Linux内核Thermal框架详解十二、Thermal Governor（2）

本文部分内容参考

万字长文 | Thermal框架源码剖析，

Linux Thermal机制源码分析之框架概述_不捡风筝的玖伍贰柒的博客-CSDN博客，

“热散由心静，凉生为室空” - linux温控的那些事儿_内核工匠的博客-CSDN博客

特此致谢！

接前一篇文章Linux内核Thermal框架详解十二、Thermal Governor（2）

二、具体温控策略

上一篇文章对于Linux 内核中的Thermal Governor进行了总体介绍，从本文开始将对于具体的5种策略的每一种进行介绍及详解。

采用由浅入深、由易到难的方法，先介绍最为简单的bang_bang策略。

1. bang_bang

bang_bang是在使用风扇进行散热的设备场景中的算法。bang_bang governor的降温策略跟它的名字一样简单，就下边两条（也可以合并成一条）：

当throttle发生，打开风扇；
当throttle解除，关闭风扇。

首先需要确定throttle即温控是否触发。触发的情况又包括了两种细分情况：（1）当前温度大于温控阈值；（2）当前温度小于温控阈值但是大于滞后温度(温控解除温度)，并且是处于降温的过程中。

如下流程所示：

bang_bang governor的代码在drivers/thermal/gov_bang_bang.c中，一共才120行，有效代码不到100行。如下所示：

#include <linux/thermal.h>#include "thermal_core.h"static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
{int trip_temp, trip_hyst;struct thermal_instance *instance;tz->ops->get_trip_temp(tz, trip, &trip_temp);if (!tz->ops->get_trip_hyst) {pr_warn_once("Undefined get_trip_hyst for thermal zone %s - ""running with default hysteresis zero\n", tz->type);trip_hyst = 0;} elsetz->ops->get_trip_hyst(tz, trip, &trip_hyst);dev_dbg(&tz->device, "Trip%d[temp=%d]:temp=%d:hyst=%d\n",trip, trip_temp, tz->temperature,trip_hyst);mutex_lock(&tz->lock);list_for_each_entry(instance, &tz->thermal_instances, tz_node) {if (instance->trip != trip)continue;/* in case fan is in initial state, switch the fan off */if (instance->target == THERMAL_NO_TARGET)instance->target = 0;/* in case fan is neither on nor off set the fan to active */if (instance->target != 0 && instance->target != 1) {pr_warn("Thermal instance %s controlled by bang-bang has unexpected state: %ld\n",instance->name, instance->target);instance->target = 1;}/** enable fan when temperature exceeds trip_temp and disable* the fan in case it falls below trip_temp minus hysteresis*/if (instance->target == 0 && tz->temperature >= trip_temp)instance->target = 1;else if (instance->target == 1 &&tz->temperature <= trip_temp - trip_hyst)instance->target = 0;dev_dbg(&instance->cdev->device, "target=%d\n",(int)instance->target);mutex_lock(&instance->cdev->lock);instance->cdev->updated = false; /* cdev needs update */mutex_unlock(&instance->cdev->lock);}mutex_unlock(&tz->lock);
}/*** bang_bang_control - controls devices associated with the given zone* @tz: thermal_zone_device* @trip: the trip point** Regulation Logic: a two point regulation, deliver cooling state depending* on the previous state shown in this diagram:**                Fan:   OFF    ON**                              |*                              |*          trip_temp:    +---->+*                        |     |        ^*                        |     |        |*                        |     |   Temperature* (trip_temp - hyst):    +<----+*                        |*                        |*                        |**   * If the fan is not running and temperature exceeds trip_temp, the fan*     gets turned on.*   * In case the fan is running, temperature must fall below*     (trip_temp - hyst) so that the fan gets turned off again.**/
static int bang_bang_control(struct thermal_zone_device *tz, int trip)
{struct thermal_instance *instance;thermal_zone_trip_update(tz, trip);mutex_lock(&tz->lock);list_for_each_entry(instance, &tz->thermal_instances, tz_node)thermal_cdev_update(instance->cdev);mutex_unlock(&tz->lock);return 0;
}static struct thermal_governor thermal_gov_bang_bang = {.name     = "bang_bang",.throttle  = bang_bang_control,
};
THERMAL_GOVERNOR_DECLARE(thermal_gov_bang_bang);

麻雀虽小，五脏俱全。别看代码行数比较少，但是背后的机制却并不简单。一段一段来进行分析。

（1）THERMAL_GOVERNOR_DECLARE相关代码

先来看THERMAL_GOVERNOR_DECLARE。它是一个宏定义，在drivers/thermal/thermal_core.h中，代码如下：

/* Init section thermal table */
extern struct thermal_governor *__governor_thermal_table[];
extern struct thermal_governor *__governor_thermal_table_end[];#define THERMAL_TABLE_ENTRY(table, name)         \static typeof(name) *__thermal_table_entry_##name  \__used __section("__" #table "_thermal_table") = &name#define THERMAL_GOVERNOR_DECLARE(name)  THERMAL_TABLE_ENTRY(governor, name)

实际上这段代码在前文Linux内核Thermal框架详解四、Thermal Core（3）中已经进行了详细分析，这里就不再赘述了。不过为了便于理解和加深印象，将bang_bang governor展开后的代码再次列出：

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

Thermal Governor都是通过THERMAL_GOVERNOR_DECLARE定义到了__governor_thermal_table这段空间内。然后在thermal core初始化时通过调用thermal_register_governors来注册到thermal_governor_list链表中。再之后通过经由“thermal_init->thermal_register_governors-> thermal_set_governor”路径和thermal zone device关联上。

（2）handle_non_critical_trips

struct thermal_governor中有一个成员throttle，其是一个函数指针：

int (*throttle)(struct thermal_zone_device *tz, int trip);

对于对象thermal_gov_bang_bang来说，指向了bang_bang_control函数。在解析bang_bang_control函数之前，有一个问题必须弄清楚：这个函数是何时被调用的？

是在drivers/thermal/thermal_core.c的handle_non_critical_trips函数中，代码如下：

static void handle_non_critical_trips(struct thermal_zone_device *tz, int trip)
{tz->governor ? tz->governor->throttle(tz, trip) :def_governor->throttle(tz, trip);
}

那么又是哪里调用的handle_non_critical_trips？是在drivers/thermal/thermal_core.c的handle_thermal_trip函数中，代码如下：

static void handle_thermal_trip(struct thermal_zone_device *tz, int trip)
{enum thermal_trip_type type;int trip_temp, hyst = 0;/* Ignore disabled trip points */if (test_bit(trip, &tz->trips_disabled))return;tz->ops->get_trip_temp(tz, trip, &trip_temp);tz->ops->get_trip_type(tz, trip, &type);if (tz->ops->get_trip_hyst)tz->ops->get_trip_hyst(tz, trip, &hyst);if (tz->last_temperature != THERMAL_TEMP_INVALID) {if (tz->last_temperature < trip_temp &&tz->temperature >= trip_temp)thermal_notify_tz_trip_up(tz->id, trip,tz->temperature);if (tz->last_temperature >= trip_temp &&tz->temperature < (trip_temp - hyst))thermal_notify_tz_trip_down(tz->id, trip,tz->temperature);}if (type == THERMAL_TRIP_CRITICAL || type == THERMAL_TRIP_HOT)handle_critical_trips(tz, trip, type);elsehandle_non_critical_trips(tz, trip);/** Alright, we handled this trip successfully.* So, start monitoring again.*/monitor_thermal_zone(tz);
}

对于handle_thermal_trip函数的详细分析有专门的文章章节，由于本篇文章专注于bang_bang governor，故在此不深入展开。

（3）bang_bang_control

再贴一下此函数代码：

/*** bang_bang_control - controls devices associated with the given zone* @tz: thermal_zone_device* @trip: the trip point** Regulation Logic: a two point regulation, deliver cooling state depending* on the previous state shown in this diagram:**                Fan:   OFF    ON**                              |*                              |*          trip_temp:    +---->+*                        |     |        ^*                        |     |        |*                        |     |   Temperature* (trip_temp - hyst):    +<----+*                        |*                        |*                        |**   * If the fan is not running and temperature exceeds trip_temp, the fan*     gets turned on.*   * In case the fan is running, temperature must fall below*     (trip_temp - hyst) so that the fan gets turned off again.**/
static int bang_bang_control(struct thermal_zone_device *tz, int trip)
{struct thermal_instance *instance;thermal_zone_trip_update(tz, trip);mutex_lock(&tz->lock);list_for_each_entry(instance, &tz->thermal_instances, tz_node)thermal_cdev_update(instance->cdev);mutex_unlock(&tz->lock);return 0;
}

函数注释已经将函数功能说得很清楚了：控制与给定thermal zone相关联的设备。调节逻辑如下：

两点调节，传递基于前一状态的冷却状态（cooling state）。前一状态参见函数代码中用字符画出的图。

若风扇并未运行并且温度超过了trip_temp（触发点温度），则风扇打开；

若风扇已在运行，温度必须降至(trip_temp - hyst)（触发点温度 - 滞回），风扇才能关闭。

（4）thermal_zone_trip_update

bang_bang_control函数中调用了thermal_zone_trip_update函数，这个函数就在bang_bang_control函数的上边，再次贴出其代码：

static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
{int trip_temp, trip_hyst;struct thermal_instance *instance;tz->ops->get_trip_temp(tz, trip, &trip_temp);if (!tz->ops->get_trip_hyst) {pr_warn_once("Undefined get_trip_hyst for thermal zone %s - ""running with default hysteresis zero\n", tz->type);trip_hyst = 0;} elsetz->ops->get_trip_hyst(tz, trip, &trip_hyst);dev_dbg(&tz->device, "Trip%d[temp=%d]:temp=%d:hyst=%d\n",trip, trip_temp, tz->temperature,trip_hyst);mutex_lock(&tz->lock);list_for_each_entry(instance, &tz->thermal_instances, tz_node) {if (instance->trip != trip)continue;/* in case fan is in initial state, switch the fan off */if (instance->target == THERMAL_NO_TARGET)instance->target = 0;/* in case fan is neither on nor off set the fan to active */if (instance->target != 0 && instance->target != 1) {pr_warn("Thermal instance %s controlled by bang-bang has unexpected state: %ld\n",instance->name, instance->target);instance->target = 1;}/** enable fan when temperature exceeds trip_temp and disable* the fan in case it falls below trip_temp minus hysteresis*/if (instance->target == 0 && tz->temperature >= trip_temp)instance->target = 1;else if (instance->target == 1 &&tz->temperature <= trip_temp - trip_hyst)instance->target = 0;dev_dbg(&instance->cdev->device, "target=%d\n",(int)instance->target);mutex_lock(&instance->cdev->lock);instance->cdev->updated = false; /* cdev needs update */mutex_unlock(&instance->cdev->lock);}mutex_unlock(&tz->lock);
}

thermal_zone_trip_update函数做了以下几件事情：

1）获取触发温度

通过tz->ops->get_trip_temp(tz, trip, &trip_temp)获取所属thermal zone的trip temp。

2）获取滞回区间

如果所属thermal zone提供了获取滞回的函数，则调用此函数tz->ops->get_trip_hyst(tz, trip, &trip_hyst)；否则滞回区间默认为0。

3）设置初始状态以及异常状态下的值

遍历该thermal zone下的各个instance，将初始状态下（instance->target为THERMAL_NO_TARGET）的值由THERMAL_NO_TARGET设置为0。THERMAL_NO_TARGET为宏定义，在drivers/thermal/thermal_core.h中，如下所示：

/* Initial state of a cooling device during binding */
#define THERMAL_NO_TARGET -1UL

并且将那些风扇状态既不是关闭（instance->target != 0）又不是打开（instance->target != 1）的状态设置为打开（instance->target = 1）。

4）按照函数说明中的策略更新状态

对应代码如下：

/** enable fan when temperature exceeds trip_temp and disable* the fan in case it falls below trip_temp minus hysteresis*/
if (instance->target == 0 && tz->temperature >= trip_temp)instance->target = 1;
else if (instance->target == 1 &&tz->temperature <= trip_temp - trip_hyst)instance->target = 0;

这段代码就对应了函数说明中的规则：

若风扇并未运行并且温度超过了trip_temp（触发点温度），则风扇打开；若风扇已在运行，温度必须降至(trip_temp - hyst)（触发点温度 - 滞回），风扇才能关闭。

不过此处还只是设置了instance->target的值，实际的操作风扇的动作要由之后的函数来完成。

5）设置instance的cdev状态

mutex_lock(&instance->cdev->lock);
instance->cdev->updated = false; /* cdev needs update */
mutex_unlock(&instance->cdev->lock);

设置instance对应的cdev的状态为false，即需要更新。为接下来控制风扇动作的函数做准备。

（5）thermal_cdev_update

thermal_cdev_update函数在drivers/thermal/thermal_helpers.c中，代码如下：

/*** thermal_cdev_update - update cooling device state if needed* @cdev:    pointer to struct thermal_cooling_device** Update the cooling device state if there is a need.*/
void thermal_cdev_update(struct thermal_cooling_device *cdev)
{mutex_lock(&cdev->lock);if (!cdev->updated) {__thermal_cdev_update(cdev);cdev->updated = true;}mutex_unlock(&cdev->lock);
}
EXPORT_SYMBOL(thermal_cdev_update);

工作实际是交给了__thermal_cdev_update函数来完成。该函数就在它的上边，代码如下：

void __thermal_cdev_update(struct thermal_cooling_device *cdev)
{struct thermal_instance *instance;unsigned long target = 0;/* Make sure cdev enters the deepest cooling state */list_for_each_entry(instance, &cdev->thermal_instances, cdev_node) {dev_dbg(&cdev->device, "zone%d->target=%lu\n",instance->tz->id, instance->target);if (instance->target == THERMAL_NO_TARGET)continue;if (instance->target > target)target = instance->target;}thermal_cdev_set_cur_state(cdev, target);trace_cdev_update(cdev, target);dev_dbg(&cdev->device, "set to state %lu\n", target);
}

thermal_cdev_set_cur_state函数又在__thermal_cdev_update函数的上边，代码如下：

static void thermal_cdev_set_cur_state(struct thermal_cooling_device *cdev,int target)
{if (cdev->ops->set_cur_state(cdev, target))return;thermal_notify_cdev_state_update(cdev->id, target);thermal_cooling_device_stats_update(cdev, target);
}

thermal_notify_cdev_state_update函数在drivers/thermal/thermal_netlink.c中，代码如下：

int thermal_notify_cdev_state_update(int cdev_id, int cdev_state)
{struct param p = { .cdev_id = cdev_id, .cdev_state = cdev_state };return thermal_genl_send_event(THERMAL_GENL_EVENT_CDEV_STATE_UPDATE, &p);
}

thermal_netlink是不是很眼熟，在“当初”介绍thermal_init函数的时候分析过，此处用上了。正所谓“草蛇灰线，伏脉千里”。

thermal_cooling_device_stats_update函数在drivers/thermal/thermal_sysfs.c中，代码如下；

void thermal_cooling_device_stats_update(struct thermal_cooling_device *cdev,unsigned long new_state)
{struct cooling_dev_stats *stats = cdev->stats;if (!stats)return;spin_lock(&stats->lock);if (stats->state == new_state)goto unlock;update_time_in_state(stats);stats->trans_table[stats->state * stats->max_states + new_state]++;stats->state = new_state;stats->total_trans++;unlock:spin_unlock(&stats->lock);
}

__thermal_cdev_update函数中的trace_cdev_update函数在include/trace/events/thermal.h中，代码如下：

TRACE_EVENT(cdev_update,TP_PROTO(struct thermal_cooling_device *cdev, unsigned long target),TP_ARGS(cdev, target),TP_STRUCT__entry(__string(type, cdev->type)__field(unsigned long, target)),TP_fast_assign(__assign_str(type, cdev->type);__entry->target = target;),TP_printk("type=%s target=%lu", __get_str(type), __entry->target)
);

后边这几个函数只列出源码了，不做详细分析。通过后边这几个函数应该能看出或者感到：具体完成控制风扇打开或关闭的动作并不是一蹴而就、在Thermal框架中完成的，而是通过Linux内核的通知机制异步来完成的。

至此，bang_bang governor就全部分析完了。