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Date: Tue, 23 Mar 2004 04:49:58 -0800
From: William Lee Irwin III <wli@holomorphy.com>
Subject: Re: can device drivers return non-ram via vm_ops->nopage?
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To: Andrea Arcangeli <andrea@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, Jeff Garzik <jgarzik@pobox.com>, James Bottomley <James.Bottomley@steeleye.com>, Russell King <rmk@arm.linux.org.uk>, Linux Arch list <linux-arch@vger.kernel.org>, Linus Torvalds <torvalds@osdl.org>, David Woodhouse <dwmw2@infradead.org>, Christoph Hellwig <hch@infradead.org>, Andrew Morton <akpm@osdl.org>
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On Tue, Mar 23, 2004 at 03:44:52AM -0800, William Lee Irwin III wrote:
>> mlock actually loops through the fault path, so in a sense it still
>> requires fault handling on the part of the driver, though AFAICT it can

On Tue, Mar 23, 2004 at 01:34:39PM +0100, Andrea Arcangeli wrote:
> it requires fault handling of course, but that's just the API with the
> driver, on the performance side (and it was the performance/latency side
> of the page faults to be complained) no page faults are generated, so
> it's not going to be a lot different from the map_sg stuff at runtime.
> anyways Linus vetoed the lazy approch so we probably should give it up
> (the one thing I like most is to avoid the branch in the fast path).

dma_mmap_coherent() being implemented via fault handling is unrelated
to ->fault() methods. It just uses the preexisting ->nopage() method
internally and transparently to the driver, and without any hooks
needed in the API either. Basically, however the arch wants to do it
so long as it fits into ->nopage(), doesn't need changes to the core, etc.


-- wli