On Wed, 26 Apr 2006, Greg KH wrote:
> On Wed, Apr 26, 2006 at 11:32:06AM -0400, Alan Stern wrote:
> > I've seen a couple of bug reports which boiled down to the fact that the
> > hub driver doesn't enable port power if the hub descriptor indicates the
> > port power isn't switchable (a la USB 1.0).
>
> 1.0 USB Hubs? They should be thrown away, due to all of the problems in
> them. 1.1 fixed a bunch of bugs in USB hubs for this very reason. I
> have only seen one 1.0 hub, and that was a pre-production version.
>
> Also, those hubs should be very old, are we sure we want to support
> them? Windows shouldn't :)
These are not USB 1.0 hubs; they are more recent than that!
The problem is that in their hub descriptors, the Logical Power Switching
Mode (LPSM) (part of wHubCharacteristics) is set to 3. According to the
USB 2.0 spec, that means:
Reserved. Used only on 1.0 compliant hubs that implement no power
switching.
Possibly because they do this because they only emulate power switching
and don't implement it fully.
In fact, lots of hubs don't implement port power switching. Even when
they say that power is turned off to some port, all they really mean is
that they will ignore any events on that port -- but the power is still
on. Relatively few brands of hubs truly do switch the power.
(This has caused difficulties for people who want to use USB bus power as
a low-cost GPIO signal. They had to try a bunch of different brands
before finding some that worked.)
Alan Stern
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