UAS: fix alignment of scatter/gather segments

From: Oliver Neukum <oneukum@suse.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: David Laight <David.Laight@ACULAB.COM>,
	"gregKH@linuxfoundation.org" <gregKH@linuxfoundation.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Subject: UAS: fix alignment of scatter/gather segments
Date: Mon, 29 Apr 2019 20:42:20 +0200	[thread overview]
Message-ID: <1556563340.20085.28.camel@suse.com> (raw)

On Mo, 2019-04-29 at 13:55 -0400, Alan Stern wrote:
> On Mon, 29 Apr 2019, Oliver Neukum wrote:
> 
> > On Mo, 2019-04-29 at 12:08 -0400, Alan Stern wrote:
> > > On Mon, 29 Apr 2019, Oliver Neukum wrote:
> > > 
> > > > On Mo, 2019-04-29 at 15:06 +0000, David Laight wrote:
> > > 
> > > > But the statement the old comment made are no longer correct.
> > > 
> > > Perhaps David would be satisfied if the comment were changed to say 
> > > that _some_ USB controller drivers have this unusual alignment 
> > > requirement.
> > 
> > It would seem to me that every controller that does not do
> > scatter/gather has this requirement. In other words, this is
> > the true requirement of USB. It does not come from the
> > controller. It comes from the protocol's need to not
> > send a short package.
> 
> Are you sure that xHCI has this requirement?  I haven't checked the

I am sure that it has not. UAS would never have worked.
Like in the case of storage this patch is necessary
for virtual controllers.

> spec.  I know that UHCI, OHCI, and EHCI do need this alignment (and
> OHCI and EHCI do in fact have hardware support for scatter-gather).
> 
> More precisely, what matters is whether the controller is able to merge
> two different DMA segments into a single packet.  UHCI can't.  OHCI and

Correct. However, we cannot blindly assume in a class driver that
certain controllers will be used.

> EHCI can, but only if the first segment ends at a page boundary and the
> second begins at a page boundary -- it's easier just to say that the
> segments have to be maxpacket-aligned.
> 
> > The second, old, comment is about controllers.
> 
> Well, if the drivers would use bounce buffers to work around the 
> controllers' issues then they wouldn't have this special requirement.  
> So it really is a combination of what the hardware can do and what the 
> driver can do.

Yes, but the point of using an API to specify restrictions to the
upper layer is to avoid using bounce buffers. Besides, bounce
buffers in block IO is interesting in terms of VM implications.

	Regards
		Oliver