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    SubjectRe: ARM unaligned MMIO access with attribute((packed))
    Date
    David Miller <davem@davemloft.net> writes:

    > From: Russell King - ARM Linux <linux@arm.linux.org.uk>
    > Date: Wed, 2 Feb 2011 16:37:02 +0000
    >
    >> 1. there's no way to tell GCC that the inline assembly is a load
    >> instruction and therefore it needs to schedule the following
    >> instructions appropriately.
    >
    > Just add a dummy '"m" (pointer)' asm input argument to the inline asm
    > statement. Just make sure "typeof(pointer)" has a size matching the
    > size of the load your are performing.

    That should be "m"(*pointer).

    >> 2. GCC will needlessly reload pointers from structures and other such
    >> behaviour because it can't be told clearly what the inline assembly
    >> is doing, so the inline asm needs to have a "memory" clobber.
    >
    > This behavior is correct, and in fact needed. Writing to chip registers
    > can trigger changes to arbitrary main memory locations.
    >
    >> 3. It seems to misses out using the pre-index addressing, prefering to
    >> create add/sub instructions prior to each inline assembly load/store.
    >
    > Yes, this is indeed a problem.

    GCC has trouble doing anything more complicated than simple indexing.
    Load/store instructions with writeback seem not to be in its
    vocabulary at all.

    > But you really need that memory clobber there whether you like it or
    > not, see above.

    I don't know of any device where the side-effects are not explicitly
    indicated by other means in the code triggering them, so it probably
    is safe without the clobber as Russel says.

    --
    Måns Rullgård
    mans@mansr.com

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