Q: how to control the TTY output queue in real time?

Hello.

I am writing an app that needs to control the serial xmit in real-time. What I need is a notification that the TTY output queue fillup (returned by TIOCOUTQ ioctl) have dropped below the specified value.
I haven't found anything that can help implementing this. If I can't get an async notification, the sync notification will do too, like, for instance, the tcdrain() call, but with the argument to specify the needed fillup, below which the function will return.
If there is nothing like this, then even the notification on every transmitted char will do.
But I've found nothing of the above. :(

Any suggestions how the real-time control can be implemented?

(please CC me the replies)

Not a supported feature basically.

Bascially even on the hardware that knows with this degree of granularity we don't propogate the information back in the manner you want.

I'm not sure its a total loss however. Currently all the code basically does stuff in the tx path or tx irq handler along the line of

if (bytes_left < constant)
write_wakeup

and I suspect if you made that adjustable and turned off the fifo and any other funnies you'd at least make it work for a sufficiently rigged demo.

We could in theory put it in the tty_port in future too if its general purpose useful.

Alan

Hi Alan, thanks, clear enough now. :)

You suggest to turn off the fifo, sounds worrysome, does this mean that tcdrain() and TIOCOUTQ do not account the fifo too?
If they do quite fine with the fifo, then maybe the new function will do too? Its basically a tcdrain(), just with the controllable watermark I guess.

I guess providing you account the fifo, and any hardware flow control it would work.