linux-kernel - Re: [HOWTO] accessing the DMA mapped data

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Date:	Thu, 10 May 2007 04:47:32 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	maheshk@...c.in
Cc:	linux-kernel@...r.kernel.org, kernelnewbies@...linux.org
Subject: Re: [HOWTO] accessing the DMA mapped data

From: Mahesh <maheshk@...c.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.
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