Timur Kristóf, a developer on Valve's open-source Linux graphics driver team, has resolved a long-standing bug preventing older AMD Radeon GPUs from working properly in Intel-based iMacs running Linux. The issue, first reported in June 2024, affected systems like the iMac with Radeon R9 M380 graphics, which would fail to boot correctly when using the AMDGPU driver and throw kernel errors related to AtomBIOS and UVD initialization.
After acquiring one of these aging iMacs for testing, Kristóf traced the problem to missing voltage dependency implementation for the display clock in the AMDGPU driver. The hardware's default voltage wasn't sufficient to properly drive the display controller, causing boot failures and graphical issues. The legacy Radeon driver worked fine, but users missed out on the performance improvements and RADV Vulkan support that come with AMDGPU.
Kristóf's fix disables memory clock dynamic power management (MCLK DPM) on problematic Sea Islands GPUs and forces the use of the highest memory clock when DPM is disabled. The patches are currently under review for inclusion in the mainline Linux kernel. This continues Kristóf's track record of improving support for older AMD hardware, following his work in 2024 that switched GCN 1.0 and 1.1 discrete GPUs to AMDGPU by default, delivering notable performance gains over the legacy driver.
