nx2: decode Switch 2 video (nn::codec::video) with FFmpeg

The decoder service used to complete every submitted frame at once
without decoding it. Each decoder service (gsv 902/906/1002 -> TIPC 1)
now decodes on a thread of its own with FFmpeg (VP9, VP8, H.264;
video_core/host1x/nx2_video_decoder), in software or on the host GPU per
the NVDEC emulation setting.

- 101 takes the codec name ("video/vp9") from its A buffer.
- 109 queues the A buffer's frame; the ready event fires while the queue
  has room.
- 311/314 reply a NativeSyncHandle {~syncpoint, 0, value}: the service
  allocates a syncpoint and increments it per finished frame, frames
  filling output images in register/release order like the client's
  ring. 314 waits for the client's release fence before reuse.
- 213 returns the frame's size, pts (TimeSpan, ns) and end of stream.
- gsv 911 creates a decoder image pool (count images in one memory) and
  gsv 961 returns the 128-byte CpuUserImageDescriptor that
  nn::codec::ReadDecoderOutputImage* copy frames out with; images are
  pitch-linear NV12, RGBA or BGRA.
- Each new video command is logged once in full; EDEN_NX2_VDEC_DUMP=<dir>
  saves every 120th decoded frame's luma.

Title 1's Mushroom Cup opening (2560x1440 VP9, 1423 frames) now decodes
in real time and the game goes on to the race. The picture on screen
still stays on the first frame: the game memcpys each frame into GPU
memory and the texture cache keeps its first upload (notes 148).

1 files changed, 67 insertions, 0 deletions

diff --git a/src/video_core/host1x/nx2_video_decoder.h b/src/video_core/host1x/nx2_video_decoder.h
new file mode 100644
index 0000000000..c979a2c801
--- /dev/null
+++ b/src/video_core/host1x/nx2_video_decoder.h
@@ -0,0 +1,67 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <memory>
+#include <span>
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include "common/common_types.h"
+
+namespace FFmpeg {
+class Frame;
+}
+
+namespace Tegra::Host1x {
+
+/// A picture decoded by Nx2VideoDecoder, in display order.
+struct Nx2Picture {
+    u32 width{};
+    u32 height{};
+    u64 pts{}; ///< As submitted with the compressed frame
+    std::shared_ptr<FFmpeg::Frame> frame;
+};
+
+/// Pixel layouts of a Switch 2 decoder output image.
+enum class Nx2PictureFormat : u32 {
+    Nv12,  ///< Luma rows, then interleaved Cb/Cr rows at chroma_offset
+    Rgba8,
+    Bgra8,
+};
+
+/// The decoder behind the Switch 2 video decoder service (nn::codec::video): FFmpeg decodes the
+/// elementary stream the game demuxed, in software or on the host GPU (NVDEC emulation setting).
+/// Used from one thread at a time.
+class Nx2VideoDecoder {
+public:
+    /// mime: the codec name nn::codec sends with Configure ("video/vp9", "video/vp8", ...).
+    explicit Nx2VideoDecoder(std::string_view mime);
+    ~Nx2VideoDecoder();
+
+    bool IsOpen() const;
+
+    const std::string& Name() const;
+
+    /// Decodes one compressed frame and appends the pictures it completes.
+    void Decode(std::span<const u8> data, u64 pts, std::vector<Nx2Picture>& pictures);
+
+    /// End of stream: appends the pictures the decoder still holds, then starts over.
+    void Drain(std::vector<Nx2Picture>& pictures);
+
+    /// Drops everything queued in the decoder (the game flushed it to seek).
+    void Reset();
+
+    /// Converts a picture to `format` in `out`: rows `pitch` bytes apart, NV12's chroma plane at
+    /// `chroma_offset`, scaled to width x height.
+    void Render(const Nx2Picture& picture, Nx2PictureFormat format, u32 width, u32 height,
+                u64 pitch, u64 chroma_offset, std::span<u8> out);
+
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl;
+};
+
+} // namespace Tegra::Host1x