[PATCH linux dev-5.15] leds: pca955x: Set blink duty cycle to fifty percent

[Eddie James]

On 4/28/22 10:12, Paul Menzel wrote:
> Dear Eddie,
>
>
> Am 27.04.22 um 20:45 schrieb Eddie James:
>> In order to blink at the specified rate, the blinking LEDs
>> need to maintain a 50% duty cycle. Therefore, don't use PWM0
>> for any attempted brightness change, and set PWM0 when
>> enabling blinking.
>
> Is that specified in some datasheet?


It is in the PCA955X datasheets 
(https://www.nxp.com/docs/en/data-sheet/PCA9552.pdf) but it's not 
exactly clear. The relationship between the PWM rate and the blink rate 
is not specified, but for example, setting maximum duty cycle and 1 
second blink period results in the LED not blinking. Setting 50% duty 
cycle blinks the LED at 1 second intervals, as expected.


>
> Nit: I’d write 50 % in the commit message summary.


Ah ok. Thanks!

Eddie


>
>
> Kind regards,
>
> Paul
>
>
>> Signed-off-by: Eddie James <eajames at linux.ibm.com>
>> ---
>>   drivers/leds/leds-pca955x.c | 17 ++++++++++-------
>>   1 file changed, 10 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/leds/leds-pca955x.c b/drivers/leds/leds-pca955x.c
>> index 2570f92b6754..da93f04e4d10 100644
>> --- a/drivers/leds/leds-pca955x.c
>> +++ b/drivers/leds/leds-pca955x.c
>> @@ -289,17 +289,12 @@ static enum led_brightness 
>> pca955x_led_get(struct led_classdev *led_cdev)
>>         switch (pca955x_ledstate(ls, pca955x_led->led_num % 4)) {
>>       case PCA955X_LS_LED_ON:
>> +    case PCA955X_LS_BLINK0:
>>           ret = LED_FULL;
>>           break;
>>       case PCA955X_LS_LED_OFF:
>>           ret = LED_OFF;
>>           break;
>> -    case PCA955X_LS_BLINK0:
>> -        ret = pca955x_read_pwm(pca955x, 0, &pwm);
>> -        if (ret)
>> -            return ret;
>> -        ret = 256 - pwm;
>> -        break;
>>       case PCA955X_LS_BLINK1:
>>           ret = pca955x_read_pwm(pca955x, 1, &pwm);
>>           if (ret)
>> @@ -332,7 +327,7 @@ static int pca955x_led_set(struct led_classdev 
>> *led_cdev,
>>               clear_bit(pca955x_led->led_num, &pca955x->active_blink);
>>               ls = pca955x_ledsel(ls, bit, PCA955X_LS_LED_OFF);
>>           } else {
>> -            ret = pca955x_write_pwm(pca955x, 0, 256 - value);
>> +            /* No variable brightness for blinking LEDs */
>>               goto out;
>>           }
>>       } else {
>> @@ -432,6 +427,14 @@ static int pca955x_led_blink(struct led_classdev 
>> *led_cdev,
>>               ret = pca955x_write_ls(pca955x, reg, ls);
>>               if (ret)
>>                   goto out;
>> +
>> +            /*
>> +             * Force 50% duty cycle to maintain the specified
>> +             * blink rate.
>> +             */
>> +            ret = pca955x_write_pwm(pca955x, 0, 128);
>> +            if (ret)
>> +                goto out;
>>           }
>>             if (pca955x->blink_period != p) {