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    SubjectRe: tty TTY_HUPPED anomaly
    On 01/04/2012 11:27 AM, Alan Cox wrote:
    >> But what has carrier dropping got to do with an TIOCSETD ioctl. For that
    >
    > When the carrier is dropped and HUPCL is set then the tty is disconnected
    > from the physical interface. It's specified behaviour and required for
    > security. So by the time you go to issue the TIOCSETD you are no longer
    > connected to the tty. That may well just be a timing change.
    >
    >> What can be done to prevent tty_hangup from being called after opening
    >> the port? And if this is really supposed to happen, why does it not
    >> always happen?
    >
    > It should only happen if the carrier is dropped.
    >
    >> Even if the first thing I do after opening the port is to clear HUPCL
    >> and set CLOCAL, this still randomly happens the first time I open the
    >> port after booting.
    >
    > I'd expect the behaviour to either be
    >
    > carrier high, stays high - open works, no hangup events seen
    >
    > or
    >
    > carrier low, stays low - open blocks, but open with O_NDELAY works,
    > hangup events not seen.
    >
    > It's the act of the drop which is a hangup not the presence of low
    > carrier if I remember the spec properly. The Synclink GT correctly does
    > this as far as I can tell (I have no hardware or docs for it) but the
    > code indicates that the hardware reports changes and it acts on them
    > properly (checking CLOCAL etc).
    >
    >
    > I would guess (given the distro change is the trigger) that you've got a
    > SuSE problem not a kernel one. The kernel behaviour and code looks
    > correct. My guess therefore is that newer SuSE is running stuff in the
    > boot which is probing serial ports and messing with the carrier wrongly
    > and in ways it didn't use to. That would fit the fact that something
    > similarly broken has apparently also appeared in the Fedora user space
    > bootup.
    >

    Yes, I never really though it was a kernel problem. All though I can't
    say I agree with the HUPCL/security thing. In any case, with the
    Synclink cards, at first open after boot of SuSE-12.1, CLOCAL is not
    set. Whereas all pre SuSE-12.1 releases do have CLOCAL set.


    Mark



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