Re: [PATCH v5 1/4] gpio: mvebu: fix pwm get_state period calculation

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Tue, Jan 05, 2021 at 02:42:28PM +0200, Baruch Siach wrote:
> The period is the sum of on and off values.
> 
> Reported-by: Russell King <linux@armlinux.org.uk>
> Fixes: 757642f9a584e ("gpio: mvebu: Add limited PWM support")
> Signed-off-by: Baruch Siach <baruch@tkos.co.il>
> ---
>  drivers/gpio/gpio-mvebu.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpio/gpio-mvebu.c b/drivers/gpio/gpio-mvebu.c
> index 672681a976f5..ac7cb6d3702e 100644
> --- a/drivers/gpio/gpio-mvebu.c
> +++ b/drivers/gpio/gpio-mvebu.c
> @@ -679,17 +679,13 @@ static void mvebu_pwm_get_state(struct pwm_chip *chip,
>  	regmap_read(mvpwm->regs, mvebu_pwmreg_blink_off_duration(mvpwm), &u);
>  	val = (unsigned long long) u * NSEC_PER_SEC;
>  	do_div(val, mvpwm->clk_rate);
> -	if (val < state->duty_cycle) {
> +	val += state->duty_cycle;
> +	if (val > UINT_MAX)
> +		state->period = UINT_MAX;
> +	else if (val)
> +		state->period = val;
> +	else
>  		state->period = 1;

Are you sure this is the correct solution? Aren't you introducing
rounding errors?

The hardware will count to (on + off) clock ticks, so the right way
to convert that is to add the two together and then convert to
nanoseconds, which may result in a single rounding error. If you
convert each individually to nanoseconds, then you can end up with
two sets of rounding errors.

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