Re: [linux-usb-devel] Re: "sr: unaligned transfer" in 2.5.2-pre1

From: David Brownell (
Date: Tue Jan 01 2002 - 14:53:31 EST

> (2) How do I pass a highmem address to the HCDs? The URB structures we use
> don't seem particularly well-suited for this.

The urb->transfer_buffer is required to be normal DMA-able memory,
following the rules in Documentation/DMA-mapping.txt ... that got cleared
up somewhere around the 2.4.5 timeframe, all the HCDs now use the
pci_map_single() calls with those buffers. kmalloc() is fine, not vmalloc(),
and so on.

Not that I've seen a writeup about highmem (linux/Documentation doesn't
seem to have one anyway) but if I infer correctly from that DMA-mapping.txt
writeup, URBs don't support it because there's no way to specify buffers as
a "struct page *" or an array of "struct scatterlist". That's the only way that
document identifies to access "highmem memory".

> (1) Do the USB HCDs support highmem? I seem to recall they do, but I'm not
> certain.

If URBs can't describe highmem, the HCD's won't support them per se;
you'd have to turn highmem to "lowmem" or whatever it's called, and
then let the HCDs manage the lowmem-to-dma_addr_t mappings.

Alternatively, in 2.5 we might add "highmem" support to USB. Now that
I've looked at it a few minutes, I suspect we must -- just to support block
devices (usb-storage) fully. Is there more to it than adding page+offset
as an alternative way to describe the transfer_buffer? (And making all
the "single" mapping calls in the HCDs use page mappings.)

- Dave

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