xref: /openbmc/u-boot/doc/README.iomux (revision 6b7c0f5e)
1/*
2 * (C) Copyright 2008
3 * Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de>
4 *
5 * See file CREDITS for list of people who contributed to this
6 * project.
7 *
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License as
10 * published by the Free Software Foundation; either version 2 of
11 * the License, or (at your option) any later version.
12 *
13 * This program is distributed in the hope that it will be useful,
14 * but WITHOUT ANY WARRANTY; without even the implied warranty of
15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
16 * GNU General Public License for more details.
17 *
18 * You should have received a copy of the GNU General Public License
19 * along with this program; if not, write to the Free Software
20 * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
21 * MA 02111-1307 USA
22 */
23
24U-Boot console multiplexing
25===========================
26
27HOW CONSOLE MULTIPLEXING WORKS
28------------------------------
29
30This functionality is controlled with CONFIG_CONSOLE_MUX in the board
31configuration file.
32
33Two new files, common/iomux.c and include/iomux.h, contain the heart
34(iomux_doenv()) of the environment setting implementation.
35
36iomux_doenv() is called in common/cmd_nvedit.c to handle setenv and in
37common/console.c in console_init_r() during bootup to initialize
38stdio_devices[].
39
40A user can use a comma-separated list of devices to set stdin, stdout
41and stderr.  For example: "setenv stdin serial,nc".  NOTE: No spaces
42are allowed around the comma(s)!
43
44The length of the list is limited by malloc(), since the array used
45is allocated and freed dynamically.
46
47It should be possible to specify any device which console_assign()
48finds acceptable, but the code has only been tested with serial and
49nc.
50
51iomux_doenv() prevents multiple use of the same device, e.g. "setenv
52stdin nc,nc,serial" will discard the second nc.  iomux_doenv() is
53not able to modify the environment, however, so that "pri stdin" still
54shows "nc,nc,serial".
55
56The major change in common/console.c was to modify fgetc() to call
57the iomux_tstc() routine in a for-loop.  iomux_tstc() in turn calls
58the tstc() routine for every registered device, but exits immediately
59when one of them returns true.  fgetc() then calls iomux_getc(),
60which calls the corresponding getc() routine.  fgetc() hangs in
61the for-loop until iomux_tstc() returns true and the input can be
62retrieved.
63
64Thus, a user can type into any device registered for stdin.  No effort
65has been made to demulitplex simultaneous input from multiple stdin
66devices.
67
68fputc() and fputs() have been modified to call iomux_putc() and
69iomux_puts() respectively, which call the corresponding output
70routines for every registered device.
71
72Thus, a user can see the ouput for any device registered for stdout
73or stderr on all devices registered for stdout or stderr.  As an
74example, if stdin=serial,nc and stdout=serial,nc then all output
75for serial, e.g. echos of input on serial, will appear on serial and nc.
76
77Just as with the old console code, this statement is still true:
78If not defined in the environment, the first input device is assigned
79to the 'stdin' file, the first output one to 'stdout' and 'stderr'.
80
81If CONFIG_SYS_CONSOLE_IS_IN_ENV is defined then multiple input/output
82devices can be set at boot time if defined in the environment.
83
84CAVEATS
85-------
86
87Note that common/iomux.c calls console_assign() for every registered
88device as it is discovered.  This means that the environment settings
89for application consoles will be set to the last device in the list.
90
91On a slow machine, such as MPC852T clocked at 66MHz, the overhead associated
92with calling tstc() and then getc() means that copy&paste will normally not
93work, even when stdin=stdout=stderr=serial.
94On a faster machine, such as a sequoia, cut&paste of longer (about 80
95characters) lines works fine when serial is the only device used.
96
97Using nc as a stdin device results in even more overhead because nc_tstc()
98is quite slow.  Even on a sequoia cut&paste does not work on the serial
99interface when nc is added to stdin, although there is no character loss using
100the ethernet interface for input. In this test case stdin=serial,nc and
101stdout=serial.
102
103In addition, the overhead associated with sending to two devices, when one of
104them is nc, also causes problems.  Even on a sequoia cut&paste does not work
105on the serial interface (stdin=serial) when nc is added to stdout (stdout=
106serial,nc).
107