3 * Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de>
5 * SPDX-License-Identifier: GPL-2.0+
8 U-Boot console multiplexing
9 ===========================
11 HOW CONSOLE MULTIPLEXING WORKS
12 ------------------------------
14 This functionality is controlled with CONFIG_CONSOLE_MUX in the board
17 Two new files, common/iomux.c and include/iomux.h, contain the heart
18 (iomux_doenv()) of the environment setting implementation.
20 iomux_doenv() is called in common/cmd_nvedit.c to handle setenv and in
21 common/console.c in console_init_r() during bootup to initialize
24 A user can use a comma-separated list of devices to set stdin, stdout
25 and stderr. For example: "setenv stdin serial,nc". NOTE: No spaces
26 are allowed around the comma(s)!
28 The length of the list is limited by malloc(), since the array used
29 is allocated and freed dynamically.
31 It should be possible to specify any device which console_assign()
32 finds acceptable, but the code has only been tested with serial and
35 iomux_doenv() prevents multiple use of the same device, e.g. "setenv
36 stdin nc,nc,serial" will discard the second nc. iomux_doenv() is
37 not able to modify the environment, however, so that "pri stdin" still
40 The major change in common/console.c was to modify fgetc() to call
41 the iomux_tstc() routine in a for-loop. iomux_tstc() in turn calls
42 the tstc() routine for every registered device, but exits immediately
43 when one of them returns true. fgetc() then calls iomux_getc(),
44 which calls the corresponding getc() routine. fgetc() hangs in
45 the for-loop until iomux_tstc() returns true and the input can be
48 Thus, a user can type into any device registered for stdin. No effort
49 has been made to demulitplex simultaneous input from multiple stdin
52 fputc() and fputs() have been modified to call iomux_putc() and
53 iomux_puts() respectively, which call the corresponding output
54 routines for every registered device.
56 Thus, a user can see the ouput for any device registered for stdout
57 or stderr on all devices registered for stdout or stderr. As an
58 example, if stdin=serial,nc and stdout=serial,nc then all output
59 for serial, e.g. echos of input on serial, will appear on serial and nc.
61 Just as with the old console code, this statement is still true:
62 If not defined in the environment, the first input device is assigned
63 to the 'stdin' file, the first output one to 'stdout' and 'stderr'.
65 If CONFIG_SYS_CONSOLE_IS_IN_ENV is defined then multiple input/output
66 devices can be set at boot time if defined in the environment.
71 Note that common/iomux.c calls console_assign() for every registered
72 device as it is discovered. This means that the environment settings
73 for application consoles will be set to the last device in the list.
75 On a slow machine, such as MPC852T clocked at 66MHz, the overhead associated
76 with calling tstc() and then getc() means that copy&paste will normally not
77 work, even when stdin=stdout=stderr=serial.
78 On a faster machine, such as a sequoia, cut&paste of longer (about 80
79 characters) lines works fine when serial is the only device used.
81 Using nc as a stdin device results in even more overhead because nc_tstc()
82 is quite slow. Even on a sequoia cut&paste does not work on the serial
83 interface when nc is added to stdin, although there is no character loss using
84 the ethernet interface for input. In this test case stdin=serial,nc and
87 In addition, the overhead associated with sending to two devices, when one of
88 them is nc, also causes problems. Even on a sequoia cut&paste does not work
89 on the serial interface (stdin=serial) when nc is added to stdout (stdout=