Gilmore on USB power

[Mitch Bradley]

John Gilmore and I were having a conversation about OLPC.  He had some
interesting comments regarding the power available to USB devices.
(Forwarded to this list with his approval.)

Mitch:

The crux of the problem is that full support for arbitrary USB devices 
blows our power budget right out of the water.

John:

OK, but devices should not end up with *flakey* power.  The USB protocol
has the device specify how much power it needs.  If the system can't
provide the amount of power that the device requests, it should turn
off the device and report the error -- rather than providing it "some
of" the power it requests.

Is the OLPC violating the USB spec by failing to supply the requested
power?  You may not be able to use the trademarked USB name, logo, or
patented connector if your device violates their specs.

When the OLPC is plugged into a wall, can it provide more power than
when it's running from a battery?  Or is the current limitation in
some intermediate circuit like a voltage regulator?  All the current
"A" boards are running on wall warts, but reporting problems anyway.  Why?

And whose "power budget" is this, anyway?  If the user would rather
have an hour of usage with a USB device, rather than have a day of
usage with no USB device, why is the design team making that choice
for the user?  Pop up a message if you insist -- but don't just say,
"Sorry, we have the power, but we won't use it for *that*!"  The
user may need to copy one minute's worth of data to or from an external
hard drive, or hook to an Ethernet for long enough to handle queued
emails, or something; the design would deny them that opportunity?

(Of course you'll run into devices that actually draw more than what
they ask for -- that's the device's bug, and should be reported
back so that they lose their USB certification if they don't fix it.
But that problem shouldn't be too widespread, though you won't know
til you start enforcing it.)