RE: [PATCH 3.12 033/118] usb: xhci: Link TRB must not occur within a USB payload burst

David Laight
2014-01-08T17:26:31Z

> From: Alan Stern
> On Wed, 8 Jan 2014, David Laight wrote:
>
> > > From: Alan Stern
> > >
> > > This may be a foolish question, but why is xhci-hcd using no-op TRBs in
> > > the first place?
> >
> > Because it can't write in a link TRB because other parts of the
> > code use link TRBs to detect the end of the ring.
> >
> > The problem is that it can't put a link TRB in the middle of
> > a chain of data fragments unless it is at a 'suitable' offset
> > from the start of the data TD. Given arbitrary input fragmentation
> > this means that you can't put a link TRB in the middle of a TD.
> > (The documented alignment might be as high as 16kB.)
> >
> > If the rest of the code used a 'ring end pointer' then a link TRB
> > could be used instead.
>
> I see. Sounds like a poor design decision in hindsight. Can it be
> changed?

Anything can be changed :-)
But it is a bit pervasive.
Padding out with nops isolated the change.

David