linux-kernel - Re: [PATCH] ARM: allow, but warn, when issuing ioremap() on RAM

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Date:	Mon, 11 Oct 2010 10:26:35 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Felipe Contreras <felipe.contreras@...il.com>,
	Arnd Hannemann <arnd@...dnet.de>,
	Hemant Pedanekar <hemantp@...com>,
	linux-main <linux-kernel@...r.kernel.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Han Jonghun <jonghun79.han@...il.com>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: allow, but warn, when issuing ioremap() on RAM

On Sat, 2010-10-09 at 17:45 +0100, Russell King - ARM Linux wrote:
> On Sat, Oct 09, 2010 at 07:07:01PM +0300, Felipe Contreras wrote:
> > On Sat, Oct 9, 2010 at 4:52 PM, Russell King - ARM Linux
> > <linux@....linux.org.uk> wrote:
> > > On Thu, Oct 07, 2010 at 12:44:22PM +0300, Felipe Contreras wrote:
> > >> For issues related to this:
> > >> http://article.gmane.org/gmane.linux.ports.arm.kernel/84454
> > >
> > > This one nicely shows some of the problems which can occur with the
> > > memory type attributes - and this is not attributable to ioremap().
> > >
> > > ioremap() is used to map devices.  It creates device memory type mappings.
> > > If what you're mapping doesn't support device memory type mappings, then
> > > accesses via an ioremap()'d region isn't going to work - as this guy is
> > > observing.
> > >
> > > That's not because ioremap() is doing something wrong.  It's doing what
> > > it's meant to do.  The use is wrong, and is completely unrelated to the
> > > issue you've raised.
> >
> > Ok, I was confused by Catalin's comment which does point to ioremap()
> > on normal RAM:
> > http://article.gmane.org/gmane.linux.ports.arm.kernel/84504
> 
> Me too - it doesn't appear to relate to the specified problem.  You
> don't want to map RAM as device nor strongly ordered, and we still
> don't know what this "MMR" is.

I don't know what MMR is either. But my comment is misleading, I think I
was thinking about dma_alloc_coherent().

We could to allow ioremap_wc() or ioremap_cached() to normal RAM but
that's probably outside the scope of ioremap* (and it would require
initial cache maintenance to remove potential dirty cache lines).

-- 
Catalin

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