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Re: USB umass hard drive "failed to create xfers" when attaching (Greg Troxel) writes:

>My impression is that something, perhaps more umass than *hci, needs a
>very large chunk of memory.

umass allocates two 64k (MAXPHYS sized) DMA buffers and a few smaller ones.
For all drivers but ehci each buffer must use contigous physical pages.

ehci learned to use multiple DMA segments some time ago, so the
error is now rare. xhci still has to learn it, other drivers may
not be able to support it due to hardware limitations.

>My fuzzy impression is that the standard wisdom is that drivers should
>not demand really large continuous chunks.

64kbyte is not really large.

                                Michael van Elst
                                "A potential Snark may lurk in every tree."

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