From: Markus Fritsche <mfritsche@reauktion.de>
Subject: media: default kAcceleratedVideoDecodeLinux to enabled when
 USE_V4L2_CODEC is the build's hardware decode path
Date: 2026-04-26

Background
----------
chromium-fourier targets mainline-Linux Wayland on Rockchip (RK3566 hantro,
RK3588 VDPU381) where the only HW video decode path is V4L2 stateless
(via the in-tree media/gpu/v4l2 stack). The build is configured with

    use_vaapi      = false
    use_v4l2_codec = true
    use_v4lplugin  = true
    use_linux_v4l2_only = true

Without this patch, GPU-process V4L2 decode is compiled in but stays
runtime-disabled by default. The runtime master gate
`media::kAcceleratedVideoDecodeLinux` (the user-visible feature name is
"AcceleratedVideoDecoder") is currently flipped to ENABLED_BY_DEFAULT only
when `BUILDFLAG(USE_VAAPI)` is set. On a USE_V4L2_CODEC-only build the
feature stays DISABLED_BY_DEFAULT, the linux gpu_mojo_media_client returns
`VideoDecoderType::kUnknown`, and `<video>` falls all the way back to
`media/filters/ffmpeg_video_decoder.cc` (software).

We confirmed this by hand on the PineTab2 (RK3566 hantro): with
`--enable-features=AcceleratedVideoDecoder` chrome correctly selects
`V4L2VideoDecoder` for h264 main, opens /dev/video1 + /dev/media0,
allocates 17 OUTPUT + 6 CAPTURE NV12 buffers, and runs SetExtCtrlsInit for
H264. Without the runtime flag, none of that happens.

Fix
---
Treat `USE_V4L2_CODEC` symmetrically with `USE_VAAPI` for the runtime
default of the master gate. A user can still disable it via
`--disable-features=AcceleratedVideoDecoder`.

This does NOT touch the `kAcceleratedVideoDecodeLinuxGL` companion gate
(already ENABLED_BY_DEFAULT) or any of the per-decoder selection logic in
`media/mojo/services/gpu_mojo_media_client_linux.cc` -- that file already
dispatches to the V4L2 decoder when `USE_V4L2_CODEC && !USE_VAAPI`, gated
behind the master flag we are flipping here.

diff --git a/media/base/media_switches.cc b/media/base/media_switches.cc
--- a/media/base/media_switches.cc
+++ b/media/base/media_switches.cc
@@ -749,7 +749,7 @@ BASE_FEATURE(kUnifiedAutoplay, base::FEATURE_ENABLED_BY_DEFAULT);
 // on chromeos, but needs an experiment on linux.
 BASE_FEATURE(kAcceleratedVideoDecodeLinux,
              "AcceleratedVideoDecoder",
-#if BUILDFLAG(USE_VAAPI)
+#if BUILDFLAG(USE_VAAPI) || BUILDFLAG(USE_V4L2_CODEC)
              base::FEATURE_ENABLED_BY_DEFAULT);
 #else
              base::FEATURE_DISABLED_BY_DEFAULT);