pwm子系统

一、系统框架

内核的PWM core，向下对实际pwm控制器驱动，提供了pwm_chip,soc厂商编程控制器驱动，只需注册结构体，配置好private_data，实例化pwm_ops操作，编写具体函数即可。向上为其他驱动调用提供了统一的接口，通过pwm_device，关联pwm_chip,其他驱动或者用户程序通过接口来操作pwm_device结构体。
pwm_chip层实际上也是核心层的一部分，其主要对应一个pwm控制器，一个pwm控制器可包含多个pwm_device，针对一个pwm_chip，提供了访问pwm 控制器的方法，通过pwm_chip提供的方法，实现对pwm 控制器的配置
在硬件层，pwm控制器驱动soc厂商已经写好，我们要做的是在设备树(或者是设备树插件)中开启控制器节点，描述pwm设备节点，然后驱动中调用内核PWM提供的接口，来实现pwm驱动控制。

驱动程序分为两部分：

device部分：目前都是设备数陪住

driver部分：创建设备节点，并且提供open，close，read，write等接口供上层应用调用

二、核心层：

PWM framework非常简单，但它同样具备framework的基本特性：对上，为内核其它driver（Consumer）提供使用PWM功能的统一接口；对下，为PWM driver（Provider）提供driver开发的通用方法和API；内部，抽象并实现公共逻辑，屏蔽技术细节。下面我们通过它所提供的API，进一步认识PWM framework。

api声明见linux\include\linux\pwm.h

PWM linux framework源代码位于内核的 drivers/pwm 目录下，具体的路径如下所示：

drivers/pwm/
├── core.c--pwm子系统核心。
├── pwm-imx.c--imx的pwm_chip驱动。
└── sysfs.c--pwm子系统的pwm_class注册，pwm_chip属性，pwm_device属性等定义。

2.1、向PWM consumer提供的APIs：

对consumer而言，关注PWM的如下参数：频率，占空比，极性，开关（使能）

/* include/linux/pwm.h */
/**
 * pwm_config() - change a PWM device configuration
 * @pwm: PWM device
 * @duty_ns: "on" time (in nanoseconds)
 * @period_ns: duration (in nanoseconds) of one cycle
 *
 * Returns: 0 on success or a negative error code on failure.
 */
static inline int pwm_config(struct pwm_device *pwm, int duty_ns,
			     int period_ns);
 
/**
 * pwm_enable() - start a PWM output toggling
 * @pwm: PWM device
 *
 * Returns: 0 on success or a negative error code on failure.
 */
static inline int pwm_enable(struct pwm_device *pwm);
 
/**
 * pwm_disable() - stop a PWM output toggling
 * @pwm: PWM device
 */
static inline void pwm_disable(struct pwm_device *pwm);
 
/**
 * pwm_set_polarity() - configure the polarity of a PWM signal
 * @pwm: PWM device
 * @polarity: new polarity of the PWM signal
 *
 * Note that the polarity cannot be configured while the PWM device is
 * enabled.
 *
 * Returns: 0 on success or a negative error code on failure.
 */
static inline int pwm_set_polarity(struct pwm_device *pwm,
				   enum pwm_polarity polarity);

上面的API都以struct pwm_device类型的指针为操作句柄，该指针抽象了一个PWM设备（consumer不需要关心其内部构成），那么怎么获得PWM句柄呢？使用如下的API：

/* include/linux/pwm.h */
 
struct pwm_device *pwm_get(struct device *dev, const char *con_id);
struct pwm_device *of_pwm_get(struct device_node *np, const char *con_id);
void pwm_put(struct pwm_device *pwm);
 
struct pwm_device *devm_pwm_get(struct device *dev, const char *con_id);
struct pwm_device *devm_of_pwm_get(struct device *dev, struct device_node *np,
                                   const char *con_id);//常用!!
void devm_pwm_put(struct device *dev, struct pwm_device *pwm);

上述api中，常用struct pwm_device *devm_of_pwm_get(struct device *dev, struct device_node *np, const char *con_id);

参数：
- pwm：PWM设备的指针
- np: 要从中检索 PWM 设备的设备树节点。
- con_id: PWM 消费者的字符串标识符。如果不需要，可以为 NULL。
返回值：
- 成功：指向表示 PWM 设备的 pwm_device 结构体的指针
- 失败：错误指针（ERR_PTR）

2.2、向PWM provider提供的APIs

使用说明

初始化pwm chip：填充ops操作函数实例，就是厂家驱动代码

通过pwmchip_add接口注册到kernel中，之后的事情，pwm driver就不用操心了

int pwmchip_add(struct pwm_chip *chip);
int pwmchip_remove(struct pwm_chip *chip);

关键结构体的层级关系如下：

pwm_device仅仅表示一个pwm通道，pwm chip表示一个pwm控制器！！

/**
 * struct pwm_device - PWM channel object
 * @label: name of the PWM device
 * @flags: flags associated with the PWM device
 * @hwpwm: per-chip relative index of the PWM device
 * @pwm: global index of the PWM device
 * @chip: PWM chip providing this PWM device
 * @chip_data: chip-private data associated with the PWM device
 * @args: PWM arguments
 * @state: curent PWM channel state
 */
struct pwm_device {
	const char *label;
	unsigned long flags;
	unsigned int hwpwm;//这是 PWM 设备的相对索引，局部于芯片
	unsigned int pwm;//这是 PWM 设备的系统全局索引
	struct pwm_chip *chip;
	void *chip_data;
 
	struct pwm_args args;
	struct pwm_state state;
};

struct pwm_args { 
   unsigned int period; /* Device's nitial period */ 
   enum pwm_polarity polarity; 
}; 

struct pwm_state { 
   unsigned int period; /* PWM period (in nanoseconds) */ 
   unsigned int duty_cycle; /* PWM duty cycle (in nanoseconds) */ 
   enum pwm_polarity polarity; /* PWM polarity */ 
   bool enabled; /* PWM enabled status */ 
}

/**
 * struct pwm_chip - abstract a PWM controller
 * @dev: device providing the PWMs
 * @list: list node for internal use
 * @ops: callbacks for this PWM controller
 * @base: number of first PWM controlled by this chip
 * @npwm: number of PWMs controlled by this chip
 * @pwms: array of PWM devices allocated by the framework
 * @of_xlate: request a PWM device given a device tree PWM specifier
 * @of_pwm_n_cells: number of cells expected in the device tree PWM specifier
 */
struct pwm_chip {
	struct device *dev;//该pwm chip对应的设备，一般由pwm driver对应的platform驱动指定
	struct list_head list;//
	const struct pwm_ops *ops;//操作PWM设备的回调函数，后面会详细介绍。必须提供
	int base;//这是由此芯片控制的第一个 PWM 的编号。如果chip->base < 0，则内核将动态分配一个基数。在将该chip下所有pwm device组成radix tree时使用，只有旧的pwm_request接口会使用，因此忽略它吧，编写pwm driver不需要关心。
	unsigned int npwm;//该pwm chip可以支持的pwm channel（也可以称作pwm device由struct pwm_device表示）个数
	struct pwm_device *pwms;//保存所有pwm device的数组，kernel根据npwm自行分配，不需要driver关心。
 
	struct pwm_device * (*of_xlate)(struct pwm_chip *pc,
					const struct of_phandle_args *args);
	unsigned int of_pwm_n_cells;
};

/**
 * struct pwm_ops - PWM controller operations
 * @request: optional hook for requesting a PWM
 * @free: optional hook for freeing a PWM
 * @config: configure duty cycles and period length for this PWM
 * @set_polarity: configure the polarity of this PWM
 * @capture: capture and report PWM signal
 * @enable: enable PWM output toggling
 * @disable: disable PWM output toggling
 * @apply: atomically apply a new PWM config. The state argument
 *	   should be adjusted with the real hardware config (if the
 *	   approximate the period or duty_cycle value, state should
 *	   reflect it)
 * @get_state: get the current PWM state. This function is only
 *	       called once per PWM device when the PWM chip is
 *	       registered.
 * @dbg_show: optional routine to show contents in debugfs
 * @owner: helps prevent removal of modules exporting active PWMs
 */
struct pwm_ops {
	int (*request)(struct pwm_chip *chip, struct pwm_device *pwm);//不再使用
	void (*free)(struct pwm_chip *chip, struct pwm_device *pwm);//不再使用。
	int (*config)(struct pwm_chip *chip, struct pwm_device *pwm,
		      int duty_ns, int period_ns);
	int (*set_polarity)(struct pwm_chip *chip, struct pwm_device *pwm,
			    enum pwm_polarity polarity);
	int (*capture)(struct pwm_chip *chip, struct pwm_device *pwm,
		       struct pwm_capture *result, unsigned long timeout);
	int (*enable)(struct pwm_chip *chip, struct pwm_device *pwm);
	void (*disable)(struct pwm_chip *chip, struct pwm_device *pwm);
	int (*apply)(struct pwm_chip *chip, struct pwm_device *pwm,
		     struct pwm_state *state);
	void (*get_state)(struct pwm_chip *chip, struct pwm_device *pwm,
			  struct pwm_state *state);
#ifdef CONFIG_DEBUG_FS
	void (*dbg_show)(struct pwm_chip *chip, struct seq_file *s);
#endif
	struct module *owner;
};

关键源码分析：

/**
 * pwmchip_add() - register a new PWM chip
 * @chip: the PWM chip to add
 *
 * Register a new PWM chip. If chip->base < 0 then a dynamically assigned base
 * will be used. The initial polarity for all channels is normal.
 *
 * Returns: 0 on success or a negative error code on failure.
 */
int pwmchip_add(struct pwm_chip *chip)
{
	return pwmchip_add_with_polarity(chip, PWM_POLARITY_NORMAL);
}

/**
 * pwmchip_add_with_polarity() - register a new PWM chip
 * @chip: the PWM chip to add
 * @polarity: initial polarity of PWM channels
 *
 * Register a new PWM chip. If chip->base < 0 then a dynamically assigned base
 * will be used. The initial polarity for all channels is specified by the
 * @polarity parameter.
 *
 * Returns: 0 on success or a negative error code on failure.
 */
int pwmchip_add_with_polarity(struct pwm_chip *chip,
			      enum pwm_polarity polarity)
{
	struct pwm_device *pwm;
	unsigned int i;
	int ret;
 
	if (!chip || !chip->dev || !chip->ops || !chip->npwm)
		return -EINVAL;
 
	if (!pwm_ops_check(chip->ops))
		return -EINVAL;
 
	mutex_lock(&pwm_lock);
 
	ret = alloc_pwms(chip->base, chip->npwm);
	if (ret < 0)
		goto out;
 
	chip->pwms = kcalloc(chip->npwm, sizeof(*pwm), GFP_KERNEL);
	if (!chip->pwms) {
		ret = -ENOMEM;
		goto out;
	}
 
	chip->base = ret;
 
	for (i = 0; i < chip->npwm; i++) {
		pwm = &chip->pwms[i];
 
		pwm->chip = chip;
		pwm->pwm = chip->base + i;
		pwm->hwpwm = i;
		pwm->state.polarity = polarity;
 
		if (chip->ops->get_state)
			chip->ops->get_state(chip, pwm, &pwm->state);
 
		radix_tree_insert(&pwm_tree, pwm->pwm, pwm);
	}
 
	bitmap_set(allocated_pwms, chip->base, chip->npwm);
 
	INIT_LIST_HEAD(&chip->list);
	list_add(&chip->list, &pwm_chips);
 
	ret = 0;
 
	if (IS_ENABLED(CONFIG_OF))
		of_pwmchip_add(chip);
 
out:
	mutex_unlock(&pwm_lock);
 
	if (!ret)
		pwmchip_sysfs_export(chip);//导出sys fs节点
 
	return ret;
}

这个也比较简单，根据这个pwm控制器的channel个数，创建相应的struct pwm_device，最终加到pwm_tree链表树中。这样所有的可以使用的pwm channel都被抽象成了struct pwm_device结构体，也是我们最终的操作结构体。

三、设备驱动层：

四、应用层：

从第一章的系统框架可以看出：应用层使用PWM有两条路径可以选择

调用设备驱动来获取核心层提供的数据
利用sysfs文件系统直接调用核心层数据，这种方法可以不进行设备树的匹配和设备驱动的编写

导出chip1通道的0设备文件：echo 0 > /sys/class/pwm/pwmchip1/export
配置chip1通道0的周期： echo 10000000 > /sys/class/pwm/pwmchip1 /pwm0/period
配置chip1通道0的占空比:echo 4000000 >/sys/class/pwm/pwmchip1/pwm0/duty_cycle
配置片chip通道0使能： echo 1 > /sys/class/pwm/pwmchip1/pwm0/enable
配置片chip通道0禁能： echo 0 > /sys/class/pwm/pwmchip1/pwm0/enable
取消导出片chip通道0设备文件： echo 0 >/sys/class/pwm/pwmchip1/unexport

PWM 通道使用从 0 到pwm<n-1>的索引编号。这些数字是相对于芯片的。每个 PWM 通道导出都会在pwmchipN中创建一个pwmX目录，该目录与使用的export文件相同。X是导出的通道号。

完整的 PWM 框架 API 和 sysfs 描述可在内核源树中的Documentation/pwm.txt文件中找到。

/sys/class/pwm/
The pwm/ class sub-directory belongs to the Generic PWM Framework and provides a sysfs interface for using PWM channels

/sys/class/pwm/pwmchipN/
A /sys/class/pwm/pwmchipN directory is created for each probed PWM controller/chip where N is the base of the PWM chip.

/sys/class/pwm/pwmchipN/npwm
The number of PWM channels supported by the PWM chip.

/sys/class/pwm/pwmchipN/export
Exports a PWM channel from the PWM chip for sysfs control.Value is between 0 and /sys/class/pwm/pwmchipN/npwm - 1.

/sys/class/pwm/pwmchipN/unexport
Unexports a PWM channel.

/sys/class/pwm/pwmchipN/pwmX
A /sys/class/pwm/pwmchipN/pwmX directory is created for each exported PWM channel where X is the exported PWM channel number.

/sys/class/pwm/pwmchipN/pwmX/period
Sets the PWM signal period in nanoseconds.

/sys/class/pwm/pwmchipN/pwmX/duty_cycle
Sets the PWM signal duty cycle in nanoseconds.

/sys/class/pwm/pwmchipN/pwmX/enable
Enable/disable the PWM signal.

p17.扩展：为什么可以通过sysyfs操作pwm_哔哩哔哩_bilibili

五、debug方法：

xxx:/ # cat /sys/kernel/debug/pwm
platform/ff680030.pwm, 1 PWM device
 pwm-0   ((null)              ): period: 0 ns duty: 0 ns polarity: inverse
 
platform/ff680020.pwm, 1 PWM device
 pwm-0   ((null)              ): period: 0 ns duty: 0 ns polarity: inverse
 
platform/ff680010.pwm, 1 PWM device
 pwm-0   (backlight           ): requested enabled period: 1000000 ns duty: 723313 ns polarity: normal
xxx:/ #

ref：

Linux内核4.14版本——PWM子系统(1)_框架分析-CSDN博客

https://www.cnblogs.com/apachecn/p/18196572

https://wiki.luckfox.com/zh/Luckfox-Pico/Luckfox-Pico-PWM/

https://doc.embedfire.com/linux/imx8mm/linux_base/zh/latest/linux_app/pwm/pwm.html

https://www.cnblogs.com/toutiegongzhu/p/17697360.html