[Bug 108521] RX 580 as eGPU amdgpu: gpu post error!

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108521

--- Comment #49 from Alex Deucher <alexdeucher at gmail.com> ---
(In reply to Dimitar Atanasov from comment #48)
> May be problem is the CPU, because it has only 16 PCIe lains, so 8 for vega
> M,
> 4 for NVME, and 4 for others, I have seen that card reader is also connected
> on PCIe.

It's the MMIO space in the CPU's address space.  The CPU (by way of the sbios)
defines a window of address space that is used for device MMIO.  By default
most platforms put a relatively small MMIO windows below 4GB for 32 bit OS
compatibility.  Having a small MMIO windows limits the amount of space for
devices and if there is not enough space some device resources can't be mapped
which is what causes the problem.  There is often a feature in the sbios config
called ">4GB MMIO" or similar which enables a bigger MMIO windows.  Some
sbioses also enable it dynamically depending on what OS is booted or conditions
in the system at boot time (legacy vs UEFI boot).  IIRC, it's a requirement for
windows 10, so there is probably something about the windows 10 OEM install
which causes it to boot with a larger MMIO window set up.

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