Subject: Re: can device drivers return non-ram via vm_ops->nopage?
From: David Woodhouse <dwmw2@infradead.org>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Christoph Hellwig <hch@infradead.org>,
       William Lee Irwin III <wli@holomorphy.com>, rmk@arm.linux.org.uk,
       Andrew Morton <akpm@osdl.org>, Andrea Arcangeli <andrea@suse.de>,
       linux-kernel@vger.kernel.org
In-Reply-To: <Pine.LNX.4.58.0403211349340.1106@ppc970.osdl.org>
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Date: Sun, 21 Mar 2004 22:23:15 +0000
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On Sun, 2004-03-21 at 13:53 -0800, Linus Torvalds wrote:
> So I really put my veto on "nopage()" returning a PFN. That's just wrong, 
> wrong, wrong. It returns a "struct page" pointer, and it has lots of 
> reasons for that.

That's fine -- I wasn't suggesting nopage() should return a PFN.

I was suggesting that if someone wants to map something they're given by
dma_alloc_coherent() into memory, they should be given a PFN to deal
with -- _not_ a "struct page". Therefore, you can't use nopage() for
mapping dma_coherent memory into userspace.

Basically, we should consider the stuff returned by dma_alloc_coherent
to be 'non-RAM' in the context of your previous statement:
	'If a driver wants to map non-RAM pages, that's perfectly ok,
	but it MUST NOT happen through "nopage()".'

There are machines where you _cannot_ sensibly use host memory for
dma_coherent() allocations, but on which there _is_ a few megabytes of
SRAM hanging off the PCI bus which was put there specifically for that
purpose. So dma_alloc_coherent() returns something for which there is
not a valid 'struct page'.

-- 
dwmw2


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