x86/mm, asm-generic: Add IOMMU ioremap_uc() variant default

We currently have no safe way of currently defining architecture
agnostic IOMMU ioremap_*() variants. The trend is for folks to
*assume* that ioremap_nocache() should be the default everywhere
and then add this mapping on each architectures -- this is not
correct today for a variety of reasons.

We have two options:

  1) Sit and wait for every architecture in Linux to get a
     an ioremap_*() variant defined before including it upstream.

  2) Gather consensus on a safe architecture agnostic ioremap_*()
     default.

Approach 1) introduces development latencies, and since 2) will
take time and work on clarifying semantics the only remaining
sensible thing to do to avoid issues is returning NULL on
ioremap_*() variants.

In order for this to work we must have all architectures declare
their own ioremap_*() variants as defined. This will take some
work, do this for ioremp_uc() to set the example as its only
currently implemented on x86. Document all this.

We only provide implementation support for ioremap_uc() as the
other ioremap_*() variants are well defined all over the kernel
for other architectures already.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: benh@kernel.crashing.org
Cc: bp@suse.de
Cc: dan.j.williams@intel.com
Cc: geert@linux-m68k.org
Cc: hch@lst.de
Cc: hmh@hmh.eng.br
Cc: jgross@suse.com
Cc: linux-mm@kvack.org
Cc: luto@amacapital.net
Cc: mpe@ellerman.id.au
Cc: mst@redhat.com
Cc: ralf@linux-mips.org
Cc: ross.zwisler@linux.intel.com
Cc: stefan.bader@canonical.com
Cc: tj@kernel.org
Cc: tomi.valkeinen@ti.com
Cc: toshi.kani@hp.com
Link: http://lkml.kernel.org/r/1436488096-3165-1-git-send-email-mcgrof@do-not-panic.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/io.h

@@ -180,6 +180,8 @@ static inline unsigned int isa_virt_to_bus(volatile void *address)
  */
 extern void __iomem *ioremap_nocache(resource_size_t offset, unsigned long size);
 extern void __iomem *ioremap_uc(resource_size_t offset, unsigned long size);
+#define ioremap_uc ioremap_uc
+
 extern void __iomem *ioremap_cache(resource_size_t offset, unsigned long size);
 extern void __iomem *ioremap_prot(resource_size_t offset, unsigned long size,
 				unsigned long prot_val);

include/asm-generic/io.h

@@ -736,6 +736,35 @@ static inline void *phys_to_virt(unsigned long address)
 }
 #endif
 
+/**
+ * DOC: ioremap() and ioremap_*() variants
+ *
+ * If you have an IOMMU your architecture is expected to have both ioremap()
+ * and iounmap() implemented otherwise the asm-generic helpers will provide a
+ * direct mapping.
+ *
+ * There are ioremap_*() call variants, if you have no IOMMU we naturally will
+ * default to direct mapping for all of them, you can override these defaults.
+ * If you have an IOMMU you are highly encouraged to provide your own
+ * ioremap variant implementation as there currently is no safe architecture
+ * agnostic default. To avoid possible improper behaviour default asm-generic
+ * ioremap_*() variants all return NULL when an IOMMU is available. If you've
+ * defined your own ioremap_*() variant you must then declare your own
+ * ioremap_*() variant as defined to itself to avoid the default NULL return.
+ */
+
+#ifdef CONFIG_MMU
+
+#ifndef ioremap_uc
+#define ioremap_uc ioremap_uc
+static inline void __iomem *ioremap_uc(phys_addr_t offset, size_t size)
+{
+	return NULL;
+}
+#endif
+
+#else /* !CONFIG_MMU */
+
 /*
  * Change "struct page" to physical address.
  *
@@ -743,7 +772,6 @@ static inline void *phys_to_virt(unsigned long address)
  * you'll need to provide your own definitions.
  */
 
-#ifndef CONFIG_MMU
 #ifndef ioremap
 #define ioremap ioremap
 static inline void __iomem *ioremap(phys_addr_t offset, size_t size)