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    SubjectRe: Serial related oops
    (trimmed tie-fei.zang from the CC, added by mistake)
    On Mon, Feb 19, 2007 at 02:35:20PM +0000, Russell King wrote:
    > > Neither did I, but introducing printk's through the function, we narrowed
    > > the problem to this part of the code. And removing it makes the problem
    > > go away. We inserted 37 printk's in the function body, and Jose bisected
    > > those until the problem went away.
    >
    > Well, there's still little clue about why this is causing a NULL pointer
    > dereference. The only thing I can think is that somehow performing
    > this test is causing a power glitch to your CPU, causing its registers
    > to get corrupted, and which results in it doing a NULL pointer deref.
    That may be the case, indeed.
    >
    > Are you saying that the NULL pointer occurred while executing this code?
    > If not, where does the NULL pointer occur?
    The thing is, the NULL pointer deref dissapeared as soon as we
    instrumented (printk'ed) the code. So it's seems to be triggered by
    check+timing+hardware.
    >
    > > > No, it's only runtime because you can't tell which ports might be
    > > > affected, and you might have a mixture of ports which are affected
    > > > and those which aren't.
    > > Hmm, ok. And what about a CONFIG_I_KNOW_MY_SERIAL_IS_BROKEN option?
    >
    > Andrew's said no (in that the thread you refer to) and suggested an
    > alternative, I've said no, how many more 'no's do you need to turn
    > you away from the wrong approach?
    One is usually sufficient once I've understood :). I missed the module
    option approach. Is it ok with you? If yes, I'll put up a patch to do
    this.
    >
    > > > > PS: CCing Andrew and Zang Roy-r61911 as they seemed to discuss this in
    > > > > http://lkml.org/lkml/2006/6/13/21
    > > >
    > > > I don't see any reference to this problem there.
    > >
    > > Sorry, I suck, I got that mixed with that one:
    > > http://lkml.org/lkml/2006/12/26/63
    > > "probing for UART_BUG_TXEN in 8250 driver leads to weird effects on some
    > > ARM boards"
    >
    > The "weird effects" were never quantified, so that's one of the reasons
    > I ignored that report (another being is that I stopped being the serial
    > maintainer a while ago, and now serial is maintainerless.)
    >
    The problem appears to be reproducible on Jose's hardware within 2-3 days.
    If you see other tests to be performed...

    Regards,
    Frederik
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