Книга: Embedded Linux Primer: A Practical, Real-World Approach

7.3.1. System Configuration: U-Boot

7.3.1. System Configuration: U-Boot

For a bootloader to be useful across many processors and architectures, some method of configuring the bootloader is necessary. As with the Linux kernel itself, configuration of a bootloader is done at compile time. This method significantly reduces the complexity of the bootloader, which, in itself, is an important characteristic.

In the case of U-Boot, board-specific configuration is driven by a single header file specific to the target platform, and a few soft links in the source tree that select the correct subdirectories based on target board, 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 options are selected using macros in the form of CONFIG_XXXX. Configuration settings are selected using macros in the form of CFG_XXXX. In general, configuration options (CONFIG_XXX) are user configurable and enable specific U-Boot operational features. Configuration settings (CFG_XXX) are usually hardware specific and require detailed knowledge of the underlying processor and/or hardware platform. Board-specific U-Boot configuration is driven by a header file dedicated to that specific platform that contains configuration options and settings appropriate for the underlying platform. The U-Boot source tree includes a directory where these board-specific configuration header files reside. They can be found in .../include/configs from the top-level U-Boot source directory.

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.

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