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

[Nicolas Pitre]

On Sat, 9 Oct 2010, Russell King - ARM Linux wrote:

> On Fri, Oct 08, 2010 at 05:00:46PM -0700, Greg KH wrote:
> > But you can't expect that you make this change, and not fix up the
> > drivers, and people would be happy, right?  The rule for API changes
> > like this, or anything, is that the person making the change fixes the
> > other drivers, and that seems to be the issue here.
> 
> Let's entirely revert the change and wait for people's data to be
> corrupted then.  I don't have the time nor the motivation to work
> through crap driver code to fix up these unreliable games which are
> already illegal on platforms such as x86.
> 
> If people want their system to be unpredictable, then let's carry on
> giving them the rope to hang themselves in that manner.
> 
> > Any pointers to patches where people have fixed up the drivers?
> 
> Despite the discussion, I'm unaware of anyone really taking the issue
> seriously and producing any patches during the last six months.
> 
> So, I say sod it, let's revert the change.

I think that the real issue here is to avoid breaking those drivers 
which were using this legitimately.  So what about this compromize 
instead:

diff --git a/arch/arm/mm/ioremap.c b/arch/arm/mm/ioremap.c
index 99627d3..4f071e4 100644
--- a/arch/arm/mm/ioremap.c
+++ b/arch/arm/mm/ioremap.c
@@ -201,6 +201,15 @@ void __iomem * __arm_ioremap_pfn_caller(unsigned long pfn,
 	if (pfn >= 0x100000 && (__pfn_to_phys(pfn) & ~SUPERSECTION_MASK))
 		return NULL;
 
+	/*
+	 * Warn if RAM is mapped to discourage this usage. Let's forbid it
+	 * outright on ARMv6+ where this became architecturally undefined
+	 * in theory and causes memory corruption in practice.
+	 */
+	if (WARN_ON(pfn_valid(pfn)))
+		if (__LINUX_ARM_ARCH__ >= 6)
+			return NULL;
+
 	type = get_mem_type(mtype);
 	if (!type)
 		return NULL;

Nicolas