From: Paul Fulghum (
Date: Mon Feb 26 2001 - 20:27:54 EST

> A customer has just brought to my attention that when you try to use the
> TIOCMIWAIT ioctl with our boards and CLOCAL is enabled, you can't check
> changes in the DCD signal. He also mentioned that that is possible with
> the regular serial ports.
> As I understood, CLOCAL meant disabling DCD sensitivity, so if CLOCAL is
> disabled, no changes in DCD will be passed from hardware driver to the
> kernel or userspace. The way the serial driver is implemented, this is not
> true (i.e. even with CLOCAL enabled, you can still see DCD changes through
> the TIOCMIWAIT command).
> My question is: what's the correct interpretation of CLOCAL?? If the
> serial driver's interpretation is the correct one, I'll be more than happy
> to change the Cyclades' driver to comply with that, I just want to make
> sure that this is the expected behavior before I patch the driver.
> Thanks in advance for your comments.
> Later,
> Ivan

I believe CLOCAL only governs how DCD is used (or ignored) when opening
a port (must be active to complete open) and maintaining a connection
(negation signals hangup).

So CLOCAL controls the driver's 'interpretation' of DCD but
TIOCMIWAIT monitors the signal transitions without regard to
a predefined interpretation (let's the application decide what
to do with DCD).

Paul Fulghum

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This archive was generated by hypermail 2b29 : Wed Feb 28 2001 - 21:00:12 EST