[REGRESSION] Too-low frequency limit for AMD GPU PCI-passed-through to Windows VM

[James D. Turner]

Hi Alex,

> I guess just querying the ATIF method does something that negatively
> influences the windows driver in the guest. Perhaps the platform
> thinks the driver has been loaded since the method has been called so
> it enables certain behaviors that require ATIF interaction that never
> happen because the ACPI methods are not available in the guest.

Do you mean the `amdgpu_atif_pci_probe_handle` function? If it would be
helpful, I could try disabling that function and testing again.

> I don't really have a good workaround other than blacklisting the
> driver since on bare metal the driver needs to use this interface for
> platform interactions.

I'm not familiar with ATIF, but should `amdgpu_atif_pci_probe_handle`
really be called for PCI devices which are bound to vfio-pci? I'd expect
amdgpu to ignore such devices.

As I understand it, starting with
f9b7f3703ff9 ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)"),
the `amdgpu_acpi_detect` function loops over all PCI devices in the
`PCI_CLASS_DISPLAY_VGA` and `PCI_CLASS_DISPLAY_OTHER` classes to find
the ATIF and ATCS handles. Maybe skipping over any PCI devices bound to
vfio-pci would fix the issue? On a related note, shouldn't it also skip
over any PCI devices with non-AMD vendor IDs?

Regards,
James