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 <[e-mail]>
Acked-by: Sam Ravnborg <[e-mail]>
Acked-by: Ingo Molnar <[e-mail]> [x86 architecture part]
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/unaligned-memory-access.txt

···
 where the source or destination (or both) are of type u8* or unsigned char*.
 Due to the byte-wise nature of this operation, unaligned accesses are avoided.
 
+
+Alignment vs. Networking
+========================
+
+On architectures that require aligned loads, networking requires that the IP
+header is aligned on a four-byte boundary to optimise the IP stack. For
+regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
+architectures this constant has the value 2 because the normal ethernet
+header is 14 bytes long, so in order to get proper alignment one needs to
+DMA to an address which can be expressed as 4*n + 2. One notable exception
+here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
+addresses can be very expensive and dwarf the cost of unaligned loads.
+
+For some ethernet hardware that cannot DMA to unaligned addresses like
+4*n+2 or non-ethernet hardware, this can be a problem, and it is then
+required to copy the incoming frame into an aligned buffer. Because this is
+unnecessary on architectures that can do unaligned accesses, the code can be
+made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:
+
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+	skb = original skb
+#else
+	skb = copy skb
+#endif
+
 --
-Author: Daniel Drake <[e-mail]>
+Authors: Daniel Drake <[e-mail]>,
+         Johannes Berg <[e-mail]>
 With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
-Johannes Berg, Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock,
-Uli Kunitz, Vadim Lobanov
+Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
+Vadim Lobanov
 

arch/Kconfig

···
 	  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

arch/powerpc/Kconfig

···
 	select HAVE_FTRACE
 	select HAVE_IDE
 	select HAVE_IOREMAP_PROT
+	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_KPROBES
 	select HAVE_ARCH_KGDB
 	select HAVE_KRETPROBES

arch/x86/Kconfig

···
 	select HAVE_FTRACE
 	select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
 	select HAVE_ARCH_KGDB if !X86_VOYAGER
+	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 
 config ARCH_DEFCONFIG
 	string