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9. PWM Use

9.1. Introduction​

Firefly-RK3288 development board has 4-way PWM outputs, which are PWM0 ~ PWM3, this chapter mainly describes how to configure PWM.

The PWM driver file:

kernel/drivers/pwm/pwm-rockchip.c

9.2. Data structure​

9.2.1. pwm_device structure​

struct pwm_device {
    const char      *label;
    unsigned long       flags;
    unsigned int        hwpwm;
    unsigned int        pwm;//pwm channel
    struct pwm_chip     *chip;
    void            *chip_data;
    unsigned int        period; /* in nanoseconds */
};

9.2.2. pwm_chip structure​

This structure is a abstract PWM controller.

struct pwm_chip {
    struct device       *dev; //provide PWM device
    struct list_head    list; //internally used node list
    const struct pwm_ops    *ops; //callback function of this PWM controller
    int  base; //number of the first PWM controlled by this device
    unsigned int        npwm; //number of PWMs controlled by this device
    struct pwm_device   *pwms;
    struct pwm_device * (*of_xlate)(struct pwm_chip *pc,const struct of_phandle_args *args);
    unsigned int of_pwm_n_cells;
    bool can_sleep;
};

9.3. Configuration steps​

Configuration of PWM mainly includes the following three steps: configure PWM DTS nodes, configure PWM kernel driver, and control PWM device.

9.3.1. Configure PWM DTS node​

The following PWM nodes are defined in kernel/arch/arm/boot/dts/rk3288.dtsi, as shown below:

pwm1: pwm@ff680010 {
      compatible = "rockchip,rk-pwm";
      reg = ;
      #pwm-cells = ;
      pinctrl-names = "default";
      pinctrl-0 = ;
      clocks = ;
      clock-names = "pclk_pwm";
      status = "okay";
};

Note: ff680010 is the address of PWM1 register.

In order to use pwm1, add it into kernel/arch/arm/boot/dts/firefly-rk3288.dts:

&pwm1 {
    status = "okay";
};

9.4. Configure PWM kernel driver​

PWM driver is located on the file kernel/drivers/pwm/pwm-rockchip.c.

Modify the following codes of this file:

static const struct of_device_id rk_pwm_of_match[] = {
    { .compatible = "rockchip,pwm",       .data = &rk_pwm_data_v1,},
    { .compatible =  "rockchip,rk-pwm",   .data = &rk_pwm_data_v2,},
    { .compatible =  "rockchip,vop-pwm",  .data = &rk_pwm_data_v3,},
    { }
};

Add the compatible = “rockchip,rk-pwm” in the previous step to the above code.

9.5. Control PWM device​

The user can use the PWM node generated from the above steps in the other driver file. The specific methods are as follows:

9.5.1. (1) Include the following header file into the device driver files to be controlled by PWM​

#include <linux/pwm.h>

This header file mainly includes PWM function interface.

9.5.2. (2) Request for PWM​

struct pwm_device *pwm_request(int pwm_id, const char *label);

Function requests for PWM. For example:

struct pwm_device * pwm0 = NULL;
pwm0 = pwm_request(0, “backlight-pwm”);

Parameter pwm_id represents the channel for which PWM is to be requested, label is the label taken by this PWM.

9.5.3. (3) Configure PWM​

int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);

Configure duty ratio of PWM, for example:

pwm_config(pwm0, 500000, 1000000);

Parameter pwm represents the pwm_device requested in the previous step. duty_ns represents the activating duration of duty ratio, and its unit is ns. period_ns represents the period of PWM, and its unit is ns.

9.5.4. (4) PWM enabling function​

int pwm_enable(struct pwm_device *pwm);

It is used for enabling PWM, for example:

pwm_enable(pwm0);

Parameter pwm represents the pwm_device to be enabled.

Controlling PWM output mainly uses the following interface functions:

struct pwm_device *pwm_request(int pwm_id, const char *label);

  • Function: for requesting pwm

  • Parameter:

    • pwm_id: pwm channel to be requested.
    • label: label taken by the pwm requested.
void pwm_free(struct pwm_device *pwm);

  • Function: for releasing the requested pwm

  • Parameter:

    • pwm: pwm structure to be released
int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);

  • Function: for configuring the duty ratio of pwm

  • Parameter:

    • pwm: pwm to be configured
    • duty_ns: activated duration of pwm duty ratio, the unit is ns
    • period_ns: period of pwm duty ratio, the unit is ns
int pwm_enable(struct pwm_device *pwm);

  • Function: enable pwm

  • Parameter:

    • pwm: pwm to be enabled
void pwm_disable(struct pwm_device *pwm);

  • Function: forbid pwm

  • Parameter:

    • pwm: pwm to be forbidden