From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
Date: Tue, 10 Jan 2006 00:44:58 +0000
Subject: Re: [Alsa-devel] Re: [OT] ALSA userspace API complexity
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To: David Lang <dlang@digitalinsight.com>
Cc: John Rigg <ad@sound-man.co.uk>, =?ISO-8859-1?Q?Ren=E9?= Rebe <rene@exactcode.de>, Hannu Savolainen <hannu@opensound.com>, Jaroslav Kysela <perex@suse.cz>, Takashi Iwai <tiwai@suse.de>, linux-sound@vger.kernel.org, ALSA development <alsa-devel@alsa-project.org>, LKML <linux-kernel@vger.kernel.org>

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.

Alan