> From: linux-usb-ow...@vger.kernel.org
> [mailto:linux-usb-ow...@vger.kernel.org] On Behalf Of David Laight
> Sent: Friday, October 03, 2014 6:08 AM
>
> From: Mathias Nyman
> > On 10/01/2014 05:50 PM, vichy wrote:
> > > hi Mathias:
> > >
> > >>
> > >> In xhci-ring.c, static int xhci_queue_isoc_tx():
> > >>
> > >> /* Calculate TRB length */
> > >> trb_buff_len = TRB_MAX_BUFF_SIZE - (addr & ((1 << TRB_MAX_BUFF_SHIFT)
> > >> - 1));
> > >>
> > >> where addr = start_addr + urb->iso_frame_desc[i].offset;
> > >>
> > >> this limits the trb_buff_len to stop the trb buffer at TRB_MAX_BUFF_SIZE
> > >> boundary
> > > Yes, you are right.
> > > the above is make sure trb buffer will not cross over
> > > TRB_MAX_BUFF_SIZE boundary.
> > > But it still possible to cross the page boundary for ISO trb, right?
> > > (suppose page boundary is 4KB, TRB_MAX_BUFF_SIZE is 64KB)
> > >
> > > appreciate your kind help,
> > >
> >
> > Hi
> >
> > As I understood it, it is ok to cross page boundaries as long as the buffer
> > is physically contiguous.
> > As our dma case should be.
> > xhci specification section 3.2.8 says that:
> >
> > "Note that no constraints are placed on the TRB Length fields in a
> > Scatter/Gather list. Classically
> > all the
> > buffers pointed to by a scatter gather list were required to be “page size”
> > in length except for the
> > first and
> > last (as illustrated by the example above). The xHCI does not require this
> > constraint. Any buffer
> > pointed to
> > by a Normal, Data Stage, or Isoch TRB in a TD may be any size between 0 and
> > 64K bytes in size. For
> > instance, if when an OS translates a virtual memory buffer into a list of
> > physical pages, some of the
> > entries
> > in the list reference multiple contiguous pages, the flexible Length fields
> > of TRBs allow a 1:1
> > mapping of list
> > entries to TRBs, i.e. a multi-page list entry does not need to be defined
> > as multiple page sized
> > TRBs."
> > "
>
> The ohci hardware requires that scatter-gather boundaries occur on USB
> message boundaries.
>
> The xhi hardware doesn't have that constraint, but has the different
> constraints:
> 1) A buffer must not cross a 64k address boundary.
> 2) A LINK TRB (at the end of the ring) must only occur at a USB message
> boundary
> (actually it is a slightly tighter constraint).
> In combination those two are a PITA.
Hi David,
May I ask where in the xHCI spec is 2) stated? I'm not finding it
(looking at "Revision 1.0 with errata to 7/12/12").
I can't find "message boundary" anywhere in the spec. Did you mean packet
boundary?
If so, there is some text in the spec which contradicts that. In 4.11.7.1,
it says:
"In Figure 26 the IOC flag only may be set in TRBs 5 and 10 ...
Note that THE LINK TRB DOES NOT LAND ON A PACKET BOUNDARY relative
to the start of TD Fragment 1, so its IOC flag may not be set."
(emphasis mine)
--
Paul
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