Re: [PATCH v6 1/4] gpio: mvebu: fix pwm get_state period calculation

[Baruch Siach <baruch@xxxxxxxxxx>]

Hi Uwe,

Thanks for your review comments.

On Thu, Jan 07 2021, Uwe Kleine-König wrote:
> On Wed, Jan 06, 2021 at 09:37:37AM +0200, Baruch Siach wrote:
>> The period is the sum of on and off values.
>> 
>> Reported-by: Russell King <linux@xxxxxxxxxxxxxxx>
>> Fixes: 757642f9a584e ("gpio: mvebu: Add limited PWM support")
>> Signed-off-by: Baruch Siach <baruch@xxxxxxxxxx>
>> ---
>> v6: divide (on + off) sum to reduce rounding error (RMK)
>> ---
>>  drivers/gpio/gpio-mvebu.c | 19 ++++++++-----------
>>  1 file changed, 8 insertions(+), 11 deletions(-)
>> 
>> diff --git a/drivers/gpio/gpio-mvebu.c b/drivers/gpio/gpio-mvebu.c
>> index 672681a976f5..a912a8fed197 100644
>> --- a/drivers/gpio/gpio-mvebu.c
>> +++ b/drivers/gpio/gpio-mvebu.c
>> @@ -676,20 +676,17 @@ static void mvebu_pwm_get_state(struct pwm_chip *chip,
>>  	else
>>  		state->duty_cycle = 1;
>>  
>> +	val = (unsigned long long) u; /* on duration */
>>  	regmap_read(mvpwm->regs, mvebu_pwmreg_blink_off_duration(mvpwm), &u);
>> -	val = (unsigned long long) u * NSEC_PER_SEC;
>> +	val += (unsigned long long) u; /* period = on + off duration */
>> +	val *= NSEC_PER_SEC;
>>  	do_div(val, mvpwm->clk_rate);
>> -	if (val < state->duty_cycle) {
>> +	if (val > UINT_MAX)
>> +		state->period = UINT_MAX;
>
> state->period is an u64, so there is no reason to not use values greater
> than UINT_MAX.

I'll post a patch for that.

>> +	else if (val)
>> +		state->period = val;
>> +	else
>>  		state->period = 1;
>
> This case assigning 1 looks strange. An explanation in a comment would
> be great. I wonder if this is a hardware property or if it is only used
> to not report 0 in case that mvpwm->clk_rate is high.

I guess that this is because 0 period does not make sense for pwm. It is
like a zero divisor. What is the common behavior?

> I found a few further shortcommings in the mvebu_pwm implementation while
> looking through it:
>
>  a) The rounding problem that RMK found is also present in .apply
>
>     There we have:
>
>     	val = clk_rate * (period - duty_cycle) / NSEC_PER_SEC
>
>     while
>
>     	val = clk_rate * period / NSEC_PER_SEC - on
>
>     would be more exact.

I'll post a patch for that.

>  b) To make
>
>  	pwm_get_state(pwm, &state);
> 	pwm_apply_state(pwm, &state);
>
>     idempotent .get_state should round up the division results.

I'll post a patch for that as well.

>  c) .apply also has a check for val being zero and configures at least 1
>     cycle for the on and off intervals. Is this a hardware imposed
>     limitation? 

Not sure what was the original intention. Maybe Andrew can explain.

On my hardware (Armada 8040), zero 'on' value does not work as
expected. There is a blink once in a long while. Maybe this is the
reason?

As I understand, all these issues should not block this patch, right?

BTW, the key you used to sign your message is expired since 2020-01-10
on the key server I use (keys.gnupg.net). Where can I find your updated
key?

Thanks,
baruch

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