Subject: introduce HAVE_UNALIGNED_ACCESS_SUPPORT Kconfig symbol In many cases, especially in networking, it can be beneficial to know at compile time whether the architecture can do unaligned accesses. This patch introduces a new Kconfig symbol HAVE_UNALIGNED_ACCESS_SUPPORT 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@xxxxxxxxxxxxxxxx> --- Erm. I should update the patch description and title too... Documentation/unaligned-memory-access.txt | 32 +++++++++++++++++++++++++++--- arch/Kconfig | 3 ++ arch/powerpc/Kconfig | 1 arch/x86/Kconfig | 1 4 files changed, 34 insertions(+), 3 deletions(-) --- everything.orig/Documentation/unaligned-memory-access.txt 2008-03-20 15:30:38.000000000 +0100 +++ everything/Documentation/unaligned-memory-access.txt 2008-03-20 19:38:30.000000000 +0100 @@ -218,9 +218,35 @@ If use of such macros is not convenient, 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 that is 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 depend on CONFIG_HAVE_UNALIGNED_ACCESS_SUPPORT like so: + +#ifdef CONFIG_HAVE_UNALIGNED_ACCESS_SUPPORT + skb = original skb +#else + skb = copy skb +#endif + -- -Author: Daniel Drake <dsd@xxxxxxxxxx> +Authors: Daniel Drake <dsd@xxxxxxxxxx>, + Johannes Berg <johannes@xxxxxxxxxxxxxxxx> 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 --- everything.orig/arch/powerpc/Kconfig 2008-03-20 15:30:38.000000000 +0100 +++ everything/arch/powerpc/Kconfig 2008-03-20 19:37:22.000000000 +0100 @@ -91,6 +91,7 @@ config PPC select HAVE_OPROFILE select HAVE_KPROBES select HAVE_KRETPROBES + select HAVE_UNALIGNED_ACCESS_SUPPORT config EARLY_PRINTK bool --- everything.orig/arch/x86/Kconfig 2008-03-20 15:30:38.000000000 +0100 +++ everything/arch/x86/Kconfig 2008-03-20 19:38:08.000000000 +0100 @@ -23,6 +23,7 @@ config X86 select HAVE_KPROBES select HAVE_KRETPROBES select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64) + select HAVE_UNALIGNED_ACCESS_SUPPORT config GENERIC_LOCKBREAK --- everything.orig/arch/Kconfig 2008-03-20 19:37:26.000000000 +0100 +++ everything/arch/Kconfig 2008-03-20 19:37:34.000000000 +0100 @@ -36,3 +36,6 @@ config HAVE_KPROBES config HAVE_KRETPROBES def_bool n + +config HAVE_UNALIGNED_ACCESS_SUPPORT + def_bool n -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
