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author | 2008-07-25 01:45:33 -0700 | |
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committer | 2008-07-25 10:53:27 -0700 | |
commit | 58340a07c194e0aed7bc58b61ff24330bb2a409f (patch) | |
tree | 907a53c71b3092e3a3a95c6641d4839e20214efd | |
parent | lists: remove a redundant conditional definition of list_add() (diff) | |
download | linux-58340a07c194e0aed7bc58b61ff24330bb2a409f.tar.gz linux-58340a07c194e0aed7bc58b61ff24330bb2a409f.tar.bz2 linux-58340a07c194e0aed7bc58b61ff24330bb2a409f.zip |
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>
-rw-r--r-- | Documentation/unaligned-memory-access.txt | 32 | ||||
-rw-r--r-- | arch/Kconfig | 19 | ||||
-rw-r--r-- | arch/powerpc/Kconfig | 1 | ||||
-rw-r--r-- | arch/x86/Kconfig | 1 |
4 files changed, 50 insertions, 3 deletions
diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt index b0472ac5226a..f866c72291bf 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 <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 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 diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig index a487671c282f..de6b49cd6be8 100644 --- a/arch/powerpc/Kconfig +++ b/arch/powerpc/Kconfig @@ -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 diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index b2ddfcf01728..66f3ab05b18c 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -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 |