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

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Date:	Fri, 8 Oct 2010 16:19:26 -0700
From:	Greg KH <greg@...ah.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 06:53:08PM +0100, Russell King - ARM Linux wrote:
> On Fri, Oct 08, 2010 at 12:32:35PM +0300, Felipe Contreras wrote:
> > I think when _you_ remove functionality from the architecture, you
> > should provide a mechanism that drivers can migrate to. Since there's
> > nothing like that, not even a guideline, you are breaking the drivers
> > willingly, and expecting other people to fix a difficult problem that
> > you yourself have no idea how to fix properly.
> 
> We can either wait for people to complain about silent data corruption
> or we can be compliant with the architecture specification.  Which is
> better - to avoid data corruption and be correct, or allow a system to
> become flakey and corrupt people's data.
> 
> What I care about is system correctness and people's data - having
> multiple mappings with different attributes is documented in very clear
> terms as being 'unpredictable' and therefore it isn't permissible to
> allow the practice that worked with previous processors (inherently
> due to their cache architecture) to continue forward onto processors
> with a different cache architecture.
> 
> As already discussed, it's nigh on impossible to unmap the existing
> direct mapped region (read the previous discussions about why this is)
> - which is precisely why there is no direct alternative solution.

Wait, let me get this straight:
  - drivers used to work on 2.6.35
  - some ARM core code changed in .36-rc to fix this iomem problem that
    you found
  - no drivers are notified of the api change as it's a run-time change,
    so the build doesn't break.
  - drivers break when run as the api stops returning valid addresses
  - no known way is around to fix the broken drivers

Um, this doesn't sound like a valid thing to be doing, how do you expect
people to fix their code if they:
	- don't realize it as the api change doesn't break the build
	- there is no way to fix their code

This sounds like a huge regression that should be reverted, or am I
missing something here?

confused,

greg k-h
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