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To: David Miller <davem@davemloft.net>
Cc: ralphc@pathscale.com, rolandd@cisco.com, openib-general@openib.org,
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Subject: Re: Suggestions for how to remove bus_to_virt()
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From: Roland Dreier <rdreier@cisco.com>
Date: Thu, 13 Jul 2006 09:02:47 -0700
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 > > A cleaner solution would be to make the dma_ API really use the device
 > > it's passed anyway, and allow drivers to override the standard PCI
 > > stuff nicely.  But that would be major surgery, I guess.

 > Clean but expensive, you should not force the rest of the kernel
 > to eat the cost of something you want to do when it's totally
 > unnecessary for most other users.

OK, fair enough.

 > For example, x86 never needs to do anything other than a direct
 > virt_to_phys translation to produce a DMA address, no matter what
 > bus the device is on.  It's a single simple integer adjustment
 > that can be done inline in about 2 or 3 instructions at most.

<pedantic>Except x86 needs to handle systems with IOMMUs now...</pedantic>

 > If you need device level DMA mapping semantics, create them for your
 > device type.  This is what USB does, btw.

Makes sense -- Ralph, I would suggest looking at USB as a model.

 - R.