contiguous memory allocation problem

Julian Elischer julian at
Mon Jul 3 01:40:45 UTC 2006

Ian Dowse wrote:

>In message <200607021138.11945.hselasky at>, Hans Petter Selasky writes:
>>But there is one problem, that has been overlooked, and that is High speed 
>>isochronous transfers, which are not supported by the existing USB system. I 
>>don't think that the EHCI specification was designed for scatter and gather, 
>>when you consider this:
>>8 transfers of 0xC00 bytes has to fit on 7 pages. If this is going to work, 
>>and I am right, one page has to contain two transfers. (see page 43 of 
>I haven't looked into the details, but the text in section 3.3.3
>seems to suggest that EHCI is designed to not require physically
>contiguous allocations here either, so the same approach of using
>bus_dmamap_load() should work:
>  This data structure requires the associated data buffer to be
>  contiguous (relative to virtual memory), but allows the physical
>  memory pages to be non-contiguous. Seven page pointers are provided
>  to support the expression of 8 isochronous transfers. The seven
>  pointers allow for 3 (transactions) * 1024 (maximum packet size)
>  * 8 (transaction records) (24576 bytes) to be moved with this
>  data structure, regardless of the alignment offset of the first
>  page.

yes, as long as the beffers are contiguous in some virtual space then 
the maximum number of pages they
can need is 7.
(they actually fit into 6 but they may start part way through the first 
page and may therefore overflow into a 7th).

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