From: Markus Fritsche Subject: [PATCH 3/4] dom/media/platforms/ffmpeg: route through libavcodec v4l2_request hwaccel for V4L2 stateless boards Date: 2026-04-27 Background ---------- Firefox's existing V4L2 init (`InitV4L2Decoder`) finds the codec by suffix lookup (`FindVideoHardwareAVCodec(mLib, mCodecID)`), which on Linux resolves to the **stateful** V4L2-M2M wrapper codec (`h264_v4l2m2m` etc). Mainline-Linux Rockchip boards (RK3399 rkvdec, RK3566/RK3588 hantro, RK3588 rkvdec2) only expose **stateless** V4L2 fourccs (`S264`, `S265`, `VP9F`); the stateful wrapper codec fails to open, Firefox falls all the way through to software. This patch adds a sibling init path, `InitV4L2RequestDecoder`, that: * uses the **generic** codec (e.g. plain `h264`, returned by `avcodec_find_decoder(AV_CODEC_ID_H264)`) rather than the stateful wrapper; * sanity-checks the codec's `hw_configs` for an `AV_HWDEVICE_TYPE_DRM` entry — that's how libavcodec surfaces the v4l2_request hwaccel upstream (no dedicated `_V4L2REQUEST` device type exists); * creates an `AV_HWDEVICE_TYPE_DRM` hwdevice context bound to `/dev/dri/renderD128` via the new `av_hwdevice_ctx_create` wrapper (patch 2/4) and attaches it to the codec context; * reuses the existing `ChooseV4L2PixelFormat` get-format callback (already returns `AV_PIX_FMT_DRM_PRIME`) and the existing `apply_cropping = 0` constraint. `InitV4L2RequestDecoder` is invoked **before** `InitV4L2Decoder` in `InitHWDecoderIfAllowed`. On Rockchip mainline it succeeds. On Pi4 / Mediatek / vendor-MPP-stateful boards the codec's `hw_configs` lacks a DRM entry (the V4L2-M2M codecs don't register one), the sanity check fails, and the existing stateful `InitV4L2Decoder` runs as before. No regression of stateful boards. `mDRMDeviceContext` is unconditionally `av_buffer_unref`'d in `ProcessShutdown` (no-op when null). Gated behind `media.ffmpeg.v4l2-request.enabled` from patch 4/4. Bug 1969297. diff --git a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.h b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.h --- a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.h 2026-03-18 19:22:14.000000000 +0000 +++ b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.h 2026-04-27 20:43:39.347992674 +0000 @@ -225,7 +225,12 @@ bool IsLinuxHDR() const; MediaResult InitVAAPIDecoder(); MediaResult InitV4L2Decoder(); + // firefox-fourier: V4L2 stateless (request API) decode path. Uses + // libavcodec's v4l2_request hwaccel, which it surfaces via + // AV_HWDEVICE_TYPE_DRM rather than a dedicated _V4L2REQUEST type. + MediaResult InitV4L2RequestDecoder(); bool CreateVAAPIDeviceContext(); + bool CreateV4L2RequestDeviceContext(); bool GetVAAPISurfaceDescriptor(VADRMPRIMESurfaceDescriptor* aVaDesc); void AddAcceleratedFormats(nsTArray& aCodecList, AVCodecID aCodecID, AVVAAPIHWConfig* hwconfig); @@ -239,7 +244,10 @@ void AdjustHWDecodeLogging(); AVBufferRef* mVAAPIDeviceContext = nullptr; + // firefox-fourier: DRM hwdevice ctx for the v4l2_request hwaccel. + AVBufferRef* mDRMDeviceContext = nullptr; bool mUsingV4L2 = false; + bool mUsingV4L2Request = false; // If video overlay is used we want to upload SW decoded frames to // DMABuf and present it as a external texture to rendering pipeline. bool mUploadSWDecodeToDMABuf = false; diff --git a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp --- a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp 2026-03-18 19:22:14.000000000 +0000 +++ b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp 2026-04-27 20:44:33.280766228 +0000 @@ -406,6 +406,105 @@ return NS_OK; } +// firefox-fourier: V4L2 stateless (request API) DRM hwdevice context. +// libavcodec's v4l2_request hwaccel binds via AV_HWDEVICE_TYPE_DRM — +// no dedicated _V4L2REQUEST type exists upstream. +bool FFmpegVideoDecoder::CreateV4L2RequestDeviceContext() { + if (!mLib->av_hwdevice_ctx_create) { + FFMPEG_LOG(" av_hwdevice_ctx_create not available (libavutil too old)"); + return false; + } + const char* drmDevice = "/dev/dri/renderD128"; + if (mLib->av_hwdevice_ctx_create(&mDRMDeviceContext, + AV_HWDEVICE_TYPE_DRM, drmDevice, + nullptr, 0) < 0) { + FFMPEG_LOG(" av_hwdevice_ctx_create(DRM, %s) failed", drmDevice); + return false; + } + mCodecContext->hw_device_ctx = mLib->av_buffer_ref(mDRMDeviceContext); + FFMPEG_LOG(" DRM hwdevice ctx created on %s", drmDevice); + return true; +} + +// firefox-fourier: try V4L2 stateless decode via libavcodec's +// v4l2_request hwaccel. Distinct from InitV4L2Decoder which uses the +// stateful h264_v4l2m2m wrapper codec. On Rockchip mainline boards +// (rkvdec / hantro / rkvdec2) only the stateless path exists. +MediaResult FFmpegVideoDecoder::InitV4L2RequestDecoder() { + FFMPEG_LOG("Initialising V4L2 stateless (request API) FFmpeg decoder"); + + StaticMutexAutoLock mon(sMutex); + + // Use the GENERIC codec (not the _v4l2m2m wrapper). libavcodec picks + // the v4l2_request hwaccel internally by walking the codec's + // hw_configs once an AV_HWDEVICE_TYPE_DRM ctx is attached. + AVCodec* codec = mLib->avcodec_find_decoder(mCodecID); + if (!codec) { + FFMPEG_LOG(" generic codec for ID %d not found", mCodecID); + return NS_ERROR_NOT_AVAILABLE; + } + + // Sanity-check: refuse when libavcodec was built without the + // v4l2_request hwaccel (no DRM hwaccel registered against this + // codec). Distro packagers occasionally drop --enable-v4l2-request. + bool hasDrmHwaccel = false; + for (int i = 0;; i++) { + const AVCodecHWConfig* cfg = mLib->avcodec_get_hw_config(codec, i); + if (!cfg) break; + if (cfg->device_type == AV_HWDEVICE_TYPE_DRM) { + hasDrmHwaccel = true; + break; + } + } + if (!hasDrmHwaccel) { + FFMPEG_LOG(" codec %s has no DRM hwaccel — libavcodec built " + "without --enable-v4l2-request?", codec->name); + return NS_ERROR_NOT_AVAILABLE; + } + FFMPEG_LOG(" V4L2 stateless: codec %s : %s", codec->name, codec->long_name); + + if (!(mCodecContext = mLib->avcodec_alloc_context3(codec))) { + FFMPEG_LOG(" couldn't init HW ffmpeg context"); + return NS_ERROR_OUT_OF_MEMORY; + } + mCodecContext->opaque = this; + + // Reuse the existing V4L2 init helpers: pixel-format selector returns + // AV_PIX_FMT_DRM_PRIME, cropping disabled (FFmpeg can't crop opaque + // DRM buffers). Same constraints as the stateful V4L2 path. + InitHWCodecContext(ContextType::V4L2); + mCodecContext->apply_cropping = 0; + + auto releaseDecoder = MakeScopeExit( + [&]() MOZ_NO_THREAD_SAFETY_ANALYSIS { ReleaseCodecContext(); }); + + if (!CreateV4L2RequestDeviceContext()) { + return NS_ERROR_NOT_AVAILABLE; + } + + MediaResult ret = AllocateExtraData(); + if (NS_FAILED(ret)) { + return ret; + } + + if (mLib->avcodec_open2(mCodecContext, codec, nullptr) < 0) { + FFMPEG_LOG(" Couldn't open V4L2 stateless decoder"); + return NS_ERROR_DOM_MEDIA_FATAL_ERR; + } + + if (mAcceleratedFormats.IsEmpty()) { + mAcceleratedFormats.AppendElement(mCodecID); + } + + AdjustHWDecodeLogging(); + + FFMPEG_LOG(" V4L2 stateless FFmpeg init successful"); + mUsingV4L2 = true; + mUsingV4L2Request = true; + releaseDecoder.release(); + return NS_OK; +} + MediaResult FFmpegVideoDecoder::InitV4L2Decoder() { FFMPEG_LOG("Initialising V4L2-DRM FFmpeg decoder"); @@ -659,6 +758,16 @@ # endif // MOZ_ENABLE_VAAPI # ifdef MOZ_ENABLE_V4L2 + // firefox-fourier: try V4L2 stateless (request API) first. On + // mainline-Linux Rockchip boards (RK3399 rkvdec, RK3566/RK3588 + // hantro, RK3588 rkvdec2) the kernel exposes only stateless + // fourccs, so the stateful path below would fail anyway. On + // stateful boards (Pi4 / vendor MPP) this gracefully falls + // through (no DRM hwaccel registered for the codec). + if (StaticPrefs::media_ffmpeg_v4l2_request_enabled() && + NS_SUCCEEDED(InitV4L2RequestDecoder())) { + return; + } // VAAPI didn't work or is disabled, so try V4L2 with DRM if (NS_SUCCEEDED(InitV4L2Decoder())) { return; @@ -2046,6 +2155,11 @@ if (IsHardwareAccelerated()) { mLib->av_buffer_unref(&mVAAPIDeviceContext); } + // firefox-fourier: release the DRM hwdevice ctx for the v4l2_request + // hwaccel. Always safe — av_buffer_unref no-ops on a null pointer. + if (mDRMDeviceContext) { + mLib->av_buffer_unref(&mDRMDeviceContext); + } #endif #ifdef MOZ_ENABLE_D3D11VA if (IsHardwareAccelerated()) {