内核initcall分析

linux中init相关内容定义在include/linux/init.h

1. initcall相关定义

先看下文件说明，此文件定义的宏主要用于初始化阶段标记函数或初始化数据，之后占用的资源会被释放掉。

/* These macros are used to mark some functions or

* initialized data (doesn't apply to uninitialized data)

* as `initialization' functions. The kernel can take this

* as hint that the function is used only during the initialization

* phase and free up used memory resources after

*

* Usage:

* For functions:

*

* You should add __init immediately before the function name, like:

*

* static void __init initme(int x, int y)

* {

*    extern int z; z = x * y;

* }

*

* If the function has a prototype somewhere, you can also add

* __init between closing brace of the prototype and semicolon:

*

* extern int initialize_foobar_device(int, int, int) __init;

*

* For initialized data:

* You should insert __initdata between the variable name and equal

* sign followed by value, e.g.:

*

* static int init_variable __initdata = 0;

* static char linux_logo[] __initdata = { 0x32, 0x36, ... };

*

* Don't forget to initialize data not at file scope, i.e. within a function,

* as gcc otherwise puts the data into the bss section and not into the init

* section.

*

* Also note, that this data cannot be "const".

*/

在内核中经常会看到arch_initcall()、subsys_initcall()、device_initcall()等，定义均在include/linux/init.h中：

#define pure_initcall(fn)       __define_initcall("0",fn,0)

#define core_initcall(fn)       __define_initcall("1",fn,1)

#define core_initcall_sync(fn)      __define_initcall("1s",fn,1s)

#define postcore_initcall(fn)       __define_initcall("2",fn,2)

#define postcore_initcall_sync(fn)  __define_initcall("2s",fn,2s)

#define arch_initcall(fn)       __define_initcall("3",fn,3)

#define arch_initcall_sync(fn)      __define_initcall("3s",fn,3s)

#define subsys_initcall(fn)     __define_initcall("4",fn,4)

#define subsys_initcall_sync(fn)    __define_initcall("4s",fn,4s)

#define fs_initcall(fn)         __define_initcall("5",fn,5)

#define fs_initcall_sync(fn)        __define_initcall("5s",fn,5s)

#define rootfs_initcall(fn)     __define_initcall("rootfs",fn,rootfs)

#define device_initcall(fn)     __define_initcall("6",fn,6)

#define device_initcall_sync(fn)    __define_initcall("6s",fn,6s)

#define late_initcall(fn)       __define_initcall("7",fn,7)

#define late_initcall_sync(fn)      __define_initcall("7s",fn,7s)

#define __initcall(fn) device_initcall(fn)

__define_initcall()宏定义：

/* initcalls are now grouped by functionality into separate

* subsections. Ordering inside the subsections is determined

* by link order.

* For backwards compatibility, initcall() puts the call in

* the device init subsection.

*

* The `id' arg to __define_initcall() is needed so that multiple initcalls

*/

#define __define_initcall(level,fn,id) \

static initcall_t __initcall_##fn##id __used \

__attribute__((__section__(".initcall" level ".init"))) = fn

typedef int (*initcall_t)(void);

typedef void (*exitcall_t)(void);

*_initcall(fn)最终都是通过__define_initcall(level,fn,id)宏定义生成，而最终所有的initcall_t型函数都存放在.initcall”level”.init section中。.initcall”level”.init定义在vmlinux.lds内。

/* arch/arm/kernel/vmlinux.lds */

__initcall_start = .;

*(.initcallearly.init) __early_initcall_end = .; *(.initcall0.init) *(.initcall0s.init) *(.initcall1.init) *(.initcall1s.init) *(.initcall2.init) *(.initcall2s.init) *(.initcall3.init) *(.initcall3s.init) *(.initcall4.init) *(.initcall4s.init) *(.initcall5.init) *(.initcall5s.init) *(.initcallrootfs.init) *(.initcall6.init) *(.initcall6s.init) *(.initcall7.init) *(.initcall7s.init)

__initcall_end = .;

正好包括了上面init.h里定义的从pure_initcall到late_initcall等8个level等级的.initcall”level”.init section. 因此通过不同的*_initcall声明的函数指针最终都会存放不同level等级的.initcall”level”.init section内。这些不同level的section按level等级高低依次存放。

附：模块相关定义

内核内置模块定义

/**

* module_init() - driver initialization entry point

* @x: function to be run at kernel boot time or module insertion

*

* module_init() will either be called during do_initcalls() (if

* builtin) or at module insertion time (if a module).  There can only

* be one per module.

*/

#define module_init(x)  __initcall(x);

/**

* module_exit() - driver exit entry point

* @x: function to be run when driver is removed

*

* module_exit() will wrap the driver clean-up code

* with cleanup_module() when used with rmmod when

* the driver is a module.  If the driver is statically

* compiled into the kernel, module_exit() has no effect.

* There can only be one per module.

*/

#define module_exit(x)  __exitcall(x);

单独模块定义#define MODULE

/* Each module must use one module_init(). */

#define module_init(initfn)                 \

static inline initcall_t __inittest(void)       \

{ return initfn; }                  \

int init_module(void) __attribute__((alias(#initfn)));

/* This is only required if you want to be unloadable. */

#define module_exit(exitfn)                 \

static inline exitcall_t __exittest(void)       \

{ return exitfn; }                  \

void cleanup_module(void) __attribute__((alias(#exitfn)));

#define __setup_param(str, unique_id, fn)   /* nothing */

#define __setup(str, func)          /* nothing */

2．initcall相关调用

内核是通过do_initcalls函数循环调用执行initcall.init section内的函数的，流程如下（init/main.c）：

start_kernel -> rest_init -> kernel_thread -> kernel_init -> do_basic_setup -> do_initcalls

kernel_thread创建一个内核线程执行kernel_init函数（linux的1号进程-init进程）

extern initcall_t __initcall_start[], __initcall_end[], __early_initcall_end[];

static void __init do_initcalls(void)

{

initcall_t *call;

for (call = __early_initcall_end; call < __initcall_end; call++)

do_one_initcall(*call);

/* Make sure there is no pending stuff from the initcall sequence */

flush_scheduled_work();

}

do_initcalls()遍历.initcall*.init段，依次执行各个级别的函数。

最后要注意的是rest_init是在start_kernel函数内最后部分才被调用执行的，rest_init前包含了kernel一系列的初始化工作。另外，这些不同level等级的initcall.init section本身有一定的执行顺序，因此如果你的驱动依赖于特定的执行顺序的话需要考虑到这一点。

参考：http://linux.chinaunix.net/techdoc/develop/2008/07/19/1018489.shtml