Re: how to get virtual address from dma address

[BALBIR SINGH]

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With Rik's reverse mapping patch, wouldn't we have the virtual address for the given
physical address ? I have no clue about how the patch works, somebody willing to explain
it?

Balbir

David S. Miller wrote:

>   From: James Bottomley <[e-mail]>
>   Date: Wed, 03 Oct 2001 17:44:18 -0500
>
>   (although I can see it may be expensive to walk iommu page tables)
>
>I know of hardware where doing the reverse mapping would not even be
>possible, the page tables are in hardware registers and are "write
>only".  This means you can't even read the PTEs back, you'd have to
>keep track of them in software and that is totally unacceptable
>overhead when it won't even be used %99 of the time.
>
>The DMA API allows us to support such hardware cleanly and
>efficiently, but once we add this feature which "everyone absolutely
>needs" we have a problem with the above mentioned piece of hardware.
>
>Franks a lot,
>David S. Miller
>[e-mail]
>
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