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To: James Bottomley <James.Bottomley@HansenPartnership.com>
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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From: Jes Sorensen <jes@sunsite.dk>
Date: 05 Oct 2001 16:06:31 +0200
In-Reply-To: James Bottomley's message of "Wed, 03 Oct 2001 17:44:18 -0500"
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>>>>> "James" == James Bottomley <James.Bottomley@HansenPartnership.com> writes:

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.

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

Because as DaveM pointed out, some hardware can't do it, and as I said
earlier, it's a lot cheaper and easier for driver writers to just
store the extra pointer in their data structures than it is to
implement a database to maintain it.

Remember you often need this address in the hot path (say TX interrupt
handler) so you don't want to introduce any unnecessary function calls.

Jes
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