Linux电源管理-Operating Performance Points（OPP）

概述

1. 什么是Operating Performance Points？

如今复杂的Soc由多个并行工作的子模块组成。在一个执行各种用例的操作系统中，不是Soc中的所有模块都一直以其最高的执行频率工作。为了实现这一目的，Soc中的子模块被分组成域，允许一些域以较低的频率和电压运行，而其他的域运行在较高的电压和频率上。将域中每个设备支持的电压和频率的离散元组的集合称为Operating Performance Points（OPP）。

举例如下：

假设一个MPU设备支持如下的电压和频率关系：

{300MHz at minimum voltage of 1V}

{800MHz at minimum voltage of 1.2V}

{1GHz at minimum voltage of 1.3V}

用OPP表示就可以用{Hz, uV}方式表示如下:

{300000000, 1000000}

{800000000, 1200000}

{1000000000, 1300000}

2. Operating Performance Points Library

OPP library提供了一系列辅助函数去管理和查询设备的OPP信息。OPP library的源代码路径在drivers/base/power/opp.c，头文件路径在include/linux/pm_opp.h中。OPP library功能可以通过kernel config: CONFOG_PM_OPP去使能。

opp library的典型用法如下：

a. 用户为设备（比如CPU）配置/注册一些默认的opp信息。

b. Soc会根据具体的运行情况，通过opp层去改变/查询设备的opp信息。

数据结构

Linux系统使用struct dev_pm_opp结构表示一个opp描述结构

struct dev_pm_opp {struct list_head node;bool available;unsigned long rate;unsigned long u_volt;struct device_opp *dev_opp;struct rcu_head head;
};

node: 用于链表管理此设备下的opp。

available: 用于判断此opp使能可以使用。

rate: 频率，单位Hz

u_volt: 电压。

dev_opp: struct device_opp类型指针，指向此opp所属的设备。

Linux系统使用struct device_opp结构表示opp设备。

struct device_opp {struct list_head node;struct device *dev;struct srcu_notifier_head head;struct list_head opp_list;
};

.node: 用于将所有的opp设备使用dev_opp_list链表管理。

.dev: 设备指针。

.head: opp设备的通知链。

.opp_list: opp设备具有的opp数据信息。

opp layer数据组织格式

Internal data structure organization with the OPP layer library is as follows:
dev_opp_list (root)
|- device 1 (represents voltage domain 1)
|   |- opp 1 (availability, freq, voltage)
|   |- opp 2 ..
... ...
|   `- opp n ..
|- device 2 (represents the next voltage domain)
...
`- device m (represents mth voltage domain)
device 1, 2.. are represented by dev_opp structure while each opp is represented by the opp structure.

可以看到所有的opp设备使用dev_opp_list组织，在每个opp设备(struct dev_opp)中存在着不同的opp数据信息，这些opp信息使用struce dev_pm_opp表示。这样就形成一个树状的结构。查找合适的opp信息，可以从树的root节点查起。

opp library维护一张内部的列表，Soc框架只需要填充和访问opp数据。但是struct dev_opp和struct dev_pm_opp结构只是在opp library内部使用，外部不需要关心其内部。

API接口说明

添加一个opp table（dev_pm_opp_add）

int dev_pm_opp_add(struct device *dev, unsigned long freq, unsigned long u_volt)
{struct device_opp *dev_opp = NULL;struct dev_pm_opp *opp, *new_opp;struct list_head *head;/* allocate new OPP node */new_opp = kzalloc(sizeof(*new_opp), GFP_KERNEL);if (!new_opp) {dev_warn(dev, "%s: Unable to create new OPP node\n", __func__);return -ENOMEM;}/* Hold our list modification lock here */mutex_lock(&dev_opp_list_lock);/* Check for existing list for 'dev' */dev_opp = find_device_opp(dev);if (IS_ERR(dev_opp)) {/** Allocate a new device OPP table. In the infrequent case* where a new device is needed to be added, we pay this* penalty.*/dev_opp = kzalloc(sizeof(struct device_opp), GFP_KERNEL);if (!dev_opp) {mutex_unlock(&dev_opp_list_lock);kfree(new_opp);dev_warn(dev,"%s: Unable to create device OPP structure\n",__func__);return -ENOMEM;}dev_opp->dev = dev;srcu_init_notifier_head(&dev_opp->head);INIT_LIST_HEAD(&dev_opp->opp_list);/* Secure the device list modification */list_add_rcu(&dev_opp->node, &dev_opp_list);}/* populate the opp table */new_opp->dev_opp = dev_opp;new_opp->rate = freq;new_opp->u_volt = u_volt;new_opp->available = true;/** Insert new OPP in order of increasing frequency* and discard if already present*/head = &dev_opp->opp_list;list_for_each_entry_rcu(opp, &dev_opp->opp_list, node) {if (new_opp->rate <= opp->rate)break;elsehead = &opp->node;}/* Duplicate OPPs ? */if (new_opp->rate == opp->rate) {int ret = opp->available && new_opp->u_volt == opp->u_volt ?0 : -EEXIST;dev_warn(dev, "%s: duplicate OPPs detected. Existing: freq: %lu, volt: %lu, enabled: %d. New: freq: %lu, volt: %lu, enabled: %d\n",__func__, opp->rate, opp->u_volt, opp->available,new_opp->rate, new_opp->u_volt, new_opp->available);mutex_unlock(&dev_opp_list_lock);kfree(new_opp);return ret;}list_add_rcu(&new_opp->node, head);mutex_unlock(&dev_opp_list_lock);/** Notify the changes in the availability of the operable* frequency/voltage list.*/srcu_notifier_call_chain(&dev_opp->head, OPP_EVENT_ADD, new_opp);return 0;
}

此函数有三个参数，第一个参数是为那个设备添加oppinfo，第二个是频率，第三个是电压值。

1. 分配一个新的dev_pm_opp结构，用于存放传入的频率和电压数据。

2. 通过find_device_opp函数在dev_opp_list链表中查找此设备是否已经注册。如果注册直接将opp info添加到设备中。如果没有注册则重新分配一个struct device_opp结构，进行一系列初始化。

3. 填充opp数据，包括电压，频率，是否使能，所属的设备。

4. 按照递增的顺序插入opp数据到opp table中，如果插入的opp数据已经存在返回相应的结果。

5. 调用通知链，通知有新的opp添加进来。

opp 查询相关接口
dev_pm_opp_find_freq_exact（返回指定freq的opp）
dev_pm_opp_find_freq_floor（返回小于或者等于指定freq的opp，返回时从参数freq中返回实际获取的freq）
dev_pm_opp_find_freq_ceil （返回大于或者等于指定freq的opp，返回时从参数freq中返回实际获取的freq）

此三个函数大致一样，不过dev_pm_opp_find_freq_exact函数会根据参数available，返回处于disable的opp。而剩余的两个函数只返回enable的opp。

struct dev_pm_opp *dev_pm_opp_find_freq_ceil(struct device *dev,unsigned long *freq)
{struct device_opp *dev_opp;struct dev_pm_opp *temp_opp, *opp = ERR_PTR(-ERANGE);if (!dev || !freq) {dev_err(dev, "%s: Invalid argument freq=%p\n", __func__, freq);return ERR_PTR(-EINVAL);}dev_opp = find_device_opp(dev);if (IS_ERR(dev_opp))return ERR_CAST(dev_opp);list_for_each_entry_rcu(temp_opp, &dev_opp->opp_list, node) {if (temp_opp->available && temp_opp->rate >= *freq) {opp = temp_opp;*freq = opp->rate;break;}}return opp;
}

1. 得opp的设备。如果没有返回错误

2. 从dev_opp的opp_list中逐一判断，如果此频率大于等于传入的参数freq，则返回此opp。同时通过参数freq返回opp的rate值。

使能/禁止设备的opp（dev_pm_opp_enable/dev_pm_opp_disable）

static int opp_set_availability(struct device *dev, unsigned long freq,bool availability_req)
{struct device_opp *tmp_dev_opp, *dev_opp = ERR_PTR(-ENODEV);struct dev_pm_opp *new_opp, *tmp_opp, *opp = ERR_PTR(-ENODEV);int r = 0;/* keep the node allocated */new_opp = kmalloc(sizeof(*new_opp), GFP_KERNEL);if (!new_opp) {dev_warn(dev, "%s: Unable to create OPP\n", __func__);return -ENOMEM;}mutex_lock(&dev_opp_list_lock);/* Find the device_opp */list_for_each_entry(tmp_dev_opp, &dev_opp_list, node) {if (dev == tmp_dev_opp->dev) {dev_opp = tmp_dev_opp;break;}}if (IS_ERR(dev_opp)) {r = PTR_ERR(dev_opp);dev_warn(dev, "%s: Device OPP not found (%d)\n", __func__, r);goto unlock;}/* Do we have the frequency? */list_for_each_entry(tmp_opp, &dev_opp->opp_list, node) {if (tmp_opp->rate == freq) {opp = tmp_opp;break;}}if (IS_ERR(opp)) {r = PTR_ERR(opp);goto unlock;}/* Is update really needed? */if (opp->available == availability_req)goto unlock;/* copy the old data over */*new_opp = *opp;/* plug in new node */new_opp->available = availability_req;list_replace_rcu(&opp->node, &new_opp->node);mutex_unlock(&dev_opp_list_lock);kfree_rcu(opp, head);/* Notify the change of the OPP availability */if (availability_req)srcu_notifier_call_chain(&dev_opp->head, OPP_EVENT_ENABLE,new_opp);elsesrcu_notifier_call_chain(&dev_opp->head, OPP_EVENT_DISABLE,new_opp);return 0;unlock:mutex_unlock(&dev_opp_list_lock);kfree(new_opp);return r;
}

通过低三个参数availability_req去判断去enable/disable opp。

1. 分配一个新的dev_pm_opp结构。

2. 从dev_opp_list中找到opp设备，找不到opp设备就返回错误。

3. 从opp_list中通过opp的freq逐一去比较freq的值。如果找到，获得到当前的opp结构。

4. 如果当前opp的状态与传入的状态一致，则退出。

5. 将找到的opp数据拷贝到分配的新的opp中，然后设置opp的状态。

6. 替换新的opp，然后释放旧的opp数据。

7. 根据参数发送相应的OPP_EVENT_ENABLE/OPP_EVENT_DISABLE通知链。

of_init_opp_table（从dt中初始化opp table）

先列举一个opp在dt中的格式：

cpu@0 { compatible = "arm,cortex-a9";reg = <0>;next-level-cache = <&L2>;operating-points = < /* kHz    uV */ 792000  1100000 396000  950000 198000  850000 >;
};

可以看到有三组频率和电压的组合，使用operating-points节点表示。从dt中解析opp的代码如下：

int of_init_opp_table(struct device *dev)
{const struct property *prop;const __be32 *val;int nr;prop = of_find_property(dev->of_node, "operating-points", NULL);if (!prop)return -ENODEV;if (!prop->value)return -ENODATA;/** Each OPP is a set of tuples consisting of frequency and* voltage like <freq-kHz vol-uV>.*/nr = prop->length / sizeof(u32);if (nr % 2) {dev_err(dev, "%s: Invalid OPP list\n", __func__);return -EINVAL;}val = prop->value;while (nr) {unsigned long freq = be32_to_cpup(val++) * 1000;unsigned long volt = be32_to_cpup(val++);if (dev_pm_opp_add(dev, freq, volt))dev_warn(dev, "%s: Failed to add OPP %ld\n",__func__, freq);nr -= 2;}return 0;
}

找到operation-points节点，调用dev_pm_opp_add函数添加opp数据。