marfrit-packages/arch/firefox-fourier/0003-ffmpegvideo-v4l2-request-route.patch

From: Markus Fritsche <mfritsche@reauktion.de>
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:

  * looks up the codec via two complementary mechanisms libavcodec
    uses for v4l2_request:
      - **named codec** (`h264_v4l2request`, `vp8_v4l2request`, etc.):
        the legacy AVCodec-per-hwaccel registration. ALARM, Debian,
        and most distros building with --enable-v4l2-request expose
        this (avcodec_find_decoder_by_name lookup).
      - **generic codec + AV_HWDEVICE_TYPE_DRM** in `hw_configs`:
        the modern hwaccel registration on some upstream-only ffmpeg
        builds.
    Probes named-codec first (explicit, portable) and falls back to
    walking the generic codec's `hw_configs` for the DRM device type;
  * 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 via either
mechanism (ALARM uses the named codec). On Pi4 / Mediatek /
vendor-MPP-stateful boards neither mechanism is registered for the
codec, the function bails out, 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<AVCodecID>& 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	2026-04-27 16:09:10.000000000 +0200
+++ b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp	2026-04-29 00:10:00.098884335 +0200
@@ -403,6 +403,129 @@
   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<LIBAV_VER>::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<LIBAV_VER>::InitV4L2RequestDecoder() {
+  FFMPEG_LOG("Initialising V4L2 stateless (request API) FFmpeg decoder");
+
+  StaticMutexAutoLock mon(sMutex);
+
+  // libavcodec exposes V4L2 stateless decoders through one of two
+  // mechanisms depending on the build:
+  //   (a) Named AVCodec entry (h264_v4l2request, vp8_v4l2request,
+  //       etc.) — the legacy mechanism. Each hwaccel is a standalone
+  //       AVCodec, looked up by name. ALARM, Debian, and most distros
+  //       building with --enable-v4l2-request expose this.
+  //   (b) Modern hwaccel registration: the generic codec advertises
+  //       AV_HWDEVICE_TYPE_DRM in its hw_configs array, and setting
+  //       hw_device_ctx on the codec context binds v4l2_request
+  //       internally. Some upstream-only builds expose this.
+  // Probe (a) first — it is the explicit, distro-portable lookup.
+  // Fall back to (b) when the named entry isn't registered.
+  const char* requestName = nullptr;
+  switch (mCodecID) {
+    case AV_CODEC_ID_H264:       requestName = "h264_v4l2request"; break;
+    case AV_CODEC_ID_HEVC:       requestName = "hevc_v4l2request"; break;
+    case AV_CODEC_ID_VP8:        requestName = "vp8_v4l2request"; break;
+    case AV_CODEC_ID_VP9:        requestName = "vp9_v4l2request"; break;
+    case AV_CODEC_ID_AV1:        requestName = "av1_v4l2request"; break;
+    case AV_CODEC_ID_MPEG2VIDEO: requestName = "mpeg2_v4l2request"; break;
+    default:
+      FFMPEG_LOG("  no v4l2_request mapping for codec ID %d", mCodecID);
+      return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  AVCodec* codec = mLib->avcodec_find_decoder_by_name(requestName);
+  if (codec) {
+    FFMPEG_LOG("  using named v4l2_request codec %s", requestName);
+  } else {
+    AVCodec* generic = mLib->avcodec_find_decoder(mCodecID);
+    if (generic) {
+      for (int i = 0;; i++) {
+        const AVCodecHWConfig* cfg = mLib->avcodec_get_hw_config(generic, i);
+        if (!cfg) break;
+        if (cfg->device_type == AV_HWDEVICE_TYPE_DRM) {
+          codec = generic;
+          FFMPEG_LOG("  using generic codec %s with DRM hwaccel", codec->name);
+          break;
+        }
+      }
+    }
+  }
+
+  if (!codec) {
+    FFMPEG_LOG("  no v4l2_request path for codec ID %d \u2014 neither named "
+               "codec %s nor generic codec with DRM hwaccel available "
+               "(libavcodec built without --enable-v4l2-request?)",
+               mCodecID, requestName);
+    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<LIBAV_VER>::InitV4L2Decoder() {
   FFMPEG_LOG("Initialising V4L2-DRM FFmpeg decoder");
 
@@ -656,6 +779,16 @@
 #  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 +2179,11 @@
   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()) {