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    SubjectRe: [linux-usb-devel] Re: USB devices fail unnecessarily on unpowered hubs
    On Thu, Jun 01, 2006 at 10:58:43AM -0400, Alan Stern wrote:
    > On Thu, 1 Jun 2006, Andrew Morton wrote:
    >
    > > On Thu, 01 Jun 2006 02:18:20 -0700
    > > David Liontooth <liontooth@cogweb.net> wrote:
    > >
    > > > Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
    > > > hubs. Alan Stern explains,
    > > >
    > > > "The idea is that the kernel now keeps track of USB power budgets. When a
    > > > bus-powered device requires more current than its upstream hub is capable
    > > > of providing, the kernel will not configure it.
    > > >
    > > > Computers' USB ports are capable of providing a full 500 mA, so devices
    > > > plugged directly into the computer will work okay. However unpowered hubs
    > > > can provide only 100 mA to each port. Some devices require (or claim they
    > > > require) more current than that. As a result, they don't get configured
    > > > when plugged into an unpowered hub."
    > > >
    > > > http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg43480.html
    > > >
    > > > This is generating a lot of grief and appears to be unnecessarily
    > > > strict. Common USB sticks with a MaxPower value just above 100mA, for
    > > > instance, typically work fine on unpowered hubs supplying 100mA.
    > > >
    > > > Is a more user-friendly solution possible? Could the shortfall
    > > > information be passed to udev, which would allow rules to be written per
    > > > device?
    >
    > I'm not sure whether we create a udev event when a new USB device is
    > connected.

    Yes we do. It's of the class "usb_device" and you can write a single
    udev rule to override the power test if you really want to.

    Of course I don't recommend someone doing this, as it is violating the
    USB power rules, and it is a good thing that we are finally testing for
    them.

    thanks,

    greg k-h
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