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    SubjectRe: [HOWTO] accessing the DMA mapped data
    On Thu, May 10, 2007 at 04:47:32AM -0700, David Miller wrote:
    > From: Mahesh <maheshk@cdac.in>
    > Date: Thu, 10 May 2007 17:11:17 +0530
    >
    > > Hi, > You haven't given an example of where this might actually
    > > happen. > > The driver is where the DMA mappings almost always
    > > occur because > that is the layer that knows the bus technology and
    > > therefore > the correct DMA interfaces to call. > > What kind of
    > > driver do you have and what is this upper layer > doing the mappings
    > > for you but not giving you a pointer to > the kernel buffer as well?
    > > > Here I am dealing with a infiniband (see www.openfabrics.org)
    > > network device driver. The layer above the driver is the standard
    > > infiniband core interface. Now I have a situation where I need to
    > > peek into the packets and do some modifications(some hacking). So I
    > > just want know whether I can access the original data region using
    > > the bus address generated by the dma_map_single.
    >
    > You can't, therefore you need to ask the Inifiniband guys to perhaps
    > tweak the infiniband driver interfaces so that you can get at the
    > buffer or provide some other mechanism by which you can accomplish
    > what you're trying to do.

    AFAICR the ipath driver had the same problem, which resulted in the
    addition of ib specific DMA-API wrappers. See usage of struct
    ipath_dma_mapping_ops in ipath_dma.c.

    Cheers,
    Muli

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