AMD IOMMU + SME + amdgpu regression

[Alex Xu (Hello71)]

Hi,

amdgpu + IOMMU + SME is now working for me on 5.7, yay! But, it is 
broken on torvalds master, boo. On boot, depending on which exact commit 
I test, it either hangs immediately (with built-in driver, before 
starting initramfs), displays some errors then hangs, or spams the 
screen with many amdgpu errors.

I bisected the black screen hang to:

commit dce8d6964ebdb333383bacf5e7ab8c27df151218
Author: Joerg Roedel <jroedel@xxxxxxx>
Date:   Wed Apr 29 15:36:53 2020 +0200

    iommu/amd: Convert to probe/release_device() call-backs

    Convert the AMD IOMMU Driver to use the probe_device() and
    release_device() call-backs of iommu_ops, so that the iommu core code
    does the group and sysfs setup.

    Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
    Link: https://lore.kernel.org/r/20200429133712.31431-16-joro@xxxxxxxxxx
    Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>

Testing torvalds master (623f6dc593) with the containing merge 
(98bdc74b36) plus the DMA mapping merge (4e94d08734) reverted allows 
amdgpu + IOMMU + SME to once again work.

I think that nobody is really working on amdgpu + SME, but it would be a 
shame if it was supported and then incidentally broken by a small 
change.

I am using an ASRock B450 Pro4 with Ryzen 1600 and ASUS RX 480. I don't 
understand this code at all, but let me know what I can do to 
troubleshoot.

Thanks,
Alex.