Initcalls
do_initcalls
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static void __init do_basic_setup(void)
{
cpuset_init_smp();
driver_init();
init_irq_proc();
do_ctors();
usermodehelper_enable();
do_initcalls();
}
Now let’s go back to previous setup function do_basic_setup. Because rest of the init functions are not important in this posting, so we will directly jump into do_initcalls function.
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static void __init do_initcalls(void)
{
int level;
size_t len = strlen(saved_command_line) + 1;
char *command_line;
command_line = kzalloc(len, GFP_KERNEL);
if (!command_line)
panic("%s: Failed to allocate %zu bytes\n", __func__, len);
for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++) {
/* Parser modifies command_line, restore it each time */
strcpy(command_line, saved_command_line);
do_initcall_level(level, command_line);
}
kfree(command_line);
}
static void __init do_initcall_level(int level, char *command_line)
{
initcall_entry_t *fn;
parse_args(initcall_level_names[level],
command_line, __start___param,
__stop___param - __start___param,
level, level,
NULL, ignore_unknown_bootoption);
trace_initcall_level(initcall_level_names[level]);
for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
do_one_initcall(initcall_from_entry(fn));
}
/* Keep these in sync with initcalls in include/linux/init.h */
static const char *initcall_level_names[] __initdata = {
"pure",
"core",
"postcore",
"arch",
"subsys",
"fs",
"device",
"late",
};
typedef int (*initcall_t)(void);
int __init_or_module do_one_initcall(initcall_t fn)
{
int count = preempt_count();
char msgbuf[64];
int ret;
if (initcall_blacklisted(fn))
return -EPERM;
do_trace_initcall_start(fn);
ret = fn();
do_trace_initcall_finish(fn, ret);
msgbuf[0] = 0;
if (preempt_count() != count) {
sprintf(msgbuf, "preemption imbalance ");
preempt_count_set(count);
}
if (irqs_disabled()) {
strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));
local_irq_enable();
}
WARN(msgbuf[0], "initcall %pS returned with %s\n", fn, msgbuf);
add_latent_entropy();
return ret;
}
For each predefined level, do_initcall invokes all the relevant init functions for that level. As shown in the initcall_level_names, there are 8 levels of init, and do_initcall_level function is invoked per level. This function actually invokes all init functions stored in a particular code section dedicated for one level.
The do_one_initcall function actually invokes the initcall functions one by one. The type of function pointer of the initcalls are defined as initcall_t. The function pointer is passed from the do_initcall_level function. Each function pointer is retrieved as a result of initcall_from_entry function, and its parameter fn is retrieved from the initcall_levels array. Let’s take a look at those structures and functions one by one.
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#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
typedef int initcall_entry_t;
static inline initcall_t initcall_from_entry(initcall_entry_t *entry)
{
return offset_to_ptr(entry);
}
#else
typedef initcall_t initcall_entry_t;
static inline initcall_t initcall_from_entry(initcall_entry_t *entry)
{
return *entry;
}
#endif
extern initcall_entry_t __initcall_start[];
extern initcall_entry_t __initcall0_start[];
extern initcall_entry_t __initcall1_start[];
extern initcall_entry_t __initcall2_start[];
extern initcall_entry_t __initcall3_start[];
extern initcall_entry_t __initcall4_start[];
extern initcall_entry_t __initcall5_start[];
extern initcall_entry_t __initcall6_start[];
extern initcall_entry_t __initcall7_start[];
extern initcall_entry_t __initcall_end[];
static initcall_entry_t *initcall_levels[] __initdata = {
__initcall0_start,
__initcall1_start,
__initcall2_start,
__initcall3_start,
__initcall4_start,
__initcall5_start,
__initcall6_start,
__initcall7_start,
__initcall_end,
};
initcall_levels array consists of multiple initcall_entry_t which is a intger value imported from linux kernel header.
include/asm-generic/vmlinux.lds.h
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#define INIT_CALLS_LEVEL(level) \
__initcall##level##_start = .; \
KEEP(*(.initcall##level##.init)) \
KEEP(*(.initcall##level##s.init)) \
#define INIT_CALLS \
__initcall_start = .; \
KEEP(*(.initcallearly.init)) \
INIT_CALLS_LEVEL(0) \
INIT_CALLS_LEVEL(1) \
INIT_CALLS_LEVEL(2) \
INIT_CALLS_LEVEL(3) \
INIT_CALLS_LEVEL(4) \
INIT_CALLS_LEVEL(5) \
INIT_CALLS_LEVEL(rootfs) \
INIT_CALLS_LEVEL(6) \
INIT_CALLS_LEVEL(7) \
__initcall_end = .;
#define INIT_DATA_SECTION(initsetup_align) \
.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { \
INIT_DATA \
INIT_SETUP(initsetup_align) \
INIT_CALLS \
CON_INITCALL \
INIT_RAM_FS \
}
Linux kernel linker script defines INIT_CALLS_LEVEL macro that defines variable that contains the starting address of memory region that have all initcall of specific level. It also provides INIT_CALLS macro that populates all the memory addresses used by the initcall_level array. We can find that each _initcall##level##_start is followed by the .initcall##level##.init and .initcall##level##s.init section. Because level is a integer from 0 to 7 the section name should be from .initcall0.init to .initcall7.init Let’s try to figure out where those sections are defined, and what content are stored in that section.
include/linux/init.h
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/*
* 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
* can point at the same handler without causing duplicate-symbol build errors.
*
* Initcalls are run by placing pointers in initcall sections that the
* kernel iterates at runtime. The linker can do dead code / data elimination
* and remove that completely, so the initcall sections have to be marked
* as KEEP() in the linker script.
*/
#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
#define ___define_initcall(fn, id, __sec) \
__ADDRESSABLE(fn) \
asm(".section \"" #__sec ".init\", \"a\" \n" \
"__initcall_" #fn #id ": \n" \
".long " #fn " - . \n" \
".previous \n");
#else
#define ___define_initcall(fn, id, __sec) \
static initcall_t __initcall_##fn##id __used \
__attribute__((__section__(#__sec ".init"))) = fn;
#endif
#define __define_initcall(fn, id) ___define_initcall(fn, id, .initcall##id)
#define pure_initcall(fn) __define_initcall(fn, 0)
#define core_initcall(fn) __define_initcall(fn, 1)
#define core_initcall_sync(fn) __define_initcall(fn, 1s)
#define postcore_initcall(fn) __define_initcall(fn, 2)
#define postcore_initcall_sync(fn) __define_initcall(fn, 2s)
#define arch_initcall(fn) __define_initcall(fn, 3)
#define arch_initcall_sync(fn) __define_initcall(fn, 3s)
#define subsys_initcall(fn) __define_initcall(fn, 4)
#define subsys_initcall_sync(fn) __define_initcall(fn, 4s)
#define fs_initcall(fn) __define_initcall(fn, 5)
#define fs_initcall_sync(fn) __define_initcall(fn, 5s)
#define rootfs_initcall(fn) __define_initcall(fn, rootfs)
#define device_initcall(fn) __define_initcall(fn, 6)
#define device_initcall_sync(fn) __define_initcall(fn, 6s)
#define late_initcall(fn) __define_initcall(fn, 7)
#define late_initcall_sync(fn) __define_initcall(fn, 7s)
Following the above macros, it is easy to understand how the .initcallX.init section is generated, and each function is located in that section.
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