Imx6q PWM drive

One : Device tree

kernel_imx/arch/arm/boot/dts/imx6q-c-sabresd.dts

kernel_imx/arch/arm/boot/dts/imx6qdl-sabresd.dtsi

pwms = <&pwm3 0 50000>; 
    pwm3： Indicates that the backlight is connected to pwm3 On 
    0： Express index by 0,pwm3 There is only 1 individual pwm, So fill in 0
    50000： Indicates that the period is 50000ns, The frequency is 20k
default-brightness-level = <60>;
     Default backlight brightness ,60

Two ：pwm drive

pwm Driving dependency backlight Subsystem .

drive ：kernel/drivers/video/backlight/pwm_bl.c kernel_imx/drivers/video/backlight/backlight.c

pwm_backlight_probe 
    pwm_backlight_parse_dt       // analysis  dts  Medium  brightness-levels、default-brightness-level
    //RK3288  It will also be parsed here  enable-gpios , however  3399  No, ,3399  Is in  probe  with  devm_gpiod_get_optional      // obtain  enable-gpio  State of 
    devm_gpiod_get_optional  // It's actually encapsulation  gpio_request_one
    devm_gpio_request_one     // Apply for backlight enable  gpio
    devm_pwm_get ->     /drivers/pwm/core.c // To obtain a pwm
        pwm_get ->
            of_pwm_get ->
                of_parse_phandle_with_args     Parse the above dts Medium pwms attribute .
                of_node_to_pwmchip
                pwm = pc->of_xlate    // The resulting struct pwm_device type .    
    pwm_request    // apply pwm, Prevent other drivers from using .
 pwm_set_period    //pb->pwm->period = data->pwm_period_ns
    pwm_get_period    // obtain period.
    dev_set_name(&pdev->dev, "rk28_bl");    //name Can't change , User space will be used :/sys/class/backlight/rk28_bl
    backlight_device_register    -> /drivers/video/baklight/backlight.c   // Register standard backlight devices 
        device_register
        backlight_register_fb ->
            fb_register_client    //callback  yes  fb_notifier_callback 
         fb_register_client   //  Register the kernel notification chain 
    backlight_update_status ->        // Update... With default values .
        bd->ops->update_status ->
            pwm_backlight_update_status ->
                compute_duty_cycle    // Calculate the duty cycle 
                pwm_config    // To configure pwm 
                pwm_backlight_power_on    //enable Backlight 
    platform_set_drvdata // Can be  pdev  Private data saved as platform bus device , When you want to use it later, you just call  platform_get_drvdata

compute_duty_cycle Calculate the duty cycle ： 

static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
{
    /* Generally, this value is 0*/
    unsigned int lth = pb->lth_brightness;
   /* Duty cycle */
    int duty_cycle;
    /*pb->levels This form is from dts node brightness-levels From ,
     Suppose that the incoming parameters brightness yes 254, So what you get duty_cycle Namely 1,
     Without this form , Then it is directly the incoming brightness value .*/
    if (pb->levels)
        duty_cycle = pb->levels[brightness];
    else
        duty_cycle = brightness;
      
    /* Suppose here lth yes 0, So the formula is duty_cycle * pb->period / pb->scale
 pb->period That is to say dts node  pwms  The third parameter period value of is  25000
    pb->scale by pb->levels The maximum value in the array 
     So this formula is based on the following formula Android The pure value of is converted into the duty cycle corresponding to the event cycle value .*/
    return (duty_cycle * (pb->period - lth) / pb->scale) + lth;
}

Either way, it calls backlight_update_status To change the backlight ,syfs It's also . look down backlight.c,

backlight_class_init -> backlight.c
class_create // establish class, The name is backlight.

among backlight_store_brightness() The final call backlight_update_status().
There is also a case where the screen is on / Call when the screen is off , Remember to register one in front fb notify callback.

static int fb_notifier_callback(struct notifier_block *self,  
                unsigned long event, void *data)  
{  
...  
    /* Only screen on and screen off events are handled .*/  
    /* If we aren't interested in this event, skip it immediately ... */  
    if (event != FB_EVENT_BLANK && event != FB_EVENT_CONBLANK)  
        return 0;  
...  
    if (bd->ops)  
        if (!bd->ops->check_fb ||  
            bd->ops->check_fb(bd, evdata->info)) {  
            bd->props.fb_blank = *(int *)evdata->data;  
            // On screen condition   
            if (bd->props.fb_blank == FB_BLANK_UNBLANK)  
                bd->props.state &= ~BL_CORE_FBBLANK;  
            // When the screen is off   
            else  
                bd->props.state |= BL_CORE_FBBLANK;  
            backlight_update_status(bd);  
        }  
...  
}

 backlight_update_status Function implementation ：

 kernel_imx/include/linux/backlight.h

The final call bd->ops->update_status(bd);bd->ops The assignment of is in pwm_bl.c In the document backlight_device_register Function assignment .

Android6.0 Backlight Drive analysis

imx6 Backlight drive commissioning

PWM, LED, BACKLIGHT, VIBRATOR Debugging notes