that build nothing reads the producer lease or splits the export apart, and clippy fails the crate on two dead-code errors. The lease has to stay a field either way: dropping it would release the producer's buffer while the descriptor is still outstanding. So it keeps an allow with the reason written down, and the method is gated on the feature that calls it.
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Replaced by #3331, which carries these commits plus the zero-copy import that was stacked on them. One PR instead of two stacked ones. |
Adds
decode/backend/vaapi.rs, 130 lines including its test, adaptingmoq_vaapi::decodeto theBackendtrait. It is the mirror of the 115-line encode backend, registered next to NVDEC and gated on Linux andvaapi.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
Output is CPU I420 via
vaDeriveImage, which works on this hardware, so the only copy is the strided row copy that strips pitch padding. There is avaCreateImageandvaGetImagefallback for drivers that cannot derive; instrumented, it never fires here.Position in the series
Depends on moq-dev/vaapi#2, which adds the decoder this calls. That is not in moq-vaapi 0.0.3, so this PR points
moq-vaapiat the branch to give CI something to build. That dependency line has to go back to a released version before merge.#3331 stacks on this one. It needs
I420::from_nv12, added here, and the samemoq-vaapidependency. This PR stands alone and can merge first.Verified
Tested on Intel Meteor Lake with iHD 26.1.5.
decode::backend::vaapi::tests::vaapi_h264_round_tripencodes a gradient with openh264, decodes it with VA-API, and compares planes by mean absolute error with exact timestamps. It self-skips rather than being#[ignore]d, so it runs wherever the hardware exists and stays silent elsewhere.Kind::Autoselectsvaapion this machine with NVDEC compiled in and refusing.The underlying decoder is verified byte-for-byte against ffmpeg in moq-dev/vaapi#2.
Decoder delay
Output trails input by
num_ref_frames, not by the reorder depth the VUI declares, because the DPB bumps only when a new picture needs the slot. With x264's defaultref=3, a short stream yields nothing until the fourth access unit. NVDEC gets zero delay from an explicit cuvid knob and VA-API has no equivalent. This is documented in the backend's module doc.