u-boot/doc/README.iomux

/*
 * (C) Copyright 2008
 * Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de>
 *
 * SPDX-License-Identifier:	GPL-2.0+
 */

U-Boot console multiplexing
===========================

HOW CONSOLE MULTIPLEXING WORKS
------------------------------

This functionality is controlled with CONFIG_CONSOLE_MUX in the board
configuration file.

Two new files, common/iomux.c and include/iomux.h, contain the heart
(iomux_doenv()) of the environment setting implementation.

iomux_doenv() is called in common/cmd_nvedit.c to handle setenv and in
common/console.c in console_init_r() during bootup to initialize
stdio_devices[].

A user can use a comma-separated list of devices to set stdin, stdout
and stderr.  For example: "setenv stdin serial,nc".  NOTE: No spaces
are allowed around the comma(s)!

The length of the list is limited by malloc(), since the array used
is allocated and freed dynamically.

It should be possible to specify any device which console_assign()
finds acceptable, but the code has only been tested with serial and
nc.

iomux_doenv() prevents multiple use of the same device, e.g. "setenv
stdin nc,nc,serial" will discard the second nc.  iomux_doenv() is
not able to modify the environment, however, so that "pri stdin" still
shows "nc,nc,serial".

The major change in common/console.c was to modify fgetc() to call
the iomux_tstc() routine in a for-loop.  iomux_tstc() in turn calls
the tstc() routine for every registered device, but exits immediately
when one of them returns true.  fgetc() then calls iomux_getc(),
which calls the corresponding getc() routine.  fgetc() hangs in
the for-loop until iomux_tstc() returns true and the input can be
retrieved.

Thus, a user can type into any device registered for stdin.  No effort
has been made to demulitplex simultaneous input from multiple stdin
devices.

fputc() and fputs() have been modified to call iomux_putc() and
iomux_puts() respectively, which call the corresponding output
routines for every registered device.

Thus, a user can see the ouput for any device registered for stdout
or stderr on all devices registered for stdout or stderr.  As an
example, if stdin=serial,nc and stdout=serial,nc then all output
for serial, e.g. echos of input on serial, will appear on serial and nc.

Just as with the old console code, this statement is still true:
If not defined in the environment, the first input device is assigned
to the 'stdin' file, the first output one to 'stdout' and 'stderr'.

If CONFIG_SYS_CONSOLE_IS_IN_ENV is defined then multiple input/output
devices can be set at boot time if defined in the environment.

CAVEATS
-------

Note that common/iomux.c calls console_assign() for every registered
device as it is discovered.  This means that the environment settings
for application consoles will be set to the last device in the list.

On a slow machine, such as MPC852T clocked at 66MHz, the overhead associated
with calling tstc() and then getc() means that copy&paste will normally not
work, even when stdin=stdout=stderr=serial.
On a faster machine, such as a sequoia, cut&paste of longer (about 80
characters) lines works fine when serial is the only device used.

Using nc as a stdin device results in even more overhead because nc_tstc()
is quite slow.  Even on a sequoia cut&paste does not work on the serial
interface when nc is added to stdin, although there is no character loss using
the ethernet interface for input. In this test case stdin=serial,nc and
stdout=serial.

In addition, the overhead associated with sending to two devices, when one of
them is nc, also causes problems.  Even on a sequoia cut&paste does not work
on the serial interface (stdin=serial) when nc is added to stdout (stdout=
serial,nc).