PWM Use

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

Data structure

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 */
   };

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;
   };

Configuration steps

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

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";
   };

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.

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:

(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.

(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.

(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.

(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