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    SubjectRe: [alsa-devel] USB transfer_buffer allocations on 64bit systems
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    On Tue, 11 May 2010 11:04:55 -0400 (EDT)
    Alan Stern <stern@rowland.harvard.edu> wrote:

    > On Tue, 11 May 2010, FUJITA Tomonori wrote:
    >
    > > On Tue, 11 May 2010 10:24:40 -0400
    > > Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
    > >
    > > > > > > Either the data isn't getting written to the buffer correctly or else
    > > > > > > the buffer isn't getting sent to the device correctly. Can anybody
    > > > > > > suggest a means of determining which is the case?
    > > > > >
    > > > > > I can't say anything about this log that including only DMA addresses.
    > > > > > I'm not familiar with how the USB core does DMA stuff. And the USB
    > > > > > stack design that the USB core does DMA stuff (allocating, mappings,
    > > > > > etc) makes debugging DMA issues really difficult.
    > > > >
    > > > > The DMA stuff is simple enough in this case. The urb->transfer_buffer
    > > > > address is passed to dma_map_single(), and the DMA address it returns
    > > > > is stored in urb->transfer_dma. Those are the two values printed out
    > > > > by the debugging patch.
    > > >
    > > > Is that address (urb->transfer_dma) the same as 'virt_to_phys(urb->transfer_buffer)'
    > > > (if not, then SWIOTLB is being utilized) and is the dma_sync_* done on the
    > > > urb->transfer_dma (to properly sync the data from the SWIOTLB to the
    > > > transfer_buffer) before you start using the urb->transfer_buffer?
    > >
    > > Or calling dma_unmap_single.
    > >
    > > Can you tell me all the exact process of DMA that the usb core and the
    > > driver do?
    >
    > 1. The audio driver stores data in urb->transfer_buffer.

    How urb->transfer_buffer is allocated?


    > 2. The audio driver calls usb_submit_urb(urb).
    >
    > 3. The USB core does
    > urb->transfer_dma = dma_map_single(controller,
    > urb->transfer_buffer,
    > urb->transfer_buffer_length,
    > DMA_TO_DEVICE);

    The mapping error is checked via dma_mapping_error? Even if an mapping
    error happens, no data corruption happens due to that (needs
    something like retrying the request)?


    > 4. The host controller driver tells the hardware to carry out the data
    > transfer.
    >
    > 5. When the hardware says the transfer is finished, the USB core does
    > dma_unmap_single(controller,
    > urb->transfer_dma,
    > urb->transfer_buffer_length,
    > DMA_TO_DEVICE);
    >
    > 6. The audio driver's completion handler is called:
    > (urb->complete)(urb);

    The driver does only DMA_TO_DEVICE? Or you see DMA problems only with
    DMA_TO_DEVICE?


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