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To: Jes Sorensen <jes@sunsite.dk>
cc: Linux Bigot <linuxopinion@yahoo.com>, linux-kernel@vger.kernel.org
Subject: Re: how to get virtual address from dma address
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Date: Wed, 03 Oct 2001 17:44:18 -0500
From: James Bottomley <James.Bottomley@HansenPartnership.com>
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> Linux> All programmers I am relatively new to linux kernel. Please
> Linux> advise what is the safe way to get the original virtaul address
> Linux> from dma address e.g.,

> You have to store the address you pass to pci_map_single() somewhere
> in your data structures together with the dma address.

Yes, but speaking as someone who had to use a large hammer to convert his 
driver from bus_to_virt et al.,  it does seem rather hard not to have the 
equivalent for the new pci_dma paradigm.  It does present an obstacle 
persuading people to convert drivers, particularly if the hardware is going to 
present a linked list of addresses (as SCSI hardware often does).

After all, whatever device maps between the io bus and the memory bus, it must 
always map a given dma_addr_t to a known physical address.  It can't be that 
hard to provide an an API in the kernel which can compute this relationship 
(although I can see it may be expensive to walk iommu page tables).  I'm only 
really asking for a dma_addr_t to virtual address by the way.  I see that 
mapping the other way would be problematic.

James Bottomley