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

[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.

[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?

[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.

[drSeehas]

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

Renoir = Vega. So: Yes.

[leipero]

I don't have any artifacts with my 2500u.

Neither on Picasso. I do have different issue with linux with iGPU clocks randomly getting stuck at X frequency, but that's not related to Mesa anyway, and I'm not reporting it as an bug until I rule out UEFI/system issue.

[agd5f]

SDMA was designed for handling transfers from system memory to vram. It's design bandwidth aligns with that. Shaders are designed to take advantage of as much bandwidth as possible. The primary use case for SDMA is in the kernel driver where it's used for paging and other memory management related tasks.

[Nille_kungen]

I don't have any artifacts with my 2500u.

[Mthw]

...rendering issues/corruption on Raven...

Like having screen artifacts after resuming from sleep? I do have this issue.

[bridgman]

I wonder if the NAVI APU's (5000 model?) will have this issue or if its specific to Vega.

Marek knows a lot more about this but my recollection is that we got better performance on Navi without SDMA and are not using it.

It's probably fair to say that for most of our chips performance improvements in GFX (shader copy, CPDMA copy) have been pretty significant while SDMA has not improved at the same pace so each year the benefit of SDMA goes down while the overhead of managing and synchronizing with a separate engine stays more or less constant.

[theriddick]

I wonder if the NAVI APU's (5000 model?) will have this issue or if its specific to Vega.