From: Arnd Bergmann <arnd@arndb.de>
To: Timur Tabi <timur.tabi@ammasso.com>
Subject: Re: Will __pa(vmalloc()) ever work?
Date: Fri, 27 May 2005 22:31:33 +0200
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Cc: Christoph Hellwig <hch@infradead.org>, linux-kernel@vger.kernel.org
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On Freedag 27 Mai 2005 21:54, Timur Tabi wrote:
> Christoph Hellwig wrote:
> 
> > It will return the correct physical address for the start of the buffer.

No, not even that. If you do __pa(vmalloc()), the result will point outside of
the physical address space on most architectures.

> > But given that vmalloc is a non-contingous allocator that's pretty useless.
> 
> So as long as the vmalloc'd memory fits inside one page, __pa() will always give the 
> correct address?  If so, then can't I just call __pa() for every page in the buffer and 
> get a list of physical addresses?  If I can do that, then how the memory be virtually 
> contiguous but not physicall contiguous?

If the vmalloc'd memory fits into one page, you should not have used
vmalloc in the first place ;-). The only reason to ever use vmalloc
is if you can't get enough memory from alloc_pages reliably.

> > As are physical addresses for anything but low-level architecture code.
> 
> I don't understand what that means.

It means that a device driver should never need to use __pa directly, because
physical addresses don't have a well-defined meaning outside of the memory
management. A driver should only need to deal with user virtual, kernel virtual
and bus virtual but never real addresses.

Also, no device driver should be using vmalloc either: vmalloc fragments
your address space, pins physical pages and eats small children.

	Arnd <><
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