Somehow this does not go to the list. On Thu, Apr 12, 2012 at 1:43 AM, Matthew Dharm <mdharm-usb@xxxxxxxxxxxxxxxxxx> wrote: > On Tue, Apr 10, 2012 at 6:40 PM, Anil Nair <anilcoll90@xxxxxxxxx> wrote: >> >> >> On Wed, Apr 11, 2012 at 6:22 AM, Matthew Dharm >> <mdharm-usb@xxxxxxxxxxxxxxxxxx> wrote: >>> >>> On Tue, Apr 10, 2012 at 5:33 PM, James Haigh <james.r.haigh@xxxxxxxxx> >>> wrote: >>> > But can a powered hub be told to restrict a specific port to 1 unit >>> > load? >>> >>> >>No, a powered hub cannot be told such a thing. The hub offers up +5V; >>> >>it is up to the device to determine how much it will draw. >> >> >> What if during USB hub enumeration to the host we specify the power >> requirements? >> that is, how much power the USB hub consumes per port. Is such >> a behavior possible? > > > The hub tells the host (for informational purposes) how much power is > available for each port. The host has no control over this value. > >> The pull-up register triggers USB detection, so if a USB is not detected >> is it still consuming power? Can we recreate the USB not detecting as a >> programming model? > > An un-detected device can still draw power. Consider all those > "goofy" devices, such as USB LED lights, or other non-intelligent > devices that can charge off of a USB port. > > In fact, detection only works if power to the port is enabled. If > power was off, the pull-up resistor wouldn't do anything. > > In real-world, practical terms, if the device is plugged in, it can > draw all the power it wants until the over-current protection circuit > trips. The host CPU could be completely crashed and this would still > be the case. There is no way to do what you want. > > Matt > > > -- > Matthew Dharm > Maintainer, USB Mass Storage driver for Linux -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
