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introduce HAVE_EFFICIENT_UNALIGNED_ACCESS Kconfig symbol

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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>
This commit is contained in:
Johannes Berg
2008-07-25 01:45:33 -07:00
committed by Linus Torvalds
parent e0ce0da9fe
commit 58340a07c1
4 changed files with 52 additions and 5 deletions
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36
Documentation/unaligned-memory-access.txt
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@@ -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.
--
Author: Daniel Drake <dsd@gentoo.org>
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
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
--
Authors: Daniel Drake <dsd@gentoo.org>,
Johannes Berg <johannes@sipsolutions.net>
With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
Vadim Lobanov