RadeonSI Resorts To Disabling SDMA For GFX9/Vega Due To APU Issues

[drSeehas]

... does this bug affect renoir apus as well?

Renoir = Vega. So: Yes.

[Venemo]

I think the only use case that gets a significant benefit from SDMA is when use an AMD GPU in an external GPU box. In this case, the SDMA is able to better utilize the bottlenecked bandwidth. In a normal desktop or an APU, the difference is probably marginal at best.

[PuckPoltergeist]

SDMA was designed for handling transfers from system memory to vram.

For APUs, VRAM is taken from system memory. Is it possible to do just a simple remapping instead of copying the whole data?

[agd5f]

For APUs, VRAM is taken from system memory. Is it possible to do just a simple remapping instead of copying the whole data?

"VRAM" on APUs is a separate region carved out at the top of system ram. So it's contiguous and the GPU has direct access to it rather than going through a remapping agent. As such it has slightly different latency characteristics compared to regular system memory. It's also marked as reserved from the perspective of the OS, so it's not automatically saved for things like suspend to disk. As such there are cases when you need to copy.

In general though, on APUs, you can use VRAM and system memory interchangeably, so when applications allocate device accessible memory, either is fine.