1 /* 2 * (C) Copyright 2008 3 * Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de> 4 * 5 * SPDX-License-Identifier: GPL-2.0+ 6 */ 7 8 U-Boot console multiplexing 9 =========================== 10 11 HOW CONSOLE MULTIPLEXING WORKS 12 ------------------------------ 13 14 This functionality is controlled with CONFIG_CONSOLE_MUX in the board 15 configuration file. 16 17 Two new files, common/iomux.c and include/iomux.h, contain the heart 18 (iomux_doenv()) of the environment setting implementation. 19 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 22 stdio_devices[]. 23 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)! 27 28 The length of the list is limited by malloc(), since the array used 29 is allocated and freed dynamically. 30 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 33 nc. 34 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 38 shows "nc,nc,serial". 39 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 46 retrieved. 47 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 50 devices. 51 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. 55 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. 60 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'. 64 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. 67 68 CAVEATS 69 ------- 70 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. 74 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. 80 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 85 stdout=serial. 86 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= 90 serial,nc). 91