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Commit 58340a07 authored by Johannes Berg's avatar Johannes Berg Committed by Linus Torvalds
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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: default avatarJohannes Berg <johannes@sipsolutions.net>
Acked-by: default avatarSam Ravnborg <sam@ravnborg.org>
Acked-by: Ingo Molnar <mingo@elte.hu> [x86 architecture part]
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e0ce0da9

Documentation/unaligned-memory-access.txt

+29 −3
Original line number Diff line number Diff line
@@ -218,9 +218,35 @@ If use of such macros is not convenient, another option is to use memcpy(), 
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 <dsd@gentoo.org>

Authors: Daniel Drake <dsd@gentoo.org>,

         Johannes Berg <johannes@sipsolutions.net>

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

+19 −0
Original line number Diff line number Diff line
@@ -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

arch/powerpc/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -112,6 +112,7 @@ config PPC 
	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

+1 −0
Original line number Diff line number Diff line
@@ -28,6 +28,7 @@ config X86 
	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