introduce HAVE_EFFICIENT_UNALIGNED_ACCESS Kconfig symbol

In many cases, especially in networking, it can be beneficial to know at
compile time whether the architecture can do unaligned accesses efficiently.
This patch introduces a new Kconfig symbol

	HAVE_EFFICIENT_UNALIGNED_ACCESS

for that purpose and adds it to the powerpc and x86 architectures.  Also add
some documentation about alignment and networking, and especially one intended
use of this symbol.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Ingo Molnar <mingo@elte.hu> [x86 architecture part]
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index 6093c0be58b0..b0fabfa864ff 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -27,6 +27,25 @@ config KPROBES
 	  for kernel debugging, non-intrusive instrumentation and testing.
 	  If in doubt, say "N".
 
+config HAVE_EFFICIENT_UNALIGNED_ACCESS
+	def_bool n
+	help
+	  Some architectures are unable to perform unaligned accesses
+	  without the use of get_unaligned/put_unaligned. Others are
+	  unable to perform such accesses efficiently (e.g. trap on
+	  unaligned access and require fixing it up in the exception
+	  handler.)
+
+	  This symbol should be selected by an architecture if it can
+	  perform unaligned accesses efficiently to allow different
+	  code paths to be selected for these cases. Some network
+	  drivers, for example, could opt to not fix up alignment
+	  problems with received packets if doing so would not help
+	  much.
+
+	  See Documentation/unaligned-memory-access.txt for more
+	  information on the topic of unaligned memory accesses.
+
 config KRETPROBES
 	def_bool y
 	depends on KPROBES && HAVE_KRETPROBES