Re: CLOCAL and TIOCMIWAIT

["Paul Fulghum" <paulkf@microgate.com>]

> 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
paulkf@microgate.com