On Tue, Oct 06, 2020 at 12:25:36PM -0700, Matthias Kaehlcke wrote:

On Tue, Oct 06, 2020 at 01:15:24PM -0400, Alan Stern wrote:

quoted

You don't need a platform device or a new driver to do this.  The code 
can go in the existing hub driver.

Maybe. IIUC currently USB drivers don't support/use suspend_late. Could that
be added? It would simplify matters, otherwise all hubs need to know their
peers and check in suspend if they are the last hub standing, only then the
power can be switched off. It would be simpler if a single instance (e.g. the
hub with the DT entries) is in control.

Adding suspend_late would be a little painful.  But you don't really 
need it; you just need to make the "master" hub wait for its peer to 
suspend, which is easy to do.

And hubs would need to know their peers in any case, because you have to 
check if any devices attached to the peer have wakeup enabled.

quoted

Incidentally, the peering information is already present in sysfs, 
although it is associated with a device's port on its upstream hub 
rather than with the device itself.

That might also help the hub driver to determine its peers without needing the
'companion-hubs' property.

It wouldn't hurt to have that property anyway.  The determination of 
peer ports doesn't always work right, because it depends on information 
provided by the firmware and that information isn't always correct.

Alan Stern