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    SubjectRe: AVR32 architecture patch against Linux 2.6.18-rc1 available
    Haavard Skinnemoen wrote:
    > Hi everyone,
    >
    > I've put up an updated set of patches for AVR32 support at
    > http://avr32linux.org/twiki/bin/view/Main/LinuxPatches
    >
    > The most interesting patch probably is
    > http://avr32linux.org/twiki/pub/Main/LinuxPatches/avr32-arch-2.patch
    >
    > which, at 544K, is too large to attach here. Please let me know if you
    > want me to do it anyway.
    >
    > Anyone want to have a look at this? I understand that a full review is
    > a huge job, but I'd appreciate a pointer or two in the general
    > direction that I need to take this in order to get it acceptable for
    > mainline.
    >

    Hi,

    +void cpu_idle(void)
    +{
    + /* endless idle loop with no priority at all */
    + while (1) {
    + /* TODO: Enter sleep mode */
    + if (need_resched())
    + schedule();
    + }
    +}

    AFAIKS, this is buggy.

    need_resched() translates to a test_bit, which doesn't have any barriers,
    so it could be optimised away completely. And if you're intending to use
    preempt, you need to have preemption disabled in the idle loop.

    Documentation/sched-arch.txt attempts to explain, and something like
    arm26's cpu_idle() is a nice, simple example to follow.

    Actually, I'm wrong about the test_bit. It casts to volatile there, which
    is probably why you don't end up with infinite loops. Still, it would be
    nicer to have an explicit barrier (eg. cpu_relax()).

    Why do we cast to volatile in places like this? Linus? I don't see why
    test_bit() should be any more "special" than the & operator. What's more,
    some architectures do cast and others don't, which is just insane.

    --
    SUSE Labs, Novell Inc.
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