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    Subject[PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR
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    armv7's flush_cache_all() flushes caches via set/way. To
    determine the cache attributes (line size, number of sets,
    etc.) the assembly first writes the CSSELR register to select a
    cache level and then reads the CCSIDR register. The CSSELR register
    is banked per-cpu and is used to determine which cache level CCSIDR
    reads. If the task is migrated between when the CSSELR is written and
    the CCSIDR is read the CCSIDR value may be for an unexpected cache
    level (for example L1 instead of L2) and incorrect cache flushing
    could occur.

    Disable interrupts across the write and read so that the correct
    cache attributes are read and used for the cache flushing
    routine. We disable interrupts instead of disabling preemption
    because the critical section is only 3 instructions and we want
    to call v7_dcache_flush_all from __v7_setup which doesn't have a
    full kernel stack with a struct thread_info.

    This fixes a problem we see in scm_call() when flush_cache_all()
    is called from preemptible context and sometimes the L2 cache is
    not properly flushed out.

    Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Nicolas Pitre <nico@fluxnic.net>
    ---

    On 02/02/12 17:18, Nicolas Pitre wrote:
    >>> If that's too much, then the simple method in assembly to quickly disable
    >>> preemption over a very few set of instructions is using mrs/msr and cpsid i.
    >>> That'll be far cheaper than fiddling about with preempt counters or
    >>> messing about with veneers in C code.
    >>
    >> I'll try the macros. So far it isn't bad, just the __v7_setup to resolve.
    >
    > If you simply disable/restore IRQs around the critical region then you
    > don't have to worry about __v7_setup. Plus this will allow for
    > v7_flush_dcache_all to still be callable from atomic context.

    Ok. Here's a patch. I still need to test it. I'll send another patch
    series to cleanup the get_thread_info stuff (there's two of them?).

    arch/arm/mm/cache-v7.S | 6 ++++++
    1 files changed, 6 insertions(+), 0 deletions(-)

    diff --git a/arch/arm/mm/cache-v7.S b/arch/arm/mm/cache-v7.S
    index 07c4bc8..654a5fc 100644
    --- a/arch/arm/mm/cache-v7.S
    +++ b/arch/arm/mm/cache-v7.S
    @@ -54,9 +54,15 @@ loop1:
    and r1, r1, #7 @ mask of the bits for current cache only
    cmp r1, #2 @ see what cache we have at this level
    blt skip @ skip if no cache, or just i-cache
    +#ifdef CONFIG_PREEMPT
    + save_and_disable_irqs r9 @ make cssr&csidr read atomic
    +#endif
    mcr p15, 2, r10, c0, c0, 0 @ select current cache level in cssr
    isb @ isb to sych the new cssr&csidr
    mrc p15, 1, r1, c0, c0, 0 @ read the new csidr
    +#ifdef CONFIG_PREEMPT
    + restore_irqs r9
    +#endif
    and r2, r1, #7 @ extract the length of the cache lines
    add r2, r2, #4 @ add 4 (line length offset)
    ldr r4, =0x3ff
    --
    Sent by an employee of the Qualcomm Innovation Center, Inc.
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.


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