feat(moq-video): add the VAAPI H.264 decoder and import its pictures zero-copy - #3331
feat(moq-video): add the VAAPI H.264 decoder and import its pictures zero-copy#3331Frando wants to merge 16 commits into
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…zero-copy (moq-dev#3331) Adds a VA-API H.264 decode backend, an NV12 DMA-BUF import that aliases each memory plane as its own texture, and `decode::Config::gpu_frames` connecting the two so a subscriber that only draws never pays for a download. Verified pixel-exact against the CPU path on Intel Meteor Lake. Squashed from `pr/zerocopy-nv12` at 7c443f8, open as moq-dev#3331.
…zero-copy (moq-dev#3331) Adds a VA-API H.264 decode backend, an end-of-stream drain, an NV12 DMA-BUF import that aliases each memory plane as its own texture, and `decode::Config::gpu_frames` connecting the two so a subscriber that only draws never pays for a download. Verified pixel-exact against the CPU path on Intel Meteor Lake. Squashed from `pr/zerocopy-nv12` at 89f4e6d, open as moq-dev#3331.
…zero-copy (moq-dev#3331) Adds a VA-API H.264 decode backend, an end-of-stream drain, an NV12 DMA-BUF import that aliases each memory plane as its own texture, and `decode::Config::gpu_frames` connecting the two so a subscriber that only draws never pays for a download. Verified pixel-exact against the CPU path on Intel Meteor Lake. Squashed from `pr/zerocopy-nv12` at 89f4e6d, open as moq-dev#3331.
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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 next moq-video release is packaged, Cargo rejects this git-only dependency because publishable dependencies must specify a crates.io version requirement. Adding version = "0.0.3" would not be sufficient because Cargo removes the git source from the packaged manifest and that release lacks the decoder API used here. Publish the decoder changes in a stable moq-vaapi release and depend on that version so moq-video remains releasable. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L108-L108
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WalkthroughAdds an opt-in Linux VAAPI H.264 decoder with CPU and GPU-resident output modes. Adds decoder flushing through the sink and consumer to release buffered frames at end of stream. Extends DMA-BUF formats and conversion paths for VAAPI surfaces. Adds planar NV12 and YUV420 Vulkan imports and enables Linux NV12 rendering with VAAPI-backed tests. Merge Risk: 🟡 Moderate · up to The change adds opt-in Linux VA-API decoding and zero-copy rendering, but it is not ready to merge while its required decoder API comes from a mutable development branch. Pinning a published release is necessary to make builds reproducible and deployment-safe. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 84.34% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 83 functions across 10 files. (2 skipped: 2 unsupported.) ✨ Finishing Touches✨ Simplify code
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In `@Cargo.toml`:
- Line 136: Update the moq-vaapi dependency to a released version that provides
the required decode API instead of the pr/decode development branch, then
regenerate Cargo.lock so it pins the selected release.
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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/decode/consumer.rsrs/moq-video/src/decode/decoder.rsrs/moq-video/src/decode/sink.rsrs/moq-video/src/frame.rsrs/moq-video/src/render/dmabuf.rsrs/moq-video/src/render/mod.rsrs/moq-video/src/render/renderer.rsrs/moq-video/src/render/source.rs
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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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📐 Maintainability & Code Quality | 🟠 Major | 🏗️ Heavy lift
🔎 Supported by static analysis
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set -eu
printf '%s\n' '--- Cargo.toml dependency context ---'
sed -n '124,142p' Cargo.toml
printf '%s\n' '--- Relevant tracked files ---'
git ls-files | rg '(^|/)(Cargo\.lock|Cargo\.toml)$|vaapi'
printf '%s\n' '--- moq-vaapi references ---'
rg -n -C 3 'moq-vaapi|vaapi|decode' Cargo.toml Cargo.lock 2>/dev/null || trueRepository: moq-dev/moq
Length of output: 4135
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get_repo_knowledge moq-dev/moq /tmp/coderabbit-repo-knowledge/moq-dev-moq-f8e40a3a/learnings /tmp/coderabbit-repo-knowledge/moq-dev-moq-f8e40a3a/conventions
Length of output: 35330
🌐 Web query:
Frando vaapi moq-vaapi decode released version GitHub tags
💡 Result:
The project moq-vaapi is a self-contained, lightweight VA-API H.264 hardware encoder for Linux, primarily used by the moq-video crate [1][2][3]. It is maintained under the moq-dev organization (GitHub: moq-dev/vaapi) [4], not by the user "Frando" (Franz Heinzmann), who is an active contributor to related projects like iroh and moq [5] but not the primary author of this specific crate [6]. As of September 3, 2026, the released versions of moq-vaapi on crates.io are [7][6]: - 0.0.3 (released 2026-07-23) - 0.0.2 (released 2026-06-16) - 0.0.1 (released 2026-06-16) The crate is described as AI-generated (via Claude) and derived from discord/cros-libva and discord/cros-codecs [1][3]. It is designed to be a standalone, publishable brick for moq-video, handling VA-API hardware encoding by dlopen-ing libva at runtime, thus avoiding build-time dependencies on libva [2][3].
Citations:
- 1: https://crates.io/crates/moq-vaapi
- 2: https://docs.rs/crate/moq-video/latest/source/src/encode/backend/vaapi.rs
- 3: https://docs.rs/crate/moq-vaapi/latest/source/Cargo.toml.orig
- 4: https://github.com/moq-dev/vaapi
- 5: https://github.com/Frando?tab=following
- 6: https://cloudfront-app.crates.io/crates/moq-vaapi
- 7: https://docs.rs/crate/moq-vaapi/latest
🌐 Web query:
site:github.com/moq-dev/vaapi/releases OR site:github.com/moq-dev/vaapi/tags moq-vaapi decode 0.0.3
💡 Result:
Search result: moq-vaapi version 0.0.3 is referenced by moq-dev/moq’s Cargo.toml. It is described as a standalone VA-API crate that dynamically loads libva; the available documentation characterizes it primarily as an H.264 hardware encoder, not a decoder. [1] [2]
The moq-video::decode API exists separately, but VA-API support is currently marked unvalidated and not listed as providing decoding. [3]
Replace the development branch before merging.
moq-vaapi resolves from pr/decode, and Cargo.lock pins a commit from that branch. Released 0.0.3 does not provide the required decoder API. Use a release that provides decode, then regenerate Cargo.lock.
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In `@Cargo.toml` at line 136, Update the moq-vaapi dependency to a released
version that provides the required decode API instead of the pr/decode
development branch, then regenerate Cargo.lock so it pins the selected release.
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…zero-copy (moq-dev#3331) Squashed from `pr/zerocopy-nv12` at 89f4e6d, open as moq-dev#3331.
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rs/moq-video/Cargo.toml (1)
8-8: 📐 Maintainability & Code Quality | 🟠 Major | ⚡ Quick winRevert the package version bump.
This change bumps
moq-videofrom0.0.21to0.0.22. The PR objectives do not ask for a version bump or a release. Revert this line, and revert the matchingversion = "0.0.22"in the rootCargo.tomlon line 142.As per coding guidelines: "Do not bump package versions unless the user explicitly asks for a version bump or release."
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Inline comments:
In `@rs/moq-video/Cargo.toml`:
- Around line 38-43: Align the VAAPI hardware-validation comments in the
affected Cargo manifests so they make the same claim; update the stale root
Cargo.toml wording to match the decoder/encoder validation status stated near
the VAAPI feature configuration, without changing dependencies or feature
behavior.
In `@rs/moq-video/src/render/renderer.rs`:
- Line 1266: Before the zero_copy/readback comparison loop, assert that
zero_copy and cpu have equal lengths, matching the sibling helper’s existing
validation, then retain the enumerate/zip iteration.
---
Outside diff comments:
In `@rs/moq-video/Cargo.toml`:
- Line 8: Revert the version bump by changing the package version in moq-video’s
Cargo.toml back to 0.0.21 and restoring the matching root Cargo.toml version
entry from 0.0.22 to 0.0.21.
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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.
`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.
wgpu-hal's importer describes one image made of one memory plane, and Mesa reports two memory planes for VK_FORMAT_G8_B8R8_2PLANE_420_UNORM under every modifier it lists, so a planar DMA-BUF had no route through it. Taken a plane at a time it does: the same driver reports one memory plane for R8_UNORM and R8G8_UNORM, so a luma import and a chroma import of the same object, each with its own offset and row pitch, alias the buffer as the two textures the NV12 shader already knew how to sample. Nothing is copied and nothing is reformatted; only the descriptor is duplicated, once per plane, since Vulkan takes ownership of the one it is handed. That makes Layout::Nv12 reachable on Linux, so it and the pipeline paired with it lose their macOS gate. YU12 comes along for the ride as three R8 planes, which is the layout the VA-API decoder produces. The re-tile keeps its place as the second chance for a modifier the driver does not list at all, and now imports its output per-plane too rather than refusing anything that comes back with more than one.
The failure this import can produce is a picture that looks plausible and is wrong: a swapped chroma pair turns blue to orange while leaving every gray alone, a plane read at the wrong offset comes out uniformly green, and a row pitch taken as the width shears the frame progressively downward. None of those show up in "the import returned Ok". So the test draws the same samples twice, once imported and once uploaded as CPU planes, reads both back, and compares every pixel. That catches all of the above at once, and a third assertion against the palette the pattern was built from covers the case where both paths misread the samples the same way. The palette is saturated and includes both blue and orange, since a gradient would pass a swapped import. It runs at three sizes, two of which the driver has to pad, so the row pitch and the chroma offset stop being derivable from the frame size: 200x120 comes back with a 256-byte pitch and its chroma plane at 256 * 128, and 62x34 with a 128-byte pitch and chroma at 128 * 64. The re-tile counter is what says the direct import ran rather than the blit, and is worth having outside the test too: a stream re-tiling every frame is paying for a GPU pass it would rather not, and both routes otherwise look identical from the outside.
The chain the per-plane import is for, end to end: openh264 encodes a gradient, VA-API decodes it, the decode surface is exported rather than downloaded, and the renderer imports its two planes. The same stream is decoded a second time the ordinary way and drawn through the CPU upload, and the two pictures have to agree. This is the case the synthetic fixture cannot reach, because the layout is the decoder's rather than one the test asked for. At 320x240 the surface comes back with a 384-byte pitch and its chroma plane at 384 * 256, so both the row padding and the height padding are the driver's and neither follows from the visible size. Needs moq-vaapi's decode_exported, which is newer than the revision the patch below pins, so the vaapi feature does not build until that lands.
`vaapi` and `pipewire` both enable `dmabuf` without `render`, and in those builds nothing reads the producer lease, splits the export apart, or constructs a plane or a descriptor. `cargo clippy --features vaapi --all-targets -- -D warnings` failed on the dead code that left behind. 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 methods and constructors are gated on the features that reach them. `DmaBufPlane::new` is built by PipeWire capture and by the importer's own tests; `DmaBuf::new` needs VA-API as well, since the tests that build one allocate through it. Verified clean under vaapi, render, vaapi+render, pipewire, capture+pipewire, and default features.
The decode backend said its output is CPU I420 because a decode target comes back at a modifier the renderer cannot import. Measured on Intel Meteor Lake with iHD, it exports at 0x100000000000009, which the renderer now imports per memory plane with no re-tile at all. What actually stands in the way of making GPU frames the default is Surface::into_i420: a CPU consumer of an exported surface has nowhere to read it back from, since the frame outlives the decoder that could map it. So it has to be something a caller asks for, which is a different problem from the one the text described.
The importer this PR adds already reaches a texture without a copy, but only for a producer that hands over a DMA-BUF, and the VAAPI decoder was not one: every picture was downloaded to CPU I420 whether the consumer read a pixel or not. What kept the export out of it was `Surface::into_i420`, which the type guarantees: an exported surface used to arrive with nothing that could map it, so a CPU consumer of one had no way back to its pixels and the export could not be offered without taking the universal arm of the `match` away. `moq-vaapi` now keeps the retired surface alongside the descriptor, so `decode::Config::gpu_frames` can ask this backend for `Surface::DmaBuf` pictures and a consumer that wants bytes still gets them, read back through `vaDeriveImage` rather than off a tiled buffer. `flush` takes the same fork, so a track that ends does not switch to downloading for its last few pictures. Off by default: exporting retires a surface from the recycling pool, which costs an allocation per picture that a CPU consumer gains nothing from. Other backends ignore the flag, as they do `resize`. A driver that decodes but will not export drops it after one warning rather than failing the stream. The renderer says once, at debug, when a frame is imported rather than uploaded, which is the only thing about a working renderer that distinguishes the two. `decoded_frames_reach_the_gpu_without_a_download` now drives this backend rather than moq-vaapi and a test fixture, so what it covers is the path the knob turns on. `gpu_frames_still_answer_into_i420` pins the read-back byte for byte against the same stream decoded to the CPU, and `flushing_returns_the_tail_of_a_gpu_stream` pins the drain, which is a separate call into moq-vaapi and would otherwise be the one path where a track's last pictures go missing. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The import module still promised a VA-API re-tile fallback for a buffer whose modifier Vulkan refuses. That fallback was dropped before this branch went up, because the counter for it read zero on every run, so what the doc described was a path no reader would find. Say what happens instead: the import fails and the renderer downloads the frame. The test helper's contract carried the same claim, and `DmaBuf::width` called its answer the coded allocation when both producers pass the visible size and the padding lives in the plane strides and offsets. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
`adopt` read the modifier off object zero and every plane offset off layer zero, then handed the consumer a descriptor for that one object. Two things it did not check can make that wrong. A driver that splits the allocation reports more than one object, and the chroma offset then addresses an object the importer never sees, which draws as a plausible-looking picture built from the wrong bytes. A driver that reports more planes than a PRIME descriptor holds four of would index past the end of the offset array and panic. `COMPOSED_LAYERS` asks for one layer in one object and Intel and AMD deliver it, but the export carries the counts and neither was read. The other half is what a failed export means. Any error from a shared decode cleared `gpu_frames` for the rest of the session and returned no frames, so one corrupt access unit cost every later picture its zero-copy path and was never reported. Once a picture has come back as a descriptor the driver has answered that question, so a later failure is a decode error and now surfaces as one. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
`read` marked the consumer drained and then awaited the flush, so a read dropped between the two answered for every read after it: the flag was already up, and the tail the decoder was still holding became an end of stream nobody could get past. Taking the tail first costs nothing, since flushing twice hands back nothing the second time. That window is narrow but the method has a wider one, so say so: the codec runs on its own thread and a read dropped mid-decode loses that access unit's pictures, which a caller reaching for `select!` needs to know before it finds out. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
`import` still read as if a second attempt followed the first: the result was bound to `direct`, and the producer lease was cloned into it rather than moved, which only makes sense when another attempt might want its own. The Vulkan guard's comment called itself belt and braces when what it is actually for is the `Ok(None)` it returns, which routes the frame to the CPU without the strike an import error would cost. Two things around the fixture while here. `va_fourcc` sat at module scope carrying its own `cfg(test)` when only the fixture calls it, and the fixture read object zero without checking there was only one, which would have compared an import of the wrong memory against a correct upload and blamed the importer. The renderer's end-to-end test named the backend by literal, where a rename would have turned into a silent skip rather than a failure. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
`Shape` splits a buffer three ways for YU12 and two for NV12, and only the two-way case had a test that runs anywhere. The three-way one needs a driver that will allocate YU12 before it is exercised at all, which is the arm most worth pinning: its chroma rows really are half as long, where NV12's are not, and that is the difference a reader is most likely to carry across from the neighbouring format. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The two picture order count derivations it corrected are on the branch this dependency points at. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Flush buffered decoder frames before ending live and fetched transcode groups. Also fix the feature-gated rustdoc link, align VAAPI validation docs, and harden zero-copy readback coverage. Co-Authored-By: GPT-5.6 Sol <noreply@openai.com>
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| Err(err) => { | ||
| tracing::warn!(%err, "VAAPI cannot hand out decoded surfaces; downloading them instead"); | ||
| self.gpu_frames = false; | ||
| Ok(Vec::new()) |
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When gpu_frames is enabled on a driver that can decode but cannot export its first output, decode_exported has already bumped pictures from the DPB, yet this branch returns an empty vector and permanently discards them before switching later calls to CPU output. The same path during flush_exported can drop the entire tail of a short stream. Download the returned exported pictures when adoption fails, or surface an error instead of silently continuing with missing frames. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L129-L129
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| [#2819](https://github.com/moq-dev/moq/issues/2819). | ||
| rejects GPU resizing returns to CPU scaling and warns once. Linux Vulkan imports | ||
| packed RGB screen frames and multi-plane NV12/YU12 DMA-BUFs one plane at a time. | ||
| An unsupported modifier falls back to a CPU download and upload. |
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For a PipeWire DMA-BUF with a non-linear modifier that Vulkan cannot import, this fallback claim is false: PipeWireDmaBuf::download_i420 explicitly returns an error whenever modifier != 0, so the renderer's attempted CPU fallback propagates an error instead of drawing the frame. Qualify this as VAAPI-only or implement a GPU/VPP download for tiled PipeWire buffers. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L129-L129
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Adds a VA-API H.264 decode backend and, with it, the missing half of Linux zero-copy rendering: moq-video imported no YUV, so every hardware-decoded frame went through system memory while the
nv12shader entry point sat compiled and unreachable.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
What is here
decode/backend/vaapi.rsadaptsmoq_vaapi::decodeto theBackendtrait, mirroring the encode backend and registered next to NVDEC, gated on Linux andvaapi.render/dmabuf.rsgains an NV12 import path.decode::Config::gpu_framesis the switch that connects the two, so a subscriber that only draws its frames never pays for a download.Depends on moq-dev/vaapi#2, which adds the decoder this calls. That is not in moq-vaapi 0.0.3, so this points
moq-vaapiat the branch to give CI something to build. That dependency line has to go back to a released version before merge.Why NV12 import looked impossible
wgpu-hal's importer builds a single
vk::SubresourceLayout, so an import describes one image made of one memory plane. Mesa reports two memory planes forVK_FORMAT_G8_B8R8_2PLANE_420_UNORMunder every modifier it lists, which reads as "NV12 cannot go through this importer" and stops the investigation there.That is the wrong question. Vulkan validates the plane count per format, and the same driver reports one memory plane for
R8_UNORMandR8G8_UNORMunder0x0,0x100000000000001and0x100000000000009alike. So a luma import and a chroma import of the same object, each at its own offset and row pitch, alias the buffer as the two textures the NV12 shader already wants. Nothing is copied and nothing is reformatted; only the descriptor is duplicated, once per plane.texture_from_dmabuf_fdalready takes an offset and a stride, so this needs no new API, no new dependency, and no change in wgpu. Vulkan takes ownership of the fd on success and closes it on failure, so each plane import gets its own duplicate.Why GPU-resident output is opt-in
Surface::into_i420is a guarantee of the type, and an exported VA-API surface used to arrive with nothing that could map it: the frame outlives the decoder whose display could reach it, and a decode target is tiled, so reading the descriptor as rows would be wrong. Handing out GPU frames unconditionally would have quietly broken every consumer that reads a pixel.moq-dev/vaapi#2 fixes that by keeping the retired surface alongside the descriptor, so the read-back goes through
vaDeriveImageon the same path an ordinary download takes. With the guarantee intact, the remaining question is cost: exporting retires a surface from the decoder's recycling pool, because a later picture decoded over it would corrupt one the consumer still holds. That is an allocation per picture, which buys nothing for a consumer that was going to download anyway. Hence a flag, off by default, that other backends ignore as they doresize. A driver that decodes but will not export drops the flag after one warning rather than failing the stream.Decoder delay, and the drain
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.The consequence is that a stream simply stopping leaves its tail in the DPB, so
Backendgrows aflushthat defaults to returning nothing, which is correct for every backend configured for zero delay, and the layers above call it when a track ends. The VAAPI backend overrides it on both forks:flushfor downloaded pictures andflush_exportedfor shared ones.Verified
On Intel Meteor Lake with iHD 26.1.5 and Mesa. The same frame is rendered twice, once through the zero-copy path and once through the existing CPU upload, then both are read back and compared per pixel across all four channels:
The padded sizes are the ones with teeth. At 200x120 the driver returns a 256-byte pitch for a 200-pixel row and puts chroma at 256128, not 256120, so neither the pitch nor the offset follows from the frame size.
Block centres are separately asserted against RGB computed in the test from the definition of BT.601 limited range, so a shared misreading of the samples cannot pass. The palette carries saturated blue and orange, which is what makes a Cb/Cr swap fail rather than quietly pass.
End to end on real decoded frames,
decoded_frames_reach_the_gpu_without_a_downloadruns this backend withgpu_frameson and a second one with it off, over the same stream: drift 0 of 255 over 76800 pixels, at modifier0x100000000000009, with the renderer's strike count confirming the CPU path was not taken.gpu_frames_still_answer_into_i420pins the read-back byte for byte,flushing_returns_the_tail_the_dpb_holdsandflushing_returns_the_tail_of_a_gpu_streampin both drains, andvaapi_h264_round_tripcompares planes by mean absolute error with exact timestamps.Those tests self-skip rather than being
#[ignore]d, so they run wherever the hardware exists and stay 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.No re-tile
An earlier version carried a VA-API post-processing fallback for buffers whose modifier Vulkan refuses. It is not here, and the tests are why: the re-tile counter read zero on every run, because this driver already allocates NV12 on an importable modifier. The fallback is a separate concern and can come later on its own merits.
Conflicts with #3332
Both register a Linux hardware decode candidate in the same list in
decode/backend/mod.rsand 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 over this branch found two things worth calling out, both fixed here.
adopttrusted the shape of the VA-API export: it read the modifier off object 0 and every plane offset off layer 0, while handing the importer only object 0's file descriptor.VA_EXPORT_SURFACE_COMPOSED_LAYERSasks for one object in one layer; it does not promise one. A driver that split the allocation would have addressed chroma in memory the importer never saw and rendered a plausible picture from the wrong bytes, with no error anywhere. Both counts were in the descriptor and unread. It also indexed a fixed four-element array with a driver-reported plane count.A bitstream error inside
decode_exportedwas reported as "this driver cannot export", which clearedgpu_framesfor the rest of the session and returned success. One corrupt access unit therefore cost the stream its zero-copy path silently. Only a failure before the first descriptor comes back is now read as the driver's verdict; after that it is a decode error.