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freebsd-nq/share/man/man4/tty.4
Ian Lepore f64342e354 Rework tty_drain() to poll the hardware for completion, and restore 
drain timeout handling to historical freebsd behavior.

The primary reason for these changes is the need to have tty_drain() call
ttydevsw_busy() at some reasonable sub-second rate, to poll hardware that
doesn't signal an interrupt when the transmit shift register becomes empty
(which includes virtually all USB serial hardware).  Such hardware hangs
in a ttyout wait, because it never gets an opportunity to trigger a wakeup
from the sleep in tty_drain() by calling ttydisc_getc() again, after
handing the last of the buffered data to the hardware.

While researching the history of changes to tty_drain() I stumbled across
some email describing the historical BSD behavior of tcdrain() and close()
on serial ports, and the ability of comcontrol(1) to control timeout
behavior.  Using that and some advice from Bruce Evans as a guide, I've
put together these changes to implement the hardware polling and restore
the historical timeout behaviors...

 - tty_drain() now calls ttydevsw_busy() in a loop at 10 Hz to accomodate
   hardware that requires polling for busy state.

 - The "new historical" behavior for draining during close(2) is retained:
   the drain timeout is "1 second without making any progress".  When the
   1-second timeout expires, if the count of bytes remaining in the tty
   layer buffer is smaller than last time, the timeout is extended for
   another second.  Unfortunately, the same logic cannot be extended all
   the way down to the hardware, because the interface to that layer is a
   simple busy/not-busy indication.

 - Due to the previous point, an application that needs a guarantee that
   all data has been transmitted must use TIOCDRAIN/tcdrain(3) before
   calling close(2).

 - The historical behavior of honoring the drainwait setting for TIOCDRAIN
   (used by tcdrain(3)) is restored.

 - The historical kern.drainwait sysctl to control the global default
   drainwait time is restored, but is now named kern.tty_drainwait.

 - The historical default drainwait timeout of 300 seconds is restored.

 - Handling of TIOCGDRAINWAIT and TIOCSDRAINWAIT ioctls is restored
   (this also makes the comcontrol(1) drainwait verb work again).

 - Manpages are updated to document these behaviors.

Reviewed by:	bde (prior version)
2017-01-12 00:48:06 +00:00

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.\" Copyright (c) 1991, 1992, 1993
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.\" without specific prior written permission.
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)tty.4 8.3 (Berkeley) 4/19/94
.\" $FreeBSD$
.\"
.Dd January 11, 2017
.Dt TTY 4
.Os
.Sh NAME
.Nm tty
.Nd general terminal interface
.Sh SYNOPSIS
.In sys/ioctl.h
.Sh DESCRIPTION
This section describes the interface to the terminal drivers
in the system.
.Ss Terminal Special Files
Each hardware terminal port on the system usually has a terminal special device
file associated with it in the directory ``/dev/'' (for
example, ``/dev/tty03'').
When a user logs into
the system on one of these hardware terminal ports, the system has already
opened the associated device and prepared the line for normal interactive
use (see
.Xr getty 8 . )
There is also a special case of a terminal file that connects not to
a hardware terminal port, but to another program on the other side.
These special terminal devices are called
.Em ptys
and provide the mechanism necessary to give users the same interface to the
system when logging in over a network (using
.Xr rlogin 1 ,
or
.Xr telnet 1
for example).
Even in these cases the details of how the terminal
file was opened and set up is already handled by special software
in the system.
Thus, users do not normally need to worry about the details of
how these lines are opened or used.
Also, these lines are often used
for dialing out of a system (through an out-calling modem), but again
the system provides programs that hide the details of accessing
these terminal special files (see
.Xr tip 1 ) .
.Pp
When an interactive user logs in, the system prepares the line to
behave in a certain way (called a
.Em "line discipline" ) ,
the particular details of which is described in
.Xr stty 1
at the command level, and in
.Xr termios 4
at the programming level.
A user may be concerned with changing
settings associated with his particular login terminal and should refer
to the preceding man pages for the common cases.
The remainder of this man page is concerned
with describing details of using and controlling terminal devices
at a low level, such as that possibly required by a program wishing
to provide features similar to those provided by the system.
.Ss Terminal File Operations
All of the following operations are invoked using the
.Xr ioctl 2
system call.
Refer to that man page for a description of the
.Em request
and
.Em argp
parameters.
In addition to the ioctl
.Em requests
defined here, the specific line discipline
in effect will define other
.Em requests
specific to it (actually
.Xr termios 4
defines them as function calls, not ioctl
.Em requests . )
The following section lists the available ioctl requests.
The name of the request, a description of its purpose, and the typed
.Em argp
parameter (if any)
are listed.
For example, the first entry says
.Pp
.D1 Em "TIOCSPGRP int *tpgrp"
.Pp
and would be called on the terminal associated with
file descriptor zero by the following code fragment:
.Bd -literal
int pgrp;
pgrp = getpgrp();
ioctl(0, TIOCSPGRP, &pgrp);
.Ed
.Ss Terminal File Request Descriptions
.Bl -tag -width TIOCGWINSZ
.It Dv TIOCSETD Fa int *ldisc
This call is obsolete but left for compatibility.
Before
.Fx 8.0 ,
it would change to the new line discipline pointed to by
.Fa ldisc .
.It Dv TIOCGETD Fa int *ldisc
Return the current line discipline in the integer pointed to by
.Fa ldisc .
.It Dv TIOCSBRK Fa void
Set the terminal hardware into BREAK condition.
.It Dv TIOCCBRK Fa void
Clear the terminal hardware BREAK condition.
.It Dv TIOCSDTR Fa void
Assert data terminal ready (DTR).
.It Dv TIOCCDTR Fa void
Clear data terminal ready (DTR).
.It Dv TIOCGPGRP Fa int *tpgrp
Return the current process group with which the terminal is associated
in the integer pointed to by
.Fa tpgrp .
This is the underlying call that implements the
.Xr termios 4
.Fn tcgetattr
call.
.It Dv TIOCSPGRP Fa int *tpgrp
Associate the terminal with the process group (as an integer) pointed to by
.Fa tpgrp .
This is the underlying call that implements the
.Xr termios 4
.Fn tcsetattr
call.
.It Dv TIOCGETA Fa struct termios *term
Place the current value of the termios state associated with the
device in the termios structure pointed to by
.Fa term .
This is the underlying call that implements the
.Xr termios 4
.Fn tcgetattr
call.
.It Dv TIOCSETA Fa struct termios *term
Set the termios state associated with the device immediately.
This is the underlying call that implements the
.Xr termios 4
.Fn tcsetattr
call with the
.Dv TCSANOW
option.
.It Dv TIOCSETAW Fa struct termios *term
First wait for any output to complete, then set the termios state
associated with the device.
This is the underlying call that implements the
.Xr termios 4
.Fn tcsetattr
call with the
.Dv TCSADRAIN
option.
.It Dv TIOCSETAF Fa struct termios *term
First wait for any output to complete, clear any pending input,
then set the termios state associated with the device.
This is the underlying call that implements the
.Xr termios 4
.Fn tcsetattr
call with the
.Dv TCSAFLUSH
option.
.It Dv TIOCOUTQ Fa int *num
Place the current number of characters in the output queue in the
integer pointed to by
.Fa num .
.It Dv TIOCSTI Fa char *cp
Simulate typed input.
Pretend as if the terminal received the character pointed to by
.Fa cp .
.It Dv TIOCNOTTY Fa void
This call is obsolete but left for compatibility.
In the past, when a process that did not have a controlling terminal (see
.Em The Controlling Terminal
in
.Xr termios 4 )
first opened a terminal device, it acquired that terminal as its
controlling terminal.
For some programs this was a hazard as they
did not want a controlling terminal in the first place, and this
provided a mechanism to disassociate the controlling terminal from
the calling process.
It
.Em must
be called by opening the file
.Pa /dev/tty
and calling
.Dv TIOCNOTTY
on that file descriptor.
.Pp
The current system does not allocate a controlling terminal to
a process on an
.Fn open
call: there is a specific ioctl called
.Dv TIOCSCTTY
to make a terminal the controlling
terminal.
In addition, a program can
.Fn fork
and call the
.Fn setsid
system call which will place the process into its own session - which
has the effect of disassociating it from the controlling terminal.
This is the new and preferred method for programs to lose their controlling
terminal.
.It Dv TIOCSTOP Fa void
Stop output on the terminal (like typing ^S at the keyboard).
.It Dv TIOCSTART Fa void
Start output on the terminal (like typing ^Q at the keyboard).
.It Dv TIOCSCTTY Fa void
Make the terminal the controlling terminal for the process (the process
must not currently have a controlling terminal).
.It Dv TIOCDRAIN Fa void
Wait until all output is drained, or until the drain wait timeout expires.
.It Dv TIOCGDRAINWAIT Fa int *timeout
Return the current drain wait timeout in seconds.
.It Dv TIOCSDRAINWAIT Fa int *timeout
Set the drain wait timeout in seconds.
A value of zero disables timeouts.
The default drain wait timeout is controlled by the tunable
.Xr sysctl 8
OID
.Va kern.tty_drainwait .
.It Dv TIOCEXCL Fa void
Set exclusive use on the terminal.
No further opens are permitted except by root.
Of course, this means that programs that are run by
root (or setuid) will not obey the exclusive setting - which limits
the usefulness of this feature.
.It Dv TIOCNXCL Fa void
Clear exclusive use of the terminal.
Further opens are permitted.
.It Dv TIOCFLUSH Fa int *what
If the value of the int pointed to by
.Fa what
contains the
.Dv FREAD
bit as defined in
.In sys/file.h ,
then all characters in the input queue are cleared.
If it contains the
.Dv FWRITE
bit, then all characters in the output queue are cleared.
If the value of the integer is zero, then it behaves as if both the
.Dv FREAD
and
.Dv FWRITE
bits were set (i.e., clears both queues).
.It Dv TIOCGWINSZ Fa struct winsize *ws
Put the window size information associated with the terminal in the
.Va winsize
structure pointed to by
.Fa ws .
The window size structure contains the number of rows and columns (and pixels
if appropriate) of the devices attached to the terminal.
It is set by user software
and is the means by which most full\&-screen oriented programs determine the
screen size.
The
.Va winsize
structure is defined in
.In sys/ioctl.h .
.It Dv TIOCSWINSZ Fa struct winsize *ws
Set the window size associated with the terminal to be the value in
the
.Va winsize
structure pointed to by
.Fa ws
(see above).
.It Dv TIOCCONS Fa int *on
If
.Fa on
points to a non-zero integer, redirect kernel console output (kernel printf's)
to this terminal.
If
.Fa on
points to a zero integer, redirect kernel console output back to the normal
console.
This is usually used on workstations to redirect kernel messages
to a particular window.
.It Dv TIOCMSET Fa int *state
The integer pointed to by
.Fa state
contains bits that correspond to modem state.
Following is a list of defined variables and the modem state they represent:
.Pp
.Bl -tag -width TIOCMXCTS -compact
.It TIOCM_LE
Line Enable.
.It TIOCM_DTR
Data Terminal Ready.
.It TIOCM_RTS
Request To Send.
.It TIOCM_ST
Secondary Transmit.
.It TIOCM_SR
Secondary Receive.
.It TIOCM_CTS
Clear To Send.
.It TIOCM_CAR
Carrier Detect.
.It TIOCM_CD
Carrier Detect (synonym).
.It TIOCM_RNG
Ring Indication.
.It TIOCM_RI
Ring Indication (synonym).
.It TIOCM_DSR
Data Set Ready.
.El
.Pp
This call sets the terminal modem state to that represented by
.Fa state .
Not all terminals may support this.
.It Dv TIOCMGET Fa int *state
Return the current state of the terminal modem lines as represented
above in the integer pointed to by
.Fa state .
.It Dv TIOCMBIS Fa int *state
The bits in the integer pointed to by
.Fa state
represent modem state as described above, however the state is OR-ed
in with the current state.
.It Dv TIOCMBIC Fa int *state
The bits in the integer pointed to by
.Fa state
represent modem state as described above, however each bit which is on
in
.Fa state
is cleared in the terminal.
.El
.Sh IMPLEMENTATION NOTES
The total number of input and output bytes
through all terminal devices
are available via the
.Va kern.tk_nin
and
.Va kern.tk_nout
read-only
.Xr sysctl 8
variables.
.Sh SEE ALSO
.Xr stty 1 ,
.Xr ioctl 2 ,
.Xr ng_tty 4 ,
.Xr pty 4 ,
.Xr termios 4 ,
.Xr getty 8
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