Re: tty loop-back device

["Matwey V. Kornilov" <matwey.kornilov@gmail.com>]

29.09.2013 07:21, Grant Edwards пишет:
> I was thinking about initially just creating a blocking ioctl() call
> that the master can use to wait for either a termios configuration
> change or a modem control line change.  I admit that's not as elegent:
> it would mean a master would need multiple threads in most cases (one
> for data and one for config and modem status).  But, it may be a less
> intrusive change [I haven't looked at the poll implementation in any
> detail, so perhaps the poll change isn't as bad as I fear.]

The simplier way is to use master in 'packet mode' (see TIOCPKT). Then 
we can distinguish stream data from termios notification for every 
master's read. It either starts with '\0' and the following is the data 
to transmit or with control symbol and then master have to use ioctl to 
read new tty->termios.

> Having to use multiple threads in a master sounds ugly at first, but
> it's still orders of magnitude easier than implementing a serial
> driver in kernel space that uses network I/O to communicate with the
> physical UART.

For sure.

We need way more ioctls. Since every new slave pty is enumerated almost 
randomly (depends on how many sessions is opened, opening order, etc), 
we have to mark somehow the pty when the daemon creates it, then udev 
should be able to create appropriate symbolic link for client user-space 
application. Then user-space client may be configured to work with 
/dev/tty/by-name/uniqname for instance.

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