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

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Date:	Sat, 9 Oct 2010 00:44:48 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Greg KH <greg@...ah.com>
Cc:	Felipe Contreras <felipe.contreras@...il.com>,
	linux-main <linux-kernel@...r.kernel.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Arnd Hannemann <arnd@...dnet.de>,
	Han Jonghun <jonghun79.han@...il.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, Hemant Pedanekar <hemantp@...com>
Subject: Re: [PATCH] ARM: allow, but warn, when issuing ioremap() on RAM

On Fri, Oct 08, 2010 at 04:25:39PM -0700, Greg KH wrote:
> I doubt that people even noticed that this was a problem.

So what you're telling me is that ARM SoC people don't read the
ARM kernel mailing list?  I might as well stop posting patches
if no one's paying attention then! :-P

Reality is that it was discussed, and I did propose solutions at
the time.

> Unless you throw a run-time warning at them, or even better, break the
> build of their drivers, they will not notice.
> 
> Also, how can we fix this in a driver, what is the proper steps to do
> so?

On April 23rd:
| I think a viable safe solution is to set aside some RAM at boot (which
| the kernel doesn't manage at all) and then use ioremap on that; that
| approach will still work with this patch in place.

On April 30th when discussing the omapfb driver:
| There's really one option for this - in the machine's fixup handler, you
| can walk the meminfo array and kick out some memory from that.  This will
| prevent the kernel mapping that memory and make pfn_valid() fail for that
| memory range.  Another advantage of this is that it won't break when we
| switch to LMB.

So, as I say, there's been six months.  It was discussed.  But still
we find that the drivers haven't been touched and now we're complaining
that drivers are breaking.

If it hadn't been discussed, if solutions hadn't been proposed, then
yes, it would be right to revert it out-right.  But that's not what
happened.

If we have to have another three months (or so), this time with a warning,
then so be it, but let's make it plainly clear that it _will_ _definitely_
be changing, and that drivers _will_ break unless they are fixed.

Unfortunately, what I fear is that nothing will happen because people
want the ioremap-on-system-RAM to just work, and then we'll hit this
exact same issue again in three months time.
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