AMD BC-250 VA-API Driver & Video Acceleration Suite (bc250-encoding-decoding-fix)

A high-performance, spec-compliant VA-API driver (bc250_drv_video.so) engineered specifically for the AMD BC-250 (Cyan Skillfish) APU on Linux.

The BC-250 is a repurposed PS5 APU (Zen 2 8-core/16-thread CPU, up to 40 unlocked RDNA 2 Compute Units) whose physical VCN (Video Core Next) hardware engine was permanently unprovisioned and eFused off at the factory. Without a working VCN block, Linux applications fail to initialize hardware video acceleration.

This project solves this by delivering:

1. GPU Compute Video Encoders: Real-time H.264 and H.265/HEVC encoding executed across the APU's 40 RDNA 2 Compute Units using custom Vulkan compute shaders with asynchronous pipelining and AVX2 CPU SIMD offloading.
2. Bit-Exact VA-API Video Decoders (VAEntrypointVLD): Threaded H.264 and HEVC decoding running on the Zen 2 CPU, verified bit-exact against reference decoders across all 302 conformance tests.
3. Low-Latency Game & VR Streaming: Pre-tuned presets and passive thread policies for Sunshine / Moonlight (1080p60/1440p) and WiVRn wireless VR streaming (~36ms motion-to-photon latency, ~190 Mbps throughput).
4. Hardware Audio Clock Fix: DKMS kernel module repairing the missing DisplayPort/HDMI audio clock.

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Performance & Conformance Highlights

1. Encoding Benchmarks (Measured on BC-250 Silicon)

• H.264 (Vulkan Compute):
  • 640x480: 267 fps
  • 720p: 179 fps
  • 1080p: 100–134 fps
  • 1440p: 67–80 fps
  • Game Streaming Overhead: Only ~4.5% total GPU impact during active 60 FPS gaming with Sunshine/Moonlight.
• H.265 / HEVC (Multi-Slice Sliced Compute):
  • 1080p: 111+ fps (with default BC250_HEVC_SLICES=4, SIMD 4x4 transforms, and MOVNTDQA streaming readback).
  • Dynamic Base QP Rate Control: Full dynamic QP targeting (12..51) matches requested bitrates and eliminates blurry over-quantization.
  • HEVC CBR Filler NALs: Conforms bit-exact to target bitrates in CBR mode.
  • Fine Detail Preservation: Tuned skip decision threshold preserves delicate textures, hair, grain, and high-frequency motion.
  • Chroma Fidelity: Bit-exact non-linear Table 8-10 QP mapping eliminates the standard chroma PSNR deficit.
• WiVRn VR Streaming:
  • Motion-to-Photon Latency: ~36 ms (down from 145 ms).
  • Headset Download Throughput: ~190 Mbits/s (surpassing software encode).
  • Host CPU Utilization: ~350% (slashed from 1300% lockup by enforcing passive OpenMP thread waiting).

2. Decoding Benchmarks & JCT-VC Conformance (VAEntrypointVLD)

Bit-exact conformance against official ITU JCT-VC test streams and reference decoders across both standalone execution and hardware VA-API on the BC-250:

Codec	Resolution	Threads / Topology	Throughput (FPS)	Conformance Status
H.264	1080p (CRF 23)	1 thread	68.4 fps	100% Bit-Exact (90/90 pass)
H.264	1080p (CRF 23)	8 threads (multi-slice)	156.2 – 181.5 fps	100% Bit-Exact (90/90 pass)
H.265 / HEVC	1080p	1 thread	66.3 fps	146 of 147 Bit-Exact (99.3%)
H.265 / HEVC	1080p	6 threads (wavefront)	97.6 fps	146 of 147 Bit-Exact (99.3%)

(Note: The sole unmapped test vector, TSUNEQBD_A_MAIN10, specifies differing bit depths for luma and chroma, which FFmpeg itself does not support).

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Supported Codec Matrix

Profile	Entrypoint	Acceleration	Max Resolution
VAProfileH264Baseline	VAEntrypointEncSlice	Vulkan Compute (40 CUs)	4096x2160 (4K)
VAProfileH264Main	VAEntrypointEncSlice	Vulkan Compute (40 CUs) + CABAC	4096x2160 (4K)
VAProfileH264High	VAEntrypointEncSlice	Vulkan Compute (40 CUs) + CABAC	4096x2160 (4K)
VAProfileHEVCMain	VAEntrypointEncSlice	Vulkan Compute ME + Host Slices	4096x2160 (4K)
VAProfileH264*	VAEntrypointVLD (Decode)	Multi-Threaded CPU Wavefront	4096x2160 (4K)
VAProfileHEVCMain	VAEntrypointVLD (Decode)	Multi-Threaded CPU Wavefront (WPP)	16384x16384 (16K)
VAProfileHEVCMain10	VAEntrypointVLD (Decode)	10-Bit CPU Wavefront (P010)	16384x16384 (16K)
VAProfileNone	VAEntrypointVideoProc	Vulkan Compute Scaler & Cropping	4096x2160 (4K)

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Quick Installation

Option A: Automated Distribution Installers

We provide native package manifests with automatic Cyan Skillfish PCI (0x1002:0x13fe) device detection:

• Arch Linux / CachyOS:

  ./tools/install_cachyos_arch.sh

  (Or use cd packaging/arch && makepkg -si)
• SteamOS / HoloISO:

  sudo ./tools/setup_steamos.sh

• Bazzite / Fedora / Silverblue:

  sudo ./tools/setup_bazzite.sh

  (Or build the RPM spec: rpmbuild -ba packaging/fedora/bc250-vaapi.spec)
• Debian / Ubuntu: Package sources available under packaging/debian/.

Option B: Pre-Compiled GitHub Release Tarballs

Download the latest release bundles directly from GitHub Releases:

tar -xzf bc250-driver-linux-x86_64.tar.gz
sudo ./install.sh

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Streaming Presets & Tuning

Sunshine / Moonlight (Game Streaming)

To configure Sunshine for zero-stutter 60/120 FPS game streaming with minimal GPU latency:

./tools/sunshine_preset/apply_sunshine_preset.sh

See docs/sunshine-guide.md for full details.

WiVRn (Wireless VR Streaming)

To configure WiVRn for ~36ms motion-to-photon latency and ~190 Mbps throughput:

./tools/wivrn_preset/apply_wivrn_preset.sh

Recommended Topology: 2 Hardware VA-API streams (left_eye and right_eye) + 1 Software stream (alpha/foveation). See docs/wivrn-guide.md.

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Key Environment Variables

Variable	Default	Purpose
LIBVA_DRIVER_NAME	(unset)	Set to bc250 to activate this driver. Handled automatically on BC-250 by systemd generator.
BC250_FAST_MODE	1	Restricts GPU compute overhead to <3–5%, preventing GPU starvation in heavy 3D games.
BC250_SLICES_PER_FRAME	4	Number of slices per H.264 frame. Use 2 for multi-stream VR to prevent CPU thread congestion.
BC250_HEVC_SLICES	4	Number of concurrent slices for HEVC encode (1..16). Yields 111+ fps at default 4.
OMP_WAIT_POLICY	PASSIVE	Critical: enforces passive wait in libgomp, cutting CPU usage from 1300% to ~350%.
GOMP_SPINCOUNT	0	Disables CPU busy-wait spin loops in worker threads.
BC250_USE_CABAC	1 (Main/High)	Toggles CABAC (10–13% smaller bitrate) vs CAVLC for H.264 encode.

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Verification & Diagnostics

1. Check VA-API profiles and entrypoints:

   LIBVA_DRIVER_NAME=bc250 vainfo

2. Run the automated hardware diagnostic suite:

   ./tools/bc250_diagnose.sh

3. Test encode and decode pipelines with FFmpeg:

   # Encode test (H.264 VA-API)
   ffmpeg -vaapi_device /dev/dri/renderD128 -f lavfi -i testsrc=size=1920x1080:rate=60 \
     -vf 'format=nv12,hwupload' -c:v h264_vaapi -b:v 15M -frames:v 300 test_enc.mp4
   
   # Decode test (Hardware VA-API decode)
   ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -i test_enc.mp4 -f null -

4. Run official JCT-VC HEVC conformance test suite (146 of 147 bitstreams):

   # Test all 147 vectors directly through the driver via VA-API:
   ./tools/test_vaapi_conformance.sh
   
   # Test standalone HEVC decoder:
   ./tools/test_hevc_conformance.sh

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Building from Source

cd approach1-compute-encoder
cmake -S . -B build -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr
cmake --build build -j$(nproc)
ctest --test-dir build --output-on-failure
sudo cmake --install build

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License & Credits

• Driver & Shader Code: Licensed under GNU General Public License v3.0.
• Audio Fix Kernel Module: Licensed under GNU General Public License v2.0.
• Normative Tables: Extracted mechanically from FFmpeg n8.1.2 under LGPL-2.1-or-later clause 3.
• Special Thanks:
  • Mattia Tadini (@MTSistemi) for authoring the complete bit-exact H.264 & HEVC decoders (VAEntrypointVLD), multi-slice HEVC, 16-bit Vulkan features, and distro packaging.
  • Shalasere for SPS crop research, Table 8-10 chroma QP mapping, and CABAC residual optimizations.
  • Community Testers: oblique99, Cosmos, land_and_air