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

David Laight
2014-01-08T16:53:02Z

> 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.

David