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    Subject[PATCH/RFC v4] introduce HAVE_UNALIGNED_ACCESS_SUPPORT 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. 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@sipsolutions.net>
    Acked-by: Sam Ravnborg <sam@ravnborg.org>
    ---
    Didn't I say I was going to fix the subject? Sorry.

    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@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

    --- 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



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