gud: set PATH connector property

[Peter Stuge]

Simon Ser wrote:
> > > Would it be possible to set the PATH connector property based on the
> > > USB port used by gud?
> > 
> > Sadly not really easily.
> > 
> > The physical topology underneath each host controller is stable but
> > bus numbers (usb1, usb2 etc.) are not.
> 
> Oh, that's news to me. So if I unplug and replug a USB device, the bus
> number and bus device number might change?

The bus number is stable as long as the bus (host controller) exists.


> Or does this happen after a power-cycle? Or is this hardware-specific?

Consider a host controller on a plug-in card, like ExpressCard (usb1)
and perhaps Thunderbolt (usb2) for a more modern example.

The bus on each new host controller gets the next available bus number.

Plug ExpressCard before Thunderbolt to get the order above. Unplug
both (usb1+usb2 disappear) then plug Thunderbolt back in before
ExpressCard; now Thunderbolt is usb1 and ExpressCard usb2.


> > For onboard host controllers it could be possible to anchor to a
> > PCI or platform bus device.
> 
> But the PCI bus device isn't enough I think? If I have a onboard host
> controller with 2 physical USB ports, the PCI bus device isn't enough
> to tell these 2 physical ports apart?

*Only* the PCI bus device is not enough, but it could serve as a
unique anchor and some host controller-specific information (e.g.
root port number) can be added to that.


> > How about using e.g. the serial number of the gud USB device instead
> > of host topology, or maybe some other information from the panel
> > behind it?
> 
> The PATH property is really about the port path, not the sink. IOW, if
> I have two USB displays, one USB port, and switch between the two
> displays on this one port, the PATH property isn't supposed to change.

Hmm. What is "port path" supposed to mean across a hot-pluggable bus?

Should PATH refer to the one ExpressCard host controller if it moves
between slots (assuming a computer with two slots) or should it
rather refer to "the upper USB port on the right hand side ExpressCard"
so I can insert another brand host controller in that slot without
PATH changing?


//Peter