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GPU Acceleration on Mac Pro 6,1

The Full Stack

The Mac Pro 6,1 ships with dual AMD FirePro GPUs. Under Linux, GPU acceleration uses a three-layer stack:

Application (OpenGL / Vulkan / OpenCL / VA-API)
    |
Mesa userspace drivers (radeonsi, RADV, rusticl)
    |
amdgpu kernel driver (SI support enabled)
    |
GPU hardware (D300 / D500 / D700)

Every layer is open-source, actively maintained, and improving with each release.

GPU Variants

GPU VRAM Codename GCN PCI ID Shader Cores
FirePro D300 2GB GDDR5 Pitcairn 1.0 (SI) 1002:6819 1280
FirePro D500 3GB GDDR5 Tahiti 1.0 (SI) 1002:6798 1792
FirePro D700 6GB GDDR5 Tahiti XT 1.0 (SI) 1002:6798 2048 (32 CUs)

All are GCN 1.0 (Southern Islands). The amdgpu kernel driver handles them with amdgpu.si_support=1. This kernel does not include the legacy radeon driver.

What Works Today

OpenGL (radeonsi)

Full OpenGL 4.6 support via Mesa's radeonsi driver. This is mature, stable, and performant.

$ glxinfo | grep "OpenGL version"
OpenGL version string: 4.6 (Compatibility Profile) Mesa 26.1.0-devel

Vulkan (RADV)

Vulkan 1.3+ via Mesa's RADV driver. Supports most modern Vulkan applications.

$ vulkaninfo --summary
GPU0: AMD RADV TAHITI (LLVM 19.1.7)
  Vulkan 1.4.341

Video Decode (VA-API / UVD)

Hardware video decode via AMD's UVD (Unified Video Decoder):

Codec Status
H.264 / AVC Hardware decode
MPEG-2 Hardware decode
VC-1 Hardware decode
H.265 / HEVC Not supported (needs GCN 2.0+)
VP9 / AV1 Not supported 
$ vainfo
Driver version: Mesa Gallium driver 26.1.0-devel for AMD Radeon R9 200 / HD 7900 Series

Video Encode (VCE)

Hardware video encode via AMD's VCE (Video Coding Engine):

  • H.264 encoding supported
  • Used by OBS, FFmpeg, and other tools

OpenCL Compute (rusticl)

OpenCL via Mesa's rusticl backend. ROCm does not support Southern Islands GPUs.

# Enable rusticl
export RUSTICL_ENABLE=radeonsi

# Verify
clinfo | grep "Device Name"

Display Output

All 6 Mini DisplayPort outputs work (3 per GPU). HDMI via adapter supported. Audio over DP/HDMI works.

What Doesn't Work

Feature Reason
ROCm / HIP AMD dropped SI support; minimum is GCN 3.0 (Fiji)
HEVC decode Hardware UVD too old (GCN 1.0)
VP9 / AV1 decode Not present in hardware
DisplayPort MST (daisy-chain) Not supported by amdgpu SI
FreeSync / VRR Requires GCN 2.0+
Power management (pp_dpm) PowerPlay sysfs not available for SI GPUs

Mesa Version Matters

Mesa improvements for GCN 1.0 have been significant in recent releases:

Mesa Version Key Improvements
24.0+ radeonsi NIR backend (replaces TGSI), better shader compilation
24.2+ RADV improvements for older GCN
25.0+ rusticl OpenCL stability
26.1 (current) Continued radeonsi/RADV refinement

Recommendation: Use the latest Mesa available. On CachyOS/Arch:

# Stable
sudo pacman -S mesa vulkan-radeon

# Bleeding edge (mesa-git from CachyOS repos)
sudo pacman -S mesa-git

This system is running mesa-git 26.1.0-devel from CachyOS.

Dual GPU Usage

Both GPUs are available. By default, GPU 1 (card1) handles display. GPU 0 (card0) is available for compute or offload:

# Check both GPUs
ls /dev/dri/renderD*
# renderD128 = GPU 0, renderD129 = GPU 1

# Run an app on the second GPU
DRI_PRIME=1 glxgears

# Assign specific GPU for Vulkan
MESA_VK_DEVICE_SELECT=1002:6798:1 vulkan-app

Performance Tuning

# Shader cache (reduces stutter on repeated workloads)
export MESA_SHADER_CACHE_DIR="$HOME/.cache/mesa_shader_cache"
export MESA_SHADER_CACHE_MAX_SIZE=4G

# Threading (significant for OpenGL)
export mesa_glthread=true

# RADV optimizations
export RADV_PERFTEST=gpl,nggc

Kernel Parameters

The linux-macpro61 kernel and ISO set these automatically:

amdgpu.si_support=1    # Enable Southern Islands in amdgpu
amdgpu.dc=0            # Display Core off (SI uses legacy display path)
acpi_mask_gpe=0x16     # Mask Thunderbolt ACPI storm (GPE16 on Mac Pro 6,1)

The modprobe config (/etc/modprobe.d/macpro-gpu.conf) ensures these persist after installation.

Acceleration Roadmap

Status Feature Details
Done OpenGL 4.6 radeonsi, mature and stable
Done Vulkan 1.3+ RADV, works for most applications
Done VA-API decode H.264, MPEG-2, VC-1 via UVD
Done VCE encode H.264 hardware encoding
Done OpenCL Via rusticl (Mesa)
Done Multi-GPU Both GPUs accessible, DRI_PRIME offload
Done DP audio HDMI/DP audio via amdgpu
Investigating macOS GPU paravirtualization PVG (ParavirtualizedGraphics) for KVM — Metal commands translated through Mesa on host. See pvg-linux.md
Not possible ROCm / HIP AMD hardware requirement: GCN 3.0+
Not possible HEVC / VP9 / AV1 decode Hardware limitation

macOS Tahoe in KVM

This kernel includes KVM support. macOS Tahoe runs in QEMU on top of the accelerated amdgpu kernel driver -- the same driver stack powering the Linux host. The guest runs on real GPU hardware through KVM.

For exposing Metal/3D to the guest (PVG -- paravirtualizing Metal commands through Mesa on the host), see pvg-linux.md.

Compared to macOS (OCLP)

Apple dropped macOS support for the 6,1. OCLP tries to bring it back by shimming 2013-era GPU kexts into modern macOS. Here's how the two approaches compare:

Linux (this project) macOS (OCLP)
GPU driver age 2026 (actively maintained) 2013 kexts (shimmed)
OpenGL 4.6 4.1 (Apple's last)
Vulkan 1.3+ via RADV MoltenVK (translation layer)
Metal N/A Shimmed, fragile
Video decode UVD (H.264) VDA (similar)
Stability Solid (native driver) Breaks on macOS updates
Future Improving every Mesa release Deprecated, OCLP may drop 6,1

Further Reading

  • mesa.md -- Mesa setup, environment variables, troubleshooting
  • pvg-linux.md -- ParavirtualizedGraphics roadmap for macOS KVM
  • kvm-macos.md -- macOS Tahoe KVM setup guide