轉自：http://blog.chinaunix.net/uid-12567959-id-161015.html

在內核代碼里到處都能看到這個subsys_initcall()，而它到底是干什么的呢？讓我們來揭開它的神秘面紗。

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先來看一段代碼：

---------------------------------------------------------------------

include/linux/init.h

174 /*

175??* Early initcalls run before initializing SMP.

176??*

177??* Only for built-in code, not modules.

178??*/

179 #define early_initcall(fn)????__define_initcall("early",fn,early)

180

181 /*

182??* A "pure" initcall has no dependencies on anything else, and purely

183??* initializes variables that couldn't be statically initialized.

184??*

185??* This only exists for built-in code, not for modules.

186??*/

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

188

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------------------

類似于subsys_initcall()還有很多，但它們都依賴于__define_initcall()，再來看__define_initcall()的定義：

---------------------------------------------------------------------

include/linux/init.h

?

131 typedef int (*initcall_t)(void);

?

165??*

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

167??* initcalls can point at the same handler without causing duplicate-symbol build errors.

168??*/

169

170 #define __define_initcall(level,fn,id) \

171???????static initcall_t __initcall_##fn##id __used \

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

173

---------------------------------------------------------------------

__define_initcall()宏只是定義一個initcall_t類型的靜態變量，并且聲明要把這個靜態變量放在特定的段里而已。上面我們看到initcall_t即是指向一個無參數有int返回值的函數的指針。

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許多的子系統都有自己的初始化函數，而這些初始化的函數又根據功能不同被分開在不同的子段里，子段的排列順序則由鏈接決定。為了向后兼容，initcall()把調用，也就是一個個指向初始化函數的函數指針放進設備初始化子段里。

?

在各個平臺的鏈接腳本文件arch/xxx/kernel/vmlinux.lds.S中，我們總能看到下面的語句：

INIT_CALLS

這個宏有如下的定義：

---------------------------------------------------------------------

include/asm-generic/vmlinux.lds.h

606 #define INIT_CALLS???????????????????????????????????????????\

607?????????????????VMLINUX_SYMBOL(__initcall_start) = .;????????\

608?????????????????INITCALLS????????????????????????????????????\

609?????????????????VMLINUX_SYMBOL(__initcall_end) = .;

---------------------------------------------------------------------

INIT_CALLS即是定義一個新的段，而定義段的字段的任務則由宏INITCALLS完成：

---------------------------------------------------------------------

include/asm-generic/vmlinux.lds.h

585 #define INITCALLS????????????????????????????????????????????\

586?????????*(.initcallearly.init)???????????????????????????????\

587?????????VMLINUX_SYMBOL(__early_initcall_end) = .;????????????\

588?????????*(.initcall0.init)???????????????????????????????????\

589?????????*(.initcall0s.init)??????????????????????????????????\

590?????????*(.initcall1.init)???????????????????????????????????\

591?????????*(.initcall1s.init)??????????????????????????????????\

592?????????*(.initcall2.init)???????????????????????????????????\

593?????????*(.initcall2s.init)??????????????????????????????????\

594?????????*(.initcall3.init)???????????????????????????????????\

595?????????*(.initcall3s.init)??????????????????????????????????\

596?????????*(.initcall4.init)???????????????????????????????????\

597?????????*(.initcall4s.init)??????????????????????????????????\

598?????????*(.initcall5.init)???????????????????????????????????\

599?????????*(.initcall5s.init)??????????????????????????????????\

600?????????*(.initcallrootfs.init)??????????????????????????????\

601?????????*(.initcall6.init)???????????????????????????????????\

602?????????*(.initcall6s.init)??????????????????????????????????\

603?????????*(.initcall7.init)???????????????????????????????????\

604?????????*(.initcall7s.init)

---------------------------------------------------------------------

?

而這些初始化函數又是在何時調用的呢？我們看到start_kernel()-> rest_init()-> kernel_init()-> do_basic_setup(void)-> do_initcalls()，而正是do_initcalls()處理了這些初始化函數，其定義為：

---------------------------------------------------------------------

init/main.c

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

766

767 static void __init do_initcalls(void)

768 {

769?????????initcall_t *fn;

770

771?????????for (fn = __early_initcall_end; fn < __initcall_end; fn++)

772?????????????????do_one_initcall(*fn);

773

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

775?????????flush_scheduled_work();

776 }

---------------------------------------------------------------------

do_initcalls()又調用do_one_initcall()函數類處理這些調用。

---------------------------------------------------------------------

init/main.c

715 static char msgbuf[64];

716 static struct boot_trace_call call;

717 static struct boot_trace_ret ret;

718

719 int do_one_initcall(initcall_t fn)

720 {

721??????int count = preempt_count();

722??????ktime_t calltime, delta, rettime;

723

724??????if (initcall_debug) {

725?????????call.caller = task_pid_nr(current);

726?????????printk("calling??%pF @ %i\n", fn, call.caller);

727?????????calltime = ktime_get();

728?????????trace_boot_call(&call, fn);

729?????????enable_boot_trace();

730?????}

731

732?????ret.result = fn();

733

734?????if (initcall_debug) {

735????????disable_boot_trace();

736????????rettime = ktime_get();

737????????delta = ktime_sub(rettime, calltime);

738????????ret.duration = (unsigned long long) ktime_to_ns(delta) >> 10;

739????????trace_boot_ret(&ret, fn);

740????????printk("initcall %pF returned %d after %Ld usecs\n", fn,

741????????ret.result, ret.duration);

742?????}

743

744?????msgbuf[0] = 0;

745

746?????if (ret.result && ret.result != -ENODEV && initcall_debug)

747????????sprintf(msgbuf, "error code %d ", ret.result);

748

749?????if (preempt_count() != count) {

750????????strlcat(msgbuf, "preemption imbalance ", sizeof(msgbuf));

751????????preempt_count() = count;

752?????}

753?????if (irqs_disabled()) {

754????????strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));

755????????local_irq_enable();

756?????}

757?????if (msgbuf[0]) {

758????????printk("initcall %pF returned with %s\n", fn, msgbuf);

759?????}

760

761?????return ret.result;

762 }

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