RX 5500 XT: PCIe link speed stuck at Gen1 2.5GT/s by default

[Bjorn Helgaas]

See https://gitlab.freedesktop.org/drm/amd/-/issues/1447

This report was against DRM, but I'm mentioning it here because we've
been talking about some link speed init issues lately, and AFAICT the
gitlab reports don't show up anywhere in lore.

Robert Hancock reported:

> I'm using a RX 5500 XT card on an Asus PRIME H270-PRO motherboard,
> Intel i5-7500 CPU, with kernel 5.10.9 under Fedora 33. I noticed
> that in Linux, "lspci -vv" always showed the GPU PCIe link running
> at 2.5GT/s link speed and never seemed to change regardless of the
> application being run, while in Windows, GPU-Z shows the link
> running at the max supported 8GT/s speed when under graphical load.
> 
> It seems like the driver thinks that 2.5GT/s is the max allowable
> speed, based on the pp_dpm_pcie file:
> 
>   > cd /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/
>   > cat pp_dpm_pcie
>   0: 2.5GT/s, x8 81Mhz *
>   1: 2.5GT/s, x8 619Mhz *
> 
> I'm assuming that something is going wrong with the PCIe link speed
> detection in the driver. Using the "amdgpu.pcie_gen_cap=0x70007"
> kernel command line option seems to result in the driver detecting
> the proper 8GT/s maximum speed.
> 
> lspci -vv output from booting without overriding the speed is
> attached.