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Date: Tue, 10 Jan 2012 14:23:42 -0800 (PST)
From: Matthew Jacob <mj@feral.com>
To: Andriy Gapon <avg@freebsd.org>
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Cc: FreeBSD current <freebsd-current@freebsd.org>, mj@feral.com
Subject: Re: bus dma: a flag/quirk for page zero
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>> At the very least, require bounce buffers.
>
> Not sure if I got this suggestion in this terse form.
> Could you please explain?

Physical address zero can be DMA'd, but via bounce buffers.
bcopy from address zero up through a pagesize to a bounce buffer, do the 
dma from there (read case), write case the opposite order