From: Dan Williams
> v1.0 hosts require that TD-fragments (portions of a TD that do not end
> on a MPB boundary) not cross a TRB segment boundary. This constraint is
> in addition to the constraint that a TRB may not specify a transfer that
> crosses a 64K boundary. This enabling permits the driver to accept
> scatterlists of nearly any geometry. "Nearly" because there is one
> unlikely remaining degenerate case of a driver submitting a transfer
> that consumes all the TRBs in a segment before hitting an MBP boundary.
> That case is trapped and the transfer is rejected.
>
> Given the multi-dimensional constraints of queuing TRBs from a
> scattelist, this implementation does not attempt to pre-calculate the
> number TRBs in a TD. Instead it attempts a dry-run of enqueuing the
> TRBs to the ring.
Presumably not really a 'dry run', it is just willing to back up to
the buffer than contained the previous boundary.
> If it discovers a TD-fragment straddling a segment
> boundary it backs up to the last MBP boundary, inserts a link-trb at
> that boundary, and restarts enqueuing in the next segment.
You have to be careful doing that.
At least some controllers don't like following a LINK and then finding
that they can't process the 'linked-to' TRB.
This means that the ownership of the LINK TRB can't be changed until
the ownership of the rest of the TRBs has been set (the controller could
be active and process the LINK at the end of the previous transfer).
This is why I ended up with a patch that moved the ownership setting
out of the callers.
> A side
> effect of not pre-calculating the number of required TRBs is that the
> ring is now expanded as the scatterlist is walked, rather than in
> prepare_ring().
>
> To simplify the math and forgo the need to track (union xhci_trb *) and
> (struct xhci_segment *) pointers, modulo-power-of-2 ring indexes are
> used. A small portion of the patch is adding infrastructure to convert
> from a (struct xhci_ring_pointer *) to an integer index.
>
> Glossary of acronyms:
> TRB: Transfer Request Buffer, 16-byte xhci-hardware scatterlist entry
>
> TD: Transfer Descriptor, the set of trbs that comprise a transfer
>
> TRB segment: A contiguous allocation of TRBs. They are of size
> PAGE_SIZE in the xhci driver. Each segment ends with a link TRB
> pointing to the next segment, but the link trb may appear at any TRB
> boundary in the segment.
>
> Ring: A linked list of segments.
>
> MBP: Max Burst Packet, is the minimum amount of data hardware expects to
> transfer before the end of a segment (assuming the TD spans a segment
> boundary).
I think you can ignore MBP, and only align to 1k boundaries.
However, I suspect that all that happens is the host controller splits
the transfer as the ring end - so provided it is at a 1k boundary
the target device can't tell the difference.
Indeed the 'end of TRB list' flag could be set on the 1k boundary.
1k boundaries were enough to make the Ge card I had (I don't have
them any more) work on the Intel Ivy bridge USB3.
David
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