1SPDX-License-Identifier: GPL-2.0+ 2/* 3 * (C) Copyright 2008 4 * Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de> 5 */ 6 7U-Boot console multiplexing 8=========================== 9 10HOW CONSOLE MULTIPLEXING WORKS 11------------------------------ 12 13This functionality is controlled with CONFIG_CONSOLE_MUX in the board 14configuration file. 15 16Two new files, common/iomux.c and include/iomux.h, contain the heart 17(iomux_doenv()) of the environment setting implementation. 18 19iomux_doenv() is called in common/cmd_nvedit.c to handle setenv and in 20common/console.c in console_init_r() during bootup to initialize 21stdio_devices[]. 22 23A user can use a comma-separated list of devices to set stdin, stdout 24and stderr. For example: "setenv stdin serial,nc". NOTE: No spaces 25are allowed around the comma(s)! 26 27The length of the list is limited by malloc(), since the array used 28is allocated and freed dynamically. 29 30It should be possible to specify any device which console_assign() 31finds acceptable, but the code has only been tested with serial and 32nc. 33 34iomux_doenv() prevents multiple use of the same device, e.g. "setenv 35stdin nc,nc,serial" will discard the second nc. iomux_doenv() is 36not able to modify the environment, however, so that "pri stdin" still 37shows "nc,nc,serial". 38 39The major change in common/console.c was to modify fgetc() to call 40the iomux_tstc() routine in a for-loop. iomux_tstc() in turn calls 41the tstc() routine for every registered device, but exits immediately 42when one of them returns true. fgetc() then calls iomux_getc(), 43which calls the corresponding getc() routine. fgetc() hangs in 44the for-loop until iomux_tstc() returns true and the input can be 45retrieved. 46 47Thus, a user can type into any device registered for stdin. No effort 48has been made to demulitplex simultaneous input from multiple stdin 49devices. 50 51fputc() and fputs() have been modified to call iomux_putc() and 52iomux_puts() respectively, which call the corresponding output 53routines for every registered device. 54 55Thus, a user can see the ouput for any device registered for stdout 56or stderr on all devices registered for stdout or stderr. As an 57example, if stdin=serial,nc and stdout=serial,nc then all output 58for serial, e.g. echos of input on serial, will appear on serial and nc. 59 60Just as with the old console code, this statement is still true: 61If not defined in the environment, the first input device is assigned 62to the 'stdin' file, the first output one to 'stdout' and 'stderr'. 63 64If CONFIG_SYS_CONSOLE_IS_IN_ENV is defined then multiple input/output 65devices can be set at boot time if defined in the environment. 66 67CAVEATS 68------- 69 70Note that common/iomux.c calls console_assign() for every registered 71device as it is discovered. This means that the environment settings 72for application consoles will be set to the last device in the list. 73 74On a slow machine, such as MPC852T clocked at 66MHz, the overhead associated 75with calling tstc() and then getc() means that copy&paste will normally not 76work, even when stdin=stdout=stderr=serial. 77On a faster machine, such as a sequoia, cut&paste of longer (about 80 78characters) lines works fine when serial is the only device used. 79 80Using nc as a stdin device results in even more overhead because nc_tstc() 81is quite slow. Even on a sequoia cut&paste does not work on the serial 82interface when nc is added to stdin, although there is no character loss using 83the ethernet interface for input. In this test case stdin=serial,nc and 84stdout=serial. 85 86In addition, the overhead associated with sending to two devices, when one of 87them is nc, also causes problems. Even on a sequoia cut&paste does not work 88on the serial interface (stdin=serial) when nc is added to stdout (stdout= 89serial,nc). 90