Re: USB devices fail unnecessarily on unpowered hubs

[Andrew Morton]

On Thu, 01 Jun 2006 02:18:20 -0700
David Liontooth <liontooth@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx/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?
> 

(added linux-usb cc)

Yes, it sounds like we're being non-real-worldly here.  This change
apparently broke things.  Did it actually fix anything as well?
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