architecture, and CPU. When configuring U-Boot for one of its supported platforms, issue this command:
$ make <platform>_config
Here, platform is one of the many platforms supported by U-Boot. These platform-configuration targets are listed in the top level U-Boot makefile. For example, to configure for the Spectrum Digital OSK, which contains a TI OMAP 5912 processor, issue this command:
$ make omap5912osk_config
This configures the U-Boot source tree with the appropriate soft links to select ARM as the target architecture, the ARM926 core, and the 5912 OSK as the target platform.
The next step in configuring U-Boot for this platform is to edit the configuration file specific to this board. This file is found in the U-Boot ../include/configs subdirectory and is called omap5912osk.h. The README file that comes with the U-Boot distribution describes the details of configuration and is the best source for this information.
Configuration of U-Boot is done using configuration variables defined in a board-specific header file. Configuration variables have two forms. Configuration
Numerous features and modes of operation can be selected by adding definitions to the board- configuration file. Listing 7-4 contains a partial configuration header file for a fictitious board based on the PPC 405GP processor.
Listing 7-4. Partial U-Boot Board-Configuration Header File
#define CONFIG_405GP /* Processor definition */
#define CONFIG_4XX /* Sub-arch specification, 4xx family */
#define CONFIG_SYS_CLK_FREQ 33333333 /* PLL Frequency */
#define CONFIG_BAUDRATE 9600
#define CONFIG_PCI /* Enable support for PCI */
...
#define CONFIG_COMMANDS (CONFIG_CMD_DFL | CFG_CMD_DHCP)
...
#define CFG_BASE_BAUD 691200
/* The following table includes the supported baudrates */
#define CFG_BAUDRATE_TABLE
{1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400}
#define CFG_LOAD_ADDR 0x100000 /* default load address */
...
/* Memory Bank 0 (Flash Bank 0) initialization */
#define CFG_EBC_PB0AP 0x9B015480
#define CFG_EBC_PB0CR 0xFFF18000
#define CFG_EBC_PB1AP 0x02815480
#define CFG_EBC_PB1CR 0xF0018000
...
Listing 7-4 gives an idea of how U-Boot itself is configured for a given board. An actual board-configuration file can contain hundreds of lines similar to those found here. In this example, you can see the definitions for the CPU, CPU family (4xx), PLL clock frequency, serial port baud rate, and PCI support. We have included examples of configuration variables (CONFIG_XXX) and configuration settings (CFG_XXX). The last few lines are actual processor register values required to initialize the external bus controller for memory banks 0 and 1. You can see that these values can come only from a detailed knowledge of the board and processor.
Many aspects of U-Boot can be configured using these mechanisms, including what functionality will be compiled into U-Boot (support for DHCP, memory tests, debugging support, and so on). This mechanism can be used to tell U-Boot how much and what kind of memory is on a given board, and where that memory is mapped. The interested reader can learn much more by looking at the U-Boot code directly, especially the excellent README file.
7.3.2. U-Boot Command Sets
U-Boot supports more than 60 standard command sets that enable more than 150 unique commands using CFG_* macros. A command set is enabled in U-Boot through the use of configuration setting (CFG_*) macros. For a complete list from a recent U-Boot snapshot, consult Appendix B, 'U-Boot Configurable Commands.' Here are just a few, to give you an idea of the capabilities available:
| Command Set | Commands |
|---|---|
| CFG_CMD_FLASH | Flash memory commands |
| CFG_CMD_MEMORY | Memory dump, fill, copy, compare, and so on |
| CFG_CMD_DHCP | DHCP Support |
| CFG_CMD_PING | Ping support |
| CFG_CMD_EXT2 | EXT2 File system support |
The following line of Listing 7-4 defines the commands enabled in a given U-Boot configuration, as driven
