freebsd-dev/sys/modules/netgraph/tty/ng_tty.8

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.\" Author: Archie Cobbs <archie@whistle.com>
.\"
.\" $FreeBSD$
.\" $Whistle: ng_tty.8,v 1.5 1999/01/25 23:46:28 archie Exp $
.\"
.Dd January 19, 1999
.Dt NG_TTY 8
.Os FreeBSD 3
.Sh NAME
.Nm ng_tty
.Nd netgraph node type that is also a line discipline
.Sh SYNOPSIS
.Fd #include <netgraph/ng_message.h>
.Fd #include <netgraph/ng_tty.h>
.Sh DESCRIPTION
The
.Nm tty
node type is both a netgraph node type and a line discipline.
A new node is created when the corresponding line discipline is
registered on a tty device (see
.Xr tty 4 ")."
.Pp
The node has a single hook called
.Dv hook .
Incoming bytes received on the tty device are sent out on this hook,
and frames received on
.Dv hook
are transmitted out on the tty device.
No modification to the data is performed in either direction.
While the line discipline is installed on a tty, the normal
read and write operations are unavailable, returning
.Er EIO .
.Pp
The node supports an optional ``hot character.'' If set to non-zero, incoming
data from the tty device is queued until this character is seen.
This avoids sending lots of mbufs containing a small number of bytes,
but introduces potentially infinite latency.
The default hot character is 0x7e, consistent with
.Dv hook
being connected to a
.Xr ng_async 8
type node. The hot character has no effect on the transmission of data.
.Pp
The node will attempt to give itself the same netgraph name as the name
of the tty device.
In any case, information about the node is available via the netgraph
.Xr ioctl 2
command
.Dv NGIOCGINFO .
This command returns a
.Dv "struct nodeinfo"
similar to the
.Dv NGM_NODEINFO
netgraph control message.
.Sh HOOKS
This node type supports the following hooks:
.Pp
.Bl -tag -width foobar
.It Dv hook
.Xr tty 4
serial data contained in
.Dv mbuf
structures, with arbitrary inter-frame boundaries.
.El
.Sh CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
.Bl -tag -width foo
.It Dv NGM_TTY_SET_HOTCHAR
This command takes an integer argument and sets the hot character
from the lower 8 bits. A hot character of zero disables queueing,
so that all received data is forwarded immediately.
.It Dv NGM_TTY_GET_HOTCHAR
Returns an integer containing the current hot character in the lower
eight bits.
.Sh SHUTDOWN
This node shuts down when the corresponding device is closed
(or the line discipline is uninstalled on the device).
The
.Dv NGM_SHUTDOWN
control message is not valid, and always returns the error
.Er EOPNOTSUPP .
.Sh BUGS
The
.Nm tty
type registers its line discipline when the type is installed,
where it is dynamically assigned an integer index.
Unfortunately, there's no way to know what this integer is
except by reading the output of
.Xr dmesg 8 .
The fix for this is to have line disciplines identified by
unique ASCII strings instead of fixed integer constants,
or else to assign one of those constants to
.Nm ng_tty .
.Pp
The serial driver code also has a notion of a ``hot character.''
Unfortunately, this value is statically defined in terms of the
line discipline and cannot be changed.
Therefore, if a hot character other than 0x7e (the default) is set for the
.Nm tty
node, the node has no way to convey this information to the
serial driver, and sub-optimal performance may result.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr netgraph 4 ,
.Xr tty 4 ,
.Xr ng_async 8 ,
.Xr ngctl 8 .
.Sh AUTHOR
Archie Cobbs <archie@whistle.com>