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 ", __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 ", __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; else head = &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 ", __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 ", __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 ", __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) ", __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); else srcu_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 . */ nr = prop->length / sizeof(u32); if (nr % 2) { dev_err(dev, "%s: Invalid OPP list ", __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 ", __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。