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

[David Laight]

> 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



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