On Tuesday, February 28th, 2023 at 16:16, Peter Stuge <peter@xxxxxxxx> wrote: > 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. Hm, right. With a first-come-first-served scheme, there is no way to have stable identifiers. I'm having a look at prior art: udev has similar needs for network interface names. For USB they use [2] a scheme with port/config/interface. I have no idea what meaning these have, but would they be useful for building a PATH KMS property? [1]: https://www.freedesktop.org/software/systemd/man/systemd.net-naming-scheme.html [2]: https://github.com/systemd/systemd/blob/7a67afe33192ce4a55e6825b80554fb4ebbb4b03/src/udev/udev-builtin-net_id.c#L758 > > > 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? I think the latter makes most sense. I'll try to send a patch to improve the PATH property docs.
