轉自:http://blog.chinaunix.net/uid-12567959-id-161015.html
在內核代碼裏到處都能看到這個subsys_initcall(),而它到底是幹什麼的呢?讓我們來揭開它的神祕面紗。
先來看一段代碼:
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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)
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類似於subsys_initcall()還有很多,但它們都依賴於__define_initcall(),再來看__define_initcall()的定義:
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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
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__define_initcall()宏只是定義一個initcall_t類型的靜態變量,並且聲明要把這個靜態變量放在特定的段裏而已。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。
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裏,子段的排列順序則由鏈接決定。爲了向後兼容,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裏。
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:
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include/asm-generic/vmlinux.lds.h
606 #define INIT_CALLS \
607 VMLINUX_SYMBOL(__initcall_start) = .; \
608 INITCALLS \
609 VMLINUX_SYMBOL(__initcall_end) = .;
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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:
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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)
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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,其定義爲:
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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 }
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do_initcalls()又調用do_one_initcall()函數類處理這些調用。
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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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