On Sun, 22 Feb 1998, Charles M. Hannum wrote:

> First, let's review how tty ioctl()s are handled.
> 
> We indirect through some magic (the nature of which is not relevant to
> the following discussion) to the device driver; e.g. comioctl().  In
> the device driver, we have a piece of code that calls the line
> discipline and the generic tty layer to handle `standard' ioctl()s.
> The way the line discpline and ttioctl() tell us to try processing an
> ioctl() through a different layer is by returning -1.  So the code
> currently looks something like:
> 
>         error = (*linesw[tp->t_line].l_ioctl)(tp, cmd, data, flag, p);
>         if (error >= 0)
>                 return (error);
> 
>         error = ttioctl(tp, cmd, data, flag, p);
>         if (error >= 0)
>                 return (error);
> 
> If both of these calls failed, then we fall through and handle our own
> specialized ioctl()s, or generic ones that have device-specific
> implemenations.  Something like this:
> 
> 	switch (cmd) {
> 	case TIOCMGET:
> 		...
> 	...
> 	}

[com driver ioctl problem]

> While this is certainly easy enough to fix, and it's arguable that
> this is a similar case to an uninitialized variable, I think it's kind
> of poor to use -1 (or, rather, ERESTART) in this case.  So, I propose
> creating another magic error value, which for lack of a better name I
> temporarily dub `ENOTHANDLED', for use in this situation.  Device
> driver ioctl() routines would then test the return value with `!=
> ENOTHANDLED' rather than `>= 0'.  In addition to making some potential
> lossage modes (like the one mentioned above) easier to diagnose, this
> would also allow ioctl()s that block to be properly interrupted and
> restarted, per SA_RESTART and all that jazz.  (I'm not currently aware
> of any case where that would be useful, but you never know...)
> 
> Any comments from the cashew gallery?

"These things are stale. Could we get some Honey-Roasted Peanuts?"

I think this is a good idea. As you note, it's a double win. We not only
can catch these problems more easily, and tty ioctls can return ERESTART.

Take care,

Bill