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    SubjectRe: [linux-usb-devel] Re: highmem and usb [was "sr: unaligned transfer" in 2.5.2-pre1]
    > > 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".
    > What kind of mapping does URBs require? A virtual mapping?

    Each URB points to one transfer buffer, address plus length, where
    the USB device drivers directly read/write those addresses. The
    requirement for drivers is that the transfer buffers can be passed to
    pci_map_single() calls by the Host Controller Drivers (HCDs). The
    device drivers, and URBs, don't expose such mappings, they only
    require that they can be created/destroyed.

    I'd be inclined to say transfer buffers must not be "virtual" mappings,
    but since terminology can differ I'll let you draw your conclusion.

    > > > (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
    > No, you can always ask to get pages low mem bounced. Highmem is no
    > requirement, and if your device really can't support it there's no point
    > in attempting to support it.

    Standard HC hardware (EHCI, OHCI, UHCI) can use 32bit addresses
    for their PCI DMA. EHCI can also, in some implementations, handle 64bit

    > > page+offset as an alternative way to describe the transfer_buffer?
    > no
    > > (And making all the "single" mapping calls in the HCDs use page
    > > mappings.)
    > exactly

    So you're saying that pci_map_page() is not necessary? And that
    highmem buffers (page+offset, or scatterlist thereof) can be turned
    into the form needed by URBs using some other mechanism?

    I'd certainly rather that be the case for the moment, for simplicity.
    Keep in mind that usb-storage seems to be the only driver that'd
    run into that issue.

    - Dave

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