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

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Message-ID: <20070511154251.GJ4523@rhun.ibm.com>
Date:	Fri, 11 May 2007 18:42:51 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	maheshk@...c.in, linux-kernel@...r.kernel.org,
	kernelnewbies@...linux.org
Subject: Re: [HOWTO] accessing the DMA mapped data

On Thu, May 10, 2007 at 04:47:32AM -0700, David Miller wrote:
> 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.

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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