From 58340a07c194e0aed7bc58b61ff24330bb2a409f Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Fri, 25 Jul 2008 01:45:33 -0700 Subject: 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 Acked-by: Sam Ravnborg Acked-by: Ingo Molnar [x86 architecture part] Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/unaligned-memory-access.txt | 32 ++++++++++++++++++++++++++++--- 1 file changed, 29 insertions(+), 3 deletions(-) (limited to 'Documentation/unaligned-memory-access.txt') diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt index b0472ac..f866c72 100644 --- a/Documentation/unaligned-memory-access.txt +++ b/Documentation/unaligned-memory-access.txt @@ -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 +Authors: Daniel Drake , + Johannes Berg 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 -- cgit v1.1