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

[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?

Alan Stern

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