Re: USB power usage during startup (was: CVS commit: src/sys/dev/usb)

[Tobias Nygren <tnn%NetBSD.org@localhost>]

On Fri, 25 Jan 2008 13:25:01 +0100
Bernd Ernesti <veego%NetBSD.org@localhost> wrote:

> On Fri, Jan 25, 2008 at 12:29:12AM +0000, Jared D. McNeill wrote:
> > 
> > Module Name:        src
> > Committed By:       jmcneill
> > Date:               Fri Jan 25 00:29:12 UTC 2008
> > 
> > Modified Files:
> >     src/sys/dev/usb: uhub.c
> > 
> > Log Message:
> > Speed up uhub attachment considerably. Rather than powering up each port
> > individually then waiting for it to become stable, power them up all
> > at once and then delay. Don't even bother delaying if we are a root hub,
> > as this is handled separately in the event thread. From OpenBSD.
> 
> Hmm, how much power does this consume if you have more then one usb device
> which needs a lot of power during startup?
> 
> Bernd
> 

USB devices must negotiate with the hub for more power than 0.5W.
If they draw more, even during startup, they violate the spec.

Additionally, I believe the specification requires hubs to
current-limit the supply for devices that have not negotiated. This is
part of the short-circuit protection.