firefox-fourier patch 3: probe v4l2_request via codec name first

Diagnostic on fresnel (RK3399 / Mali-T860 / mainline) showed
InitV4L2RequestDecoder running at runtime but failing the
hw_configs sanity check:

  FFMPEG: Initialising V4L2 stateless (request API) FFmpeg decoder
  FFMPEG:   codec h264 has no DRM hwaccel —
            libavcodec built without --enable-v4l2-request?
  FFMPEG: Initialising V4L2-DRM FFmpeg decoder      ← falls through to stateful
  FFMPEG:   V4L2 codec h264_v4l2m2m : V4L2 mem2mem H.264 decoder wrapper
  FFMPEG:   Couldn't initialise V4L2 decoder        ← stateful also fails

…despite the system libavcodec.so being built with
--enable-v4l2-request and exposing h264_v4l2request, vp8_v4l2request,
etc. as named AVCodec entries.

Root cause: libavcodec exposes v4l2_request through one of two
mechanisms depending on the build:
  (a) Named AVCodec entry (legacy, distro-portable): looked up via
      avcodec_find_decoder_by_name("h264_v4l2request"). ALARM,
      Debian, most distros use this.
  (b) hw_configs entry on the generic codec (modern, upstream): the
      generic codec's AVCodecHWConfig array advertises
      AV_HWDEVICE_TYPE_DRM. Setting hw_device_ctx binds the hwaccel.

Patch 3 originally only probed (b). On builds that ship (a) — the
common case — the check failed even though v4l2_request was fully
functional.

Fix: probe (a) first via the codec-name lookup table, fall back to
(b) walking hw_configs. Both shapes work; the decoder is opened with
whichever AVCodec the probe selected. The DRM hwdevice ctx
attachment is unchanged (both mechanisms need it for surface
allocation).

Patch description and the in-code comment block updated to document
the dual mechanism. Behaviour on stateful-only boards (Pi4 / vendor
MPP) preserved: neither named codec nor hw_configs DRM is registered,
the function bails out, the existing InitV4L2Decoder runs as before.

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

@@ -15,12 +15,16 @@ 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);
+  * 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, used by ALARM /
+        Debian / most distros that build with --enable-v4l2-request;
+      - **generic codec + AV_HWDEVICE_TYPE_DRM** in `hw_configs`:
+        the modern hwaccel registration, used by some upstream-only
+        builds.
+    Probes named-codec first (explicit, portable) and falls back to
+    walking the generic codec's hw_configs.
   * 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;
@@ -104,30 +108,55 @@ diff --git a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp b/dom/media/platf
 +
 +  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);
++  // 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's 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;
 +  }
 +
-+  // 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;
++  AVCodec* codec = mLib->avcodec_find_decoder_by_name(requestName);
++  if (codec) {
++    FFMPEG_LOG("  using named v4l2_request codec %s", requestName);
++  } else {
++    // Fallback path (b): generic codec + DRM hwaccel.
++    AVCodec* generic = mLib->avcodec_find_decoder(mCodecID);
++    if (generic) {
 +      for (int i = 0;; i++) {
-+    const AVCodecHWConfig* cfg = mLib->avcodec_get_hw_config(codec, i);
++        const AVCodecHWConfig* cfg = mLib->avcodec_get_hw_config(generic, i);
 +        if (!cfg) break;
 +        if (cfg->device_type == AV_HWDEVICE_TYPE_DRM) {
-+      hasDrmHwaccel = true;
++          codec = generic;
++          FFMPEG_LOG("  using generic codec %s with DRM hwaccel", codec->name);
 +          break;
 +        }
 +      }
-+  if (!hasDrmHwaccel) {
-+    FFMPEG_LOG("  codec %s has no DRM hwaccel — libavcodec built "
-+               "without --enable-v4l2-request?", codec->name);
++    }
++  }
++
++  if (!codec) {
++    FFMPEG_LOG("  no v4l2_request path for codec ID %d — 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);