On Thursday 24 February 2005 2:20 pm, Glenn Maynard wrote: > > The exception is that some (self-powered) root hubs have limits on the > > amount of power they put out. For example, they might not be able to > > put more than 250 mA out on their single port, or they might have two > > ports each limited to 200mA. (I picked those examples because I happen > > to have boards with those specific limitations...) > > Ick. That would mean that attaching a bus-powered hub, which uses 100mA > itself, would only have 100-150mA available for the total of all devices, > right?
Right. And for an OTG root hub, with 8mA total, you may not be able to hook any kind of hub up without blowing the power budget ... in fact, you probably can't fit any non-OTG device at all. (Except by relying on the fact that most powered hubs overreport their power demands...) Root hub power budgets need to be treated a bit differently. For the moment such configs are rejected entirely by the logic which rejects connection of consecutive bus-powered hubs. > Is this information correctly reported by the USB host? Right now it'd sit in board-specific code, part of the bus glue that sets up the root hub. Power budgets don't appear in sysfs though, ISTR. > If some boards can't supply enough power to get at least 100mA per port > (to two ports, not four) on our bus-powered hub, that's an important point > for us when evaluating replacement boards. I don't think PCs generally need need to worry about this stuff, it's just boards designed for embedded boxes with power limits of a handful of Watts total ... which pretty much rules out most x86 systems, or for that matter non-custom add-on boards they'd be "mother" to. (Those boards tend to need another handful of Watts of power ...) The systems I've seen with this issue don't have the luxury of switching board designs. - Dave ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
