Re: Char devices drivers

David Lawyer (
Thu, 28 Mar 2002 13:39:43 -0800

On Mon, Mar 11, 2002 at 09:59:14AM -0800, Ed Vance wrote:
> On Mon, Mar 11, 2002, Henrique Gobbi wrote:
> >
> > Can anyone explain what is the utility of the callout devices
> > in the char drivers ???
> To further confuse things, most vendors have a "back door" port
> configuration mechanism outside of termio(s)/sgtty. For example, Linux has
> the setserial command. There is less need for separate callout devices when
> the user can set a port to open with the desired flag values.

Setserial doesn't have facilities to set open flags for the ports. The
reason cua isn't needed is that the programmer can make a ttyS port look
like a cua port by using the O_NONBLOCK or O_NDELAY flags. However the
non-blocking flag will not only force an open when CD is asserted, but
will make all reads of the port non-blocking. So the programmer can
change this if he wants after the port has opened.

So eliminating cua means more work for the programmer but less confusion
for users. Overall, it's a good thing since there are many more users
than programmers.

David Lawyer
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at