UAS: fix alignment of scatter/gather segments

[David Laight <David.Laight@ACULAB.COM>]

From: Oliver Neukum
> Sent: 29 April 2019 14:38
> On Mo, 2019-04-29 at 13:31 +0000, David Laight wrote:
> > From: Oliver Neukum
> > >
> > > +	 * USB has unusual scatter-gather requirements: the length of each
> > > +	 * scatterlist element except the last must be divisible by the
> > > +	 * Bulk maxpacket value.  Fortunately this value is always a
> > > +	 * power of 2.  Inform the block layer about this requirement.
> > > +	 */
> >
> > That isn't the correct restriction for XHCI.
> > It has its own perverse restrictions.
> > I think they are all handled within the xhci driver.
> 
> Yes, but that does not matter. You just cannot assume that only
> XHCI will be used with UAS. In particular virtual drivers will
> be used.

True, but there is no need to enforce a 2k (IIRC) alignment for XHCI.
Perhaps you need a different property from the controller.

Even if you decide the code is 'good enough' (I don't know what the
cost is of enforcing a 2k alignment instead of 512 bytes)
the comment is just plain wrong.

	David

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