AMDGPU Linux 5.5 Fixes 8K / 4K120 Output, Hits Sync Object Timeline Support For Vulkan

While on the back-half of the Linux 5.5 kernel cycle, sent in on Wednesday were an interesting batch of AMDGPU driver fixes that are quite notable.

First up, the reverting of the retry faults behavior is part of this pull request. This code revert is about helping the stability of recent AMD APUs that for some Linux users has meant frequent hangs unless this functionality was disabled. The alternative to try this workaround is booting the kernel with amdgpu.noretry=0 to achieve the same effect on existing kernel releases.

Besides that Raven Ridge APU stability fix, there is also a change to reduce the HDMI pixel encoding if the max clock is exceeded. What this means in practice is for capable GPUs of being able to achieve 8K output or 4K120/HFR display modes that up to now were disabled by the AMDGPU driver, either not allowing 8K or limiting the refresh rate. The pixel encoding is dropped from 4:2:2 to 4:2:0 when the TMDS clock is exceeded.

Rounding out these notable fixes to the AMDGPU Direct Rendering Manager driver for Linux 5.5 is another big one: exposing the SYNCOBJ_TIMELINE feature. While this is exposing a new feature and not a "fix", it should be let into Linux 5.5 as the feature was already plumbed into the driver and it was just a matter of exposing the bit for user-space to know the functionality is supported. The synchronization object timeline support by the kernel driver is necessary for the AMDVLK / RADV drivers to expose Vulkan's VK_KHR_timeline_semaphore support. The AMDGPU support was plumbed into the kernel driver for a while but was waiting on the Vulkan extension to be firmed up and released.

So while this "fixes" pull request is small with just three patches, all of them are quite notable.