U-Boot console handling

Introduction

At system-startup U-Boot initializes a serial console. When U-Boot relocates itself to RAM, all console drivers are initialized (they will register all detected console devices to the system for further use).

If not defined in the environment, the first input device is assigned to the ‘stdin’ file, the first output one to ‘stdout’ and ‘stderr’.

You can use the command coninfo to see all registered console devices and their flags. You can assign a standard file (stdin, stdout or stderr) to any device you see in that list simply by assigning its name to the corresponding environment variable. For example:

# Use the serial input
setenv stdin serial

# Use the video console
setenv stdout vidconsole

Do a simple saveenv to save the console settings in the environment and get them working on the next startup, too.

How to output text to the console

You can use the following functions to access the console:

stdout

  • putc() - write a char to stdout

  • puts() - write a string to stdout

  • printf() - format and write a string to stdout

stdin

  • tstc() - test for the presence of a char in stdin

  • getchar() - get a char from stdin

stderr

  • eputc() - write a char to stderr

  • eputs() - write a string to stderr

  • eprintf() - format and write a string to stderr

file (‘stdin’, ‘stdout’ or ‘stderr’)

  • fputc() - write a char to a file

  • fputs() - write a string to a file

  • fprintf() - format and write a string to a file

  • ftstc() - test for the presence of a char in file

  • fgetc() - get a char from a file

Remember that FILE-related functions CANNOT be used before U-Boot relocation, which is done in board_init_r().

Pager

U-Boot has a simple pager feature, enabled with CONFIG_CONSOLE_PAGER. It is only available if CONFIG_CONSOLE_MUX is also enabled.

When activated, the pager pauses at the end of each ‘page’ (screenful) of output, shows a prompt “: Press SPACE to continue” and lets the user read the output. To continue to the next page, press the SPACE key. To quit paging without seeing further output from the current command, press ‘q’.

The pager can be bypassed by pressing ‘Q’ at the prompt. This disables the pager until the ‘pager’ environment variable is given a new value.

Page Size Configuration

The number of lines per page is determined in the following order of priority:

  1. Environment variable: The pager environment variable (hex value) takes highest priority. Set to 0 to disable paging.

  2. Video console detection: If no environment variable is set and a video console is active, the pager uses the number of rows from the video console.

  3. Serial TTY detection: For serial consoles, the pager checks if the output is connected to a terminal (TTY). If not connected to a TTY, paging is disabled. This check works by sending a few special characters to the terminal and (hopefully) receiving a reply. If you are logging the output of U-Boot, you may see these characters in the log. Disable CONFIG_SERIAL_TERM_PRESENT is this is unwanted.

  4. Configuration default: If none of the above apply, falls back to CONFIG_CONSOLE_PAGER_LINES.

Examples:

# Set page size to 30 lines (hex value 1e)
setenv pager 1e

# Set page size to 24 lines (hex value 18)
setenv pager 18

# Disable paging
setenv pager 0

For developer documentation, please see Console.