u-boot-2016.09 transplantation (2)-uboot startup simple analysis

Note: This article and the following about the transplantation of uboot refer to "Embedded Linux Learning Notes (Based on S5PV210, TQ210)"

No matter what version of uboot, the entry function ENTRY(_start) is defined in arch/arm/cpu/u-boot-spl.lds and arch/arm/cpu/u-boot.lds. u-boot-2016.09 is no exception. The search found that _start is implemented in arch/arm/lib/vectors.S:

_start first executes reset, then defines the exception vector table, reset is implemented in arch/arm/cpu/armv7/start.S

Only one jump instruction is executed in reset, the function of save_boot_params is to jump to the save_boot_params_ret function

In this program, there is a macro CONFIG_SKIP_LOWLEVEL_INIT to determine whether to perform some low-level initialization (skip if necessary, and perform initialization if not). cpu_init_cp15 is used to implement the initialization of cp15. You can refer to the "ARM Architecture Reference Manual". We don't need to know too much about this. cpu_init_crit calls the lowlevel_init function, which is implemented in board/samsung/tq210/lowlevel_init.S. It can perform low-level initialization of clocks, memory, etc. Then jump to _main, which is implemented in arch/arm/lib/crt0.S.

Here we mainly focus on board_init_f, which is implemented in common/board_f.c. It performs some basic hardware initialization to prepare for entering DRAM memory. After the initialization is completed, continue to execute

Here, relocate_code is called to relocate and move u-boot.bin into DRAM. Continue to execute

Here, the BSS clearing operation is performed, and then the link address of the function board_init_r is assigned to the PC pointer, and the execution is directly jumped to the DRAM memory. In board_init_r, some hardware initialization is continued, and the init_sequence_r function array is called in board_init_r, and finally the execution is executed to run_main_loop:

run_main_loop finally executes to main_loop, which is implemented in common/board_r.c

In main_loop, the user input will be detected. If no user presses the button within the specified delay time, the kernel image will be copied from the SD card or FLASH to DRAM according to the environment variables, and then jump to the OS starting address to execute the OS. At this point, the uboot startup is completed.