Re: [PATCH v2 4/9] PCI: mark USB4 devices as removable

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi Mario,

On Thu, Feb 10, 2022 at 04:43:24PM -0600, Mario Limonciello wrote:
> USB4 class devices are also removable like Intel Thunderbolt devices.
> 
> Drivers of downstream devices use this information to declare functional
> differences in how the drivers perform by knowing that they are connected
> to an upstream TBT/USB4 port.

This may not be covering the integrated controllers. For discrete, yes
but if it is the PCIe root ports that start the PCIe topology (over the
PCIe tunnels) this does not work.

For integrated we have the "usb4-host-interface" ACPI property that
tells this for each port:

https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#mapping-native-protocols-pcie-displayport-tunneled-through-usb4-to-usb4-host-routers

and for discrete there is the PCIe DVSEC that can be used (see the USB4
spec archive it includes the "USB4 DVSEC Version 1.0.pdf" that has more
information). I would expect AMD controller (assuming it is discrete)
implements this too.

So I'm proposing that we mark the devices that are below  PCIe ports
(root, downstream) that fall in the above categories as "removable".
This is then not dependent on checking the USB4 controller and how it is
setup in a particular system.