Linux内核Thermal框架详解五、Thermal Core（4）
本文部分内容参考Linux Thermal 学习笔记 - 爱码网。特此致谢！
接前一篇文章Linux内核Thermal框架详解四、Thermal Core（3）
三、相关源码及分析
2. thermal_register_governors
上一回说到这一段代码：
for (__governor = __governor_thermal_table; \__governor < __governor_thermal_table_end; \__governor++) {ret = thermal_register_governor(*governor);if (ret) {pr_err("Failed to register governor: '%s'",(*governor)->name);break;}pr_info("Registered thermal governor '%s'",(*governor)->name);
}
__governor_thermal_table上一回仔细分析了其来龙去脉，现在该关注__governor这个变量了。
__governor这个变量在之前代码中出现过，如下：
struct thermal_governor **governor;这是一个二重指针，即指向指针的指针。在上述代码中，它一开始指向了__governor_thermal_table，还记得__governor_thermal_table是怎样定义的吗？
在drivers/thermal/thermal_core.h中：
extern struct thermal_governor *__governor_thermal_table[];
extern struct thermal_governor *__governor_thermal_table_end[];虽然这里是extern，但是这比include/asm-generic/vmlinux.lds.h中真正定义__governor_thermal_table的地方好理解。__governor_thermal_table实际上是一个指针数组。所以使用二重指针governor指向它也就合情合理了。
再来回顾一下各种governor策略：
- step_wise
drivers/thermal/gov_step_wise.c中：
static struct thermal_governor thermal_gov_step_wise = {.name = "step_wise",.throttle = step_wise_throttle,
};static struct thermal_governor *__thermal_table_entry_thermal_gov_step_wise \__used __section("__governor_thermal_table") = &thermal_gov_step_wise
- power_allocator
drivers/thermal/gov_power_allocator.c中：
static struct thermal_governor thermal_gov_power_allocator = {.name = "power_allocator",.bind_to_tz = power_allocator_bind,.unbind_from_tz = power_allocator_unbind,.throttle = power_allocator_throttle,
};static struct thermal_governor *__thermal_table_entry_thermal_gov_power_allocator \__used __section("__governor_thermal_table") = &thermal_gov_power_allocator
- fair_share
drivers/thermal/gov_fair_share.c中：
static struct thermal_governor thermal_gov_fair_share = {.name = "fair_share",.throttle = fair_share_throttle,
};static struct thermal_governor *__thermal_table_entry_thermal_gov_fair_share \__used __section("__governor_thermal_table") = &thermal_gov_fair_share
- user_space
drivers/thermal/gov_user_space.c中：
static struct thermal_governor thermal_gov_user_space = {.name = "user_space",.throttle = notify_user_space,.bind_to_tz = user_space_bind,
};static struct thermal_governor *__thermal_table_entry_thermal_gov_user_space \__used __section("__governor_thermal_table") = &thermal_gov_user_space
- bang_bang
drivers/thermal/gov_bang_bang.c中：
static struct thermal_governor thermal_gov_bang_bang = {.name = "bang_bang",.throttle = bang_bang_control,
};static struct thermal_governor *__thermal_table_entry_thermal_gov_bang_bang \__used __section("__governor_thermal_table") = &thermal_gov_bang_bang
弄清楚了以上细节后就能知道，本文开头的代码的意义是：遍历所有的governor策略并进行注册。注册具体都完成了哪些工作？下边接着来看。
（2）thermal_register_governor
thermal_register_governor函数同样在drivers/thermal/thermal_core.c中实现，代码如下：
int thermal_register_governor(struct thermal_governor *governor)
{int err;const char *name;struct thermal_zone_device *pos;if (!governor)return -EINVAL;mutex_lock(&thermal_governor_lock);err = -EBUSY;if (!__find_governor(governor->name)) {bool match_default;err = 0;list_add(&governor->governor_list, &thermal_governor_list);match_default = !strncmp(governor->name,DEFAULT_THERMAL_GOVERNOR,THERMAL_NAME_LENGTH);if (!def_governor && match_default)def_governor = governor;}mutex_lock(&thermal_list_lock);list_for_each_entry(pos, &thermal_tz_list, node) {/** only thermal zones with specified tz->tzp->governor_name* may run with tz->govenor unset*/if (pos->governor)continue;name = pos->tzp->governor_name;if (!strncasecmp(name, governor->name, THERMAL_NAME_LENGTH)) {int ret;ret = thermal_set_governor(pos, governor);if (ret)dev_err(&pos->device,"Failed to set governor %s for thermal zone %s: %d\n",governor->name, pos->type, ret);}}mutex_unlock(&thermal_list_lock);mutex_unlock(&thermal_governor_lock);return err;
}函数虽然不算太长，但也不算太短，还是有一些内容的。逐段来看：
一开始是判断并确保入参governor不为空。
接下来加锁mutex_lock(&thermal_governor_lock)。thermal_governor_lock在同文件（drivers/thermal/thermal_core.c）中定义并初始化，代码如下：
static DEFINE_MUTEX(thermal_governor_lock);接下来是一个判断if (!__find_governor(governor->name))。__find_governor函数同样在drivers/thermal/thermal_core.c中，代码如下：
/** Governor section: set of functions to handle thermal governors** Functions to help in the life cycle of thermal governors within* the thermal core and by the thermal governor code.*/static struct thermal_governor *__find_governor(const char *name)
{struct thermal_governor *pos;if (!name || !name[0])return def_governor;list_for_each_entry(pos, &thermal_governor_list, governor_list)if (!strncasecmp(name, pos->name, THERMAL_NAME_LENGTH))return pos;return NULL;
}要弄清楚这个函数的功能，就必须弄清楚list_for_each_entry的含义。list_for_each_entry是一个宏，在include/linux/list.h中，代码如下：
/*** list_for_each_entry - iterate over list of given type* @pos: the type * to use as a loop cursor.* @head: the head for your list.* @member: the name of the list_head within the struct.*/
#define list_for_each_entry(pos, head, member) \for (pos = list_first_entry(head, typeof(*pos), member); \!list_entry_is_head(pos, head, member); \pos = list_next_entry(pos, member))
list_first_entry当然也在include/linux/list.h中，代码如下：
/*** list_first_entry - get the first element from a list* @ptr: the list head to take the element from.* @type: the type of the struct this is embedded in.* @member: the name of the list_head within the struct.** Note, that list is expected to be not empty.*/
#define list_first_entry(ptr, type, member) \list_entry((ptr)->next, type, member)list_entry也在include/linux/linux.h中，就在list_first_entry宏定义的上边，代码如下：
/*** list_entry - get the struct for this entry* @ptr: the &struct list_head pointer.* @type: the type of the struct this is embedded in.* @member: the name of the list_head within the struct.*/
#define list_entry(ptr, type, member) \container_of(ptr, type, member)由上，list_first_entry展开为：
#define list_first_entry container_of((ptr)->next, type, member)list_entry_is_head同样在include/linux/linux.h中，代码如下：
/*** list_entry_is_head - test if the entry points to the head of the list* @pos: the type * to cursor* @head: the head for your list.* @member: the name of the list_head within the struct.*/
#define list_entry_is_head(pos, head, member) \(&pos->member == (head))
list_next_entry同样在include/linux/linux.h中，代码如下：
/*** list_next_entry - get the next element in list* @pos: the type * to cursor* @member: the name of the list_head within the struct.*/
#define list_next_entry(pos, member) \list_entry((pos)->member.next, typeof(*(pos)), member)