Right now, in drm-kmod, MSI is ifdef'd out. (Even though it was used in the old in-tree DRM port.)
Which doesn't seem to cause any problems on amd64, but legacy interrupts and ARM/aarch64 don't mix very well.
That is, on my MACCHIATObin with a Radeon RX 480, I've been seeing an interrupt rate of >150000 with just console and >300000 with GUI. 25% of a CPU core (!) was wasted on interrupts and desktop responsiveness suffered.
Enabling MSI in amdgpu produces a much more reasonable interrupt rate — I'm seeing 160 as I'm typing this :)
I've tried to use the dev.msix field but I couldn't figure out how the MSI-X assignment works, so I introduced a new flag. Suggestions for a simpler/better way are welcome of course.
also, this field is currently not initialized, where is the best place to do that?