Re: kern/60073: xhci(4): NetBSD xHCI roothub uses ud_addr=0 instead of 1?

["Jonathan A. Kollasch" <jakllsch%kollasch.net@localhost>]

Using address 0 for the NetBSD xhci(4) root hub is precisely what I
intended to do.

An xHCI hardware/firmware implementation has exclusive discretion as to
how it allocates bus addresses to devices.  The only address it will
never give a device is 0, as that is the un-addressed initial state of
a new device.  An xHCI hardware/firmware implementation is allowed to
use identical USB addresses for real devices on different bus interfaces
(think multiple USB1/2 bus interfaces to increase available bandwidth
across multiple USB1/2 root ports).  Hardware/firmware implementations
are also allowed to allocate address 1 to a device on the bus(es).

The OS's USB root hub is *not* a real device.  USB Transactions to the
OS-implemented root hub never reach the data lines of the bus, and there
is no mechanism to notify the xHCI hardware/firmware that the OS has
reserved a bus address for a virtual (root hub) device.

Any solution to 60074 should take these constraints into account.

I agree this should be documented in comments in the code.