[PATCH v2 0/3] USB: add generic onboard USB HUB driver

On Tue, 5 Jan 2016, Rob Herring wrote:

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It's not clear (to me, anyway) how this is meant to work.  USB is a
completely discoverable bus: There is no way to represent devices
statically; they have to be discovered.  But a device can't be
discovered unless it is functional, so if an onboard hub (or any other
sort of USB device) requires power, clocks, or something similar in
order to function, it won't be discovered.  There won't be any device
structure to probe, and "forcing driver probe" won't accomplish
anything.

It seems to me that the only way something like this could be made to
work is if the necessary actions are keyed off the host controller (and
in particular, _not_ the hub driver), presumably at the time the
controller is probed.

The problem with putting this in the the host controller driver is it
assumes the initialization sequence is generic enough to be described
in DT and that initialization is the only time we need DT data. The
MFD example is a perfect example of another case. Suspend/resume also
comes to mind. Even if we could put the control in both places, that
is poor design IMO. I'd rather just make it a requirement that the
bootloader do enough setup that devices can be discovered.

That would be okay with me.  It would make things a lot simpler (but it 
would also waste energy in situations where the devices weren't being 
used).

Alan Stern