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    SubjectRe: USB transfer_buffer allocations on 64bit systems
    On Wed, Apr 07, 2010 at 10:59:47AM -0400, Alan Stern wrote:
    > On Wed, 7 Apr 2010, Daniel Mack wrote:
    > > Depending on the condition of the memory management, things might work
    > > or not, and especially right after a reboot, there's a better chance to
    > > get lower memory.
    > >
    > > The fix is to use usb_buffer_alloc() for that purpose which ensures
    > > memory that is suitable for DMA. And on x86_64, this also means that the
    > > upper 32 bits of the address returned are all 0's.
    > That is not a good fix. usb_buffer_alloc() provides coherent memory,
    > which is not what we want. I believe the correct fix is to specify the
    > GFP_DMA32 flag in the kzalloc() call.
    > Of course, some EHCI hardware _is_ capable of using 64-bit addresses.
    > But not all, and other controller types aren't. In principle we could
    > create a new allocation routine, which would take a pointer to the USB
    > bus as an additional argument and use it to decide whether the memory
    > needs to lie below 4 GB. I'm not sure adding this extra complexity
    > would be worthwhile.

    Well, I thought this is exactly what the usb_buffer_alloc() abstraction
    functions are there for. We already pass a pointer to a struct
    usb_device, so the routine knows which host controller it operates on.
    So we can safely dispatch all the magic inside this function, no?

    If not, I would rather introduce a new function than adding GFP_ flags
    to all existing drivers.

    I vote for a clean solution, a fixup of existing implementations and
    a clear note about how to allocate buffers for USB drivers. I believe
    faulty allocations of this kind can explain quite a lot of problems on
    x86_64 machines.

    > > If what I've stated is true, there are quite some more drivers affected
    > > by this issue.
    > Practically every USB driver, I should think. And maybe a lot of
    > non-USB drivers too.

    I found many such problems in drivers/media/{dvb,video},
    drivers/usb/serial, sound/usb and even in the USB core. If we agree on
    how to fix that nicely, I can take some of them.


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