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>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index 83ec9b1d77cc..de25aad07853 100644
--- a/arch/x86/include/asm/io.h
+++ b/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);