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Subject: Re: can device drivers return non-ram via vm_ops->nopage?
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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To: William Lee Irwin III <wli@holomorphy.com>
Cc: 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>, Andrea Arcangeli <andrea@suse.de>
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> On Mon, Mar 22, 2004 at 05:18:30PM -0500, Jeff Garzik wrote:
> > You need both kernel and userspace...  for audio drivers, mmap(2) is 
> > direct to userspace, but read(2) and write(2) must copy_from_user() into 
> > the allocated DMA area.
> 
> This is burned into silicon, so supporting it's not an option. Frankly
> I think what's best is another device interface for userspace to fall
> back to when this coherent userspace mmap() is unimplementable, e.g.
> read()/write() on some device node.

Exactly. We can implement the simple/nice interface discussed here, and
just not support it on those platforms, they'll have to fall back to
read/write or simply not support those drivers who require that
functionality.

Eventually a nopage variant may be worth for things doing really
large mappings, but I tend to think that when we need to do that mapping
to userland, it is because we need short latencies, which is the opposite
of what a nopage implementation provides, dunno if it's worth the pain
(though it's not _that_ painful).

Ben.