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Subject: Re: [Alsa-devel] Re: [OT] ALSA userspace API complexity
From: Lee Revell <rlrevell@joe-job.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: David Lang <dlang@digitalinsight.com>, John Rigg <ad@sound-man.co.uk>,
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Date: Mon, 09 Jan 2006 20:56:21 -0500
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On Tue, 2006-01-10 at 00:44 +0000, Alan Cox wrote:
> On Llu, 2006-01-09 at 16:29 -0800, David Lang wrote:
> > I was under the (apparently mistaken) impression that you couldn't DMA 
> > from userspace (something to do with the possibility that the userspace 
> > memory pages could be swapped out in the middle of the DMA)
> 
> Drivers can choose to support this two different ways. One is to have a
> buffer of kernel memory mapped into user space and shared with the
> hardware (this is how OSS did it), the other is to use the 2.6
> get_user_pages API to get the physical address of a set of pages and
> lock them down so they don't wander off during DMA.
> 
> Both have advantages for different uses.

ALSA would appear to use the first method (get_user_pages does not
appear in the source), presumably because new ALSA versions still
support 2.4 (and 2.2, maybe even 2.0).

Lee