10.4. BSP Initialization

The bootloader jumps or calls the RTEMS executable’s entry point, normally a fixed address. The BSP entry point or start up code performs:

  1. Low level processor specific initialization that such as setting control registers so the processor is operating in a mode RTEMS is built for

  2. Cache flushing, clearing and invalidation

  3. Memory management unit (MMU) set up if required

  4. Clear the uninitialized data section

  5. Process a command line if supported by the bootloader

  6. Call bootcard which disabled interrupts, saves away a command line if the BSP supports it then call the RTEMS kernel early initialize entry point rtems_initialize_executive. This call never returns.

Further BSP initialization happens as part of RTEMS kernel’s System Initialization process. The following handlers are declared and if provided are placed at the beginning of the initialization handler list. The BSP can provides:

bsp_work_area_initialize

This function determines the amount of memory that can be given to RTEMS for the workspace and the C library heap which malloc uses. The call typically uses the bsp_work_area_initialize_default to perform actually perform the initialization.

bsp_start

This function is specialized for each architecture and even for some BSPs. It performs the low level initialization RTEMS needs so it can run on the architecture and BSP.

bsp_predriver_hook

This function can be used to initialize hardware drivers depend on such as configuring an interrupt controller. The default version is empty and does nothing.

BSPs all perform similar operations with common functionality and the RTEMS kernel provides common code that can be shared between BSPs. The use of the common code is encouraged for all new BSPs.

10.5. RTEMS Initialization

The RTEMS kernel initialization is:

  1. Invoke the registered system initialization handlers

  2. Set the system state to up

  3. If the kernel supports SMP request multitasking start. All online cores are transferred to the ready to start multitasking state.

  4. Start threaded multitasking. RTEMS starts multitasking by getting the first thread to run and dispatching it.

C++ static object constructors are called in the context of the first running thread before the thread body is entered.

10.5.1. System Initialization Handlers

RTEMS supports the automatic registration of services used in applications. This method of initialization automatically configures RTEMS with only the services used in an application. There is no manual configuration of services used and no updating of initialization function tables.

RTEMS uses specialized sections in the ELF executable to perform this task. The system is based on the FreeBSD SYSINT Framework. Ordered initialization is performed before multitasking is started.

The RTEMS Tool rtems-exeinfo can provide some detail about the registered handlers. The following shows the initialization handlers for the Hello World sample application in the RTEMS kernel’s testsuite:

$ rtems-exeinfo --init arm-rtems6/c/xilinx_zynq_zedboard/testsuites/samples/hello.exe
RTEMS Executable Info 6.5416cfa39dd6
exe: arm-rtems6/c/xilinx_zynq_zedboard/testsuites/samples/hello.exe

Compilation:
 Producers: 2
  |  GNU AS 2.31.1: 14 objects
  |  GNU C11 7.3.0 20180125 (RTEMS 6, RSB e55769c64cf1a201588565a5662deafe3f1ccdcc, Newlib 103b055035fea328f8bc7826801760fb1c055683): 284 objects
 Common flags: 4
  | -march=armv7-a -mthumb -mfpu=neon -mfloat-abi=hard

Init sections: 2
 .init_array
  0x001047c1 frame_dummy
 .rtemsroset
  0x00104c05 bsp_work_area_initialize
  0x00104c41 bsp_start
  0x0010eb45 zynq_debug_console_init
  0x0010ec19 rtems_counter_sysinit
  0x0010b779 _User_extensions_Handler_initialization
  0x0010c66d rtems_initialize_data_structures
  0x00107751 _RTEMS_tasks_Manager_initialization
  0x0010d4f5 _POSIX_Keys_Manager_initialization
  0x0010dd09 _Thread_Create_idle
  0x0010cf01 rtems_libio_init
  0x001053a5 rtems_filesystem_initialize
  0x0010546d _Console_simple_Initialize
  0x0010c715 _IO_Initialize_all_drivers
  0x001076d5 _RTEMS_tasks_Initialize_user_tasks_body
  0x0010cfa9 rtems_libio_post_driver

The section .rtemsroset lists the handlers called in order. The handlers can be split into the BSP initialization handlers that start the BSP:

  • bsp_work_area_initialize

  • bsp_start

  • zynq_debug_console_init

  • rtems_counter_sysinit

And the remainder are handlers for services used by the application. The list varies based on the services the application uses.