Boot

摘自 Embedded Linux System Design and Development

Linux 開機大致上可分成三個階段：

1. Boot loader 階段
2. Kernel 初始化階段
3. User space 初始化階段

Boot loader 階段

• 硬體初始化及測試
• 載入 kernel image：Downloading Kernel Image and Initial Ram Disk: 從 flash 或網路載入到記憶體, 需要時作解壓縮。
• 設定 Linux kernel 需要的開機參數。
• 跳到 Kernel Entry Point，控制權轉移到 Linux kernel。

Kernel 初始化階段

does the platform-specific 初始化，brings up the kernel subsystems, turns on multitasking, mounts the root file system, and jumps to user space.

• CPU/Platform-specific 初始化：
  • setting up the environment for the first C routine -- start_kernel(): turn on MMU, initialize cache, zero out BSS, set up stack
  • The setup_arch() function: recognize the processor/board, analysis command-line parameters, identify the ram disk that can be mount as root file system, initial and reserve memory, call the paging initialization function
  • initialization of exception -- the trap_init() function
  • 中斷處理程序初始化 -- the init_IRQ() function (interrupt not enabled)
  • timer tick 初始化 -- the time_init()
  • console 初始化 -- console_init() -- and the start-up messages appear on the screen
  • 計算 delay loop -- calibrate_delay() -- for udelay() to use
• 子系統初始化：scheduler, memory manager, and VFS initialization, and finally create the init process to do the rest of the startup
• 驅動程式初始化
• 掛載 rootfs: rootfs 位置可寫死在 kernel 裡或用開機參數 "root=" 指定。有 3 種 root file systems 常用在embedded systems: initrd, NFS, and Flash-based file systems.
• Doing initcall and freeing initial memory: The init section marked between __init_begin and __init_end contains text and data such as driver initialization functions that can be thrown away after they are used once during the system start-up. A use-and-throw function or variable is declared using the __init directive. Once all the driver and subsystem initialization is done, the start-up code frees all the memory. This is done just before moving to user space. Linux also provides a way of grouping functions that should be called at system start-up time. This can be done by declaring the function with __initcall directive. These functions are automatically called during kernel start-up, so you need not insert them into system start-up code.
• moving to user space: The kernel that is executing in the context of the init process jumps to the user space by overlaying itself (using execve) with the executable image of a special program also referred to as init. This executable normally resides in the root file system in the directory /sbin. Note that the user can specify the init program using a command line argument to the kernel. However if the kernel is unable to load either the user-specified init program or the default one, it enters the panic state.

參考：

1. Embedded Linux System Design and Development