[PATCH v3 05/12] PCI: Detect root port of internal USB4 devices by `usb4-host-interface`

[Bjorn Helgaas]

On Mon, Feb 14, 2022 at 12:56:58PM +0200, Mika Westerberg wrote:
> On Mon, Feb 14, 2022 at 09:52:02AM +0100, Lukas Wunner wrote:
> > On Mon, Feb 14, 2022 at 09:34:26AM +0200, Mika Westerberg wrote:
> > > On Fri, Feb 11, 2022 at 03:45:46PM -0600, Bjorn Helgaas wrote:
> > > > My expectation is that "USB" (like "PCI" and "PCIe") tells me
> > > > something about how a device is electrically connected and how
> > > > software can operate it.  It doesn't really tell me anything about
> > > > whether those electrical connections are permanent, made through an
> > > > internal slot, or made through an external connector and cable.
> > > 
> > > It is used to identify "tunneled" ports (whether PCIe, USB 3.x or
> > > DisplayPort). Tunnels are created by software (in Linux it is the
> > > Thunderbolt driver) and are dynamic in nature. The USB4 links go over
> > > USB Type-C cable which also is something user can plug/unplug freely.
> > > 
> > > I would say it is reasonable expectation that anything behind these
> > > ports can be assumed as "removable".
> > 
> > USB gadgets may be soldered to the mainboard.  Those cannot be
> > unplugged freely.  It is common practice to solder USB Ethernet
> > or USB FTDI serial ports and nothing's preventing a vendor to solder
> > USB4/Thunderbolt gadgets.
> 
> Right, that's why I say it is "reasonable expectation" that anything
> behind these ports can be assumed "removable" :) Of course they don't
> have to be but if we assume that in the driver where this actually
> matters we should be on the safe side, no?

Spec citations help maintain things over the long term.  Reasonable
expectations that are part of today's folklore but are not written
down and shared leads to things that work today but not tomorrow.

Bjorn