RE: [PATCH] lsusb: Show USB 3.0 U1, U2, and LTM status.

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On Wed, 27 Jun 2012, Paul Zimmerman wrote:

> > > Just FYI, a device cannot have a bcdUSB of 0x300 in its device descriptor
> > > if it's not running at super speed, because a USB 3.0 device descriptor
> > > is incompatible with earlier versions, due to the bMaxPacketSize0 field
> > > being encoded as a power of 2 instead of a byte count.
> > 
> > I don't see any problem.  If bcdUSB is 0x0300 and the device is running
> > at, say, high speed then the maxpacket size for ep0 would be 64 bytes,
> > which would be represented by bMaxPacketSize0 = 6.  See the description
> > of bMaxPacketSize0 in table 9-8 of the USB-3 spec.
> 
> Ummm... how is a USB 2.0 host, released before the USB 3.0 spec was even
> written, supposed to know that? The device does not know what version of
> host it is connecting to.

A USB 2.0 host is supposed to recognize that a bcdUSB value of 0x0300
means the device is not compliant with the USB-2 spec.  Therefore the
host should know not to misinterpret the bMaxPacketSize0 field.

On the other hand, it is true that a device which wants to be 
compatible with older hosts had better not use a USB-3 descriptor.

Alan Stern

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