b81b50199a
`make_converter_for_video_caps` builds the chain webrtcsink inserts ahead of an encoder it selected, special-casing hardware it knows — NVMM, D3D11, CUDA, GL, VA, and on main also v4l2h264enc — and falling back to software `videoconvert ! videoscale` for anything else. Rockchip's MPP encoder takes NV12, I420, YUY2 and more directly and converts on the SoC's 2D accelerator, so the fallback adds a full CPU pass over every frame to do work the hardware was going to do anyway, on the four A55s robotd's 50 Hz loop shares. The reason this matters more than CPU: the robot currently avoids the whole question by pre-encoding and handing webrtcsink finished H.264. That works, and it means webrtcsink cannot reach the encoder — so congestion control cannot adapt the bitrate to the link, and a peer's PLI cannot produce a keyframe, which leaves a viewer that lost one broken until the next periodic GOP. Letting webrtcsink own the encoder fixes both. This patch is what makes that affordable. **This repository is no longer patch-free, and says so.** MPL-2.0 asks that modifications be identifiable, so the README states it, patches/README.md gives each patch's reasoning, and build.sh records every applied patch in the release MANIFEST beside the upstream ref. `git apply --check` runs first, so a patch that stops applying fails the build naming itself rather than yielding a plugin quietly missing the change it was carried for. The trade-off is written down rather than glossed: without videoscale the bin cannot resize, so the negotiated resolution must be one the source produces. True on this robot, which pins its caps upstream of the tee — and the reason upstream may want RGA-backed scaling instead of nothing before taking it. The v4l2h264enc arm on main is the same shape for another hardware encoder, so the precedent exists, and if it lands this file is deleted at the next bump. Verified to apply cleanly against a real 0.15.3 checkout. Assisted-by: Claude:claude-opus-5[1m] shellcheck
43 lines
2.7 KiB
Markdown
43 lines
2.7 KiB
Markdown
# Patches
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Applied to the upstream checkout by `scripts/build.sh`, in filename order, and recorded in every
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release's `MANIFEST` so a binary can be traced to the exact source that produced it.
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**Carrying a patch is a cost, so each one has to say what it buys and how it ends.** A patch with no
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route upstream is a fork with extra steps: it has to be re-cut at every version bump, and the
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binary stops being something anyone else can reproduce from a public ref alone.
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`build.sh` runs `git apply --check` first, so a patch that no longer applies fails the build naming
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itself, rather than silently producing a plugin missing the change it was carried for.
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---
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## `0001-webrtcsink-no-converter-for-mpph264enc.patch`
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**What it changes.** `make_converter_for_video_caps` in `net/webrtc/src/webrtcsink/imp.rs` builds
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the chain `webrtcsink` inserts in front of an encoder it selected. It special-cases hardware it
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knows — NVMM, D3D11, CUDA, GL, VA, and on `main` also `v4l2h264enc` — and falls back to software
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`videoconvert ! videoscale` for anything else. This adds an arm for `mpph264enc` that inserts
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nothing.
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**Why.** Rockchip's MPP encoder takes NV12, I420, YUY2 and more directly, and converts on the SoC's
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2D accelerator rather than the CPU. A software convert-and-scale pass in front of it costs a full
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CPU traversal of every frame on four Cortex-A55s — which is exactly what the hardware encoder is
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there to avoid, and which shares those cores with `robotd`'s 50 Hz control loop.
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**Why not just keep pre-encoding.** Because the robot did, and it costs more than it looks.
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Handing `webrtcsink` finished H.264 means it cannot reach the encoder, so two things it normally
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does silently do not happen: congestion control cannot adapt the bitrate to the link, and a peer's
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PLI cannot produce a keyframe — a viewer that loses one stays broken until the next periodic GOP.
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Letting `webrtcsink` own the encoder fixes both, and this patch is what makes that affordable here.
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**The trade-off it makes.** Without `videoscale` the bin cannot resize, so the negotiated
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resolution has to be one the source already produces. True on this robot, which pins its caps
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upstream of its tee — and the honest reason this may need discussion before upstream takes it, since
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a general fix would want RGA-backed scaling rather than none. `mpph264enc` has `width` and `height`
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properties that scale on the VPU, but nothing in `webrtcsink` sets them.
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**How it ends.** Upstream. The `v4l2h264enc` arm on `main` is the same shape for another hardware
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encoder, so the precedent exists; if it is taken, this file is deleted at the next version bump.
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Until then it is re-cut per bump, which `--check` will demand rather than let slide.
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