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    SubjectRe: Different behaviour when using "nosmp" parameter on SMP and UP
    2011/12/1 Kumar Gala <>:
    > On Dec 1, 2011, at 3:57 AM, Jean-Michel Hautbois wrote:
    >> Hi,
    >> I have a P2020 CPU (powerpc) and I compiled it with two different defconfigs.
    >> The first one is a SMP, 2 cores, launched with the "nosmp" kernel
    >> parameter, the other one is an UP kernel.
    >> My driver behaviour is not the same whether launching one or the
    >> other. It is hard to explain more precisely, as it deals only with
    >> ioctl which only does ioread32/iowrite32 on a PCIe device.
    >> But I can tell that it never works the same way when UP (not working
    >> correctly), or SMP "nosmp" (working) or even SMP (not working).
    >> AFAIK, the "nosmp" parameter should tell the kernel to act the same is
    >> if it is an UP kernel, and it disables IO APIC, which is not an issue
    >> in my case.
    >> Can you think about anything that would explain it, or would help me
    >> debugging it ?
    > This is a bit odd, hard to say w/o more details on what exactly your code is trying to do and wait the failure is.

    I understand that it is difficult to give more details, but the driver
    is not mine, and I can't give all the code associated.

    > The larger differences between SMP & UP build would be setting of memory attribute for cache coherency.  In UP case we don't set it.
    > Between SMP 'nosmp' and SMP 'on' cases you seem like you're hitting some locking condition.
    > Guessing your running into a timing & locking issue that you're getting lucky on the SMP 'nosmp' case such that it just happens to work.

    Any idea on how to debug this ? I am using a 2.6.35 kernel.

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