The uart(4) driver is an universal driver for various UART hardware.

It improves on sio(4) in the following areas:
o  Fully newbusified to allow for memory mapped I/O. This is a must
   for ia64 and sparc64,
o  Machine dependent code to take full advantage of machine and firm-
   ware specific ways to define serial consoles and/or debug ports.
o  Hardware abstraction layer to allow the driver to be used with
   various UARTs, such as the well-known ns8250 family of UARTs, the
   Siemens sab82532 or the Zilog Z8530. This is especially important
   for pc98 and sparc64 where it's common to have different UARTs,
o  The notion of system devices to unkludge low-level consoles and
   remote gdb ports and provides the mechanics necessary to support
   the keyboard on sparc64 (which is UART based).
o  The notion of a kernel interface so that a UART can be tied to
   something other than the well-known TTY interface. This is needed
   on sparc64 to present the user with a device and ioctl handling
   suitable for a keyboard, but also allows us to cleanly hide an
   UART when used as a debug port.

Following is a list of features and bugs/flaws specific to the ns8250
family of UARTs as compared to their support in sio(4):
o  The uart(4) driver determines the FIFO size and automaticly takes
   advantages of larger FIFOs and/or additional features. Note that
   since I don't have sufficient access to 16[679]5x UARTs, hardware
   flow control has not been enabled. This is almost trivial to do,
   provided one can test. The downside of this is that broken UARTs
   are more likely to not work correctly with uart(4). The need for
   tunables or knobs may be large enough to warrant their creation.
o  The uart(4) driver does not share the same bumpy history as sio(4)
   and will therefore not provide the necessary hooks, tweaks, quirks
   or work-arounds to deal with once common hardware. To that extend,
   uart(4) supports a subset of the UARTs that sio(4) supports. The
   question before us is whether the subset is sufficient for current
   hardware.
o  There is no support for multiport UARTs in uart(4). The decision
   behind this is that uart(4) deals with one EIA RS232-C interface.
   Packaging of multiple interfaces in a single chip or on a single
   expansion board is beyond the scope of uart(4) and is now mostly
   left for puc(4) to deal with. Lack of hardware made it impossible
   to actually implement such a dependency other than is present for
   the dual channel SAB82532 and Z8350 SCCs.

The current list of missing features is:
o  No configuration capabilities. A set of tunables and sysctls is
   being worked out. There are likely not going to be any or much
   compile-time knobs. Such configuration does not fit well with
   current hardware.
o  No support for the PPS API. This is partly dependent on the
   ability to configure uart(4) and partly dependent on having
   sufficient information to implement it properly.

As usual, the manpage is present but lacks the attention the
software has gotten.

1 files changed, 115 insertions, 0 deletions

diff --git a/share/man/man4/uart.4 b/share/man/man4/uart.4
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+.\"
+.\" Copyright (c) 2003 Marcel Moolenaar
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\"
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 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.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd August 25, 2003
+.Dt UART 4
+.Os
+.\"
+.Sh NAME
+.Nm uart
+.Nd driver for Universal Asynchronous Receiver/Transmitter (UART) devices
+.\"
+.Sh SYNOPSIS
+.Cd "device uart"
+.Pp
+.Cd "device puc"
+.Cd "device uart"
+.Pp
+.\"
+.Sh DESCRIPTION
+The
+.Nm
+device driver provides support for various classes of UARTs implementing the
+EIA RS-232C (CCITT V.24) serial communications interface.
+Each such interface is controlled by a seperate and independent instance of
+the
+.Nm
+driver.
+The primary support for devices that contain multiple serial interfaces or
+that contain other functionality besides one or more serial interfaces is
+provided by the
+.Xr puc 4
+device driver.
+However, the serial interfaces of those devices that are managed by the
+.Xr puc 4
+driver are controlled by the
+.Nm
+driver.
+As such, the
+.Xr puc 4
+driver provides umbrella functionality for the
+.Nm
+driver and hides the complexities that are inherent when elementary components
+are packaged together.
+.Pp
+The
+.Nm
+driver has a modular design to allow it to be used on differing hardware and
+for various purposes.
+In the following sections the components are discussed in detail.
+Options are described in the section that covers the component to which each
+option applies.
+.\"
+.Ss CORE COMPONENT
+At the heart of the
+.Nm
+driver is the core component. It contains the bus attachments and the low-level
+interrupt handler.
+.\"
+.Ss HARDWARE DRIVERS
+The core component and the kernel interfaces talk to the hardware through the
+hardware interface.
+This interface serves as an abstraction of the hardware and allows varying
+UARTs to be used for serial communications.
+.\"
+.Ss SYSTEM DEVICES
+System devices are UARTs that have a special purpose by way of hardware
+design or software setup.
+For example, Sun UltraSparc machines use UARTs as their keyboard interface.
+Such an UART cannot be used for general purpose communications.
+Likewise, when the kernel is configured for a serial console, the
+corresponding UART will in turn be a system device so that the kernel can
+output boot messages early on in the boot process.
+.\"
+.Ss KERNEL INTERFACES
+The last but not least of the components is the kernel interface.
+This component ultimately determines how the UART is made visible to the
+kernel in particular and to users in general.
+The default kernel interface is the TTY interface.
+This allows the UART to be used for terminals, modems and serial line IP
+applications.
+System devices, with the notable exception of serial consoles, generally
+have specialized kernel interfaces.
+.\"
+.Sh SEE ALSO
+.Xr puc 4
+.\"
+.Sh HISTORY
+The
+.Nm
+device driver first appeared in
+.Fx 5.2 .