[v3] pwm: iqs620a: Fix overflow and optimize calculations

If state->duty_cycle is 0x100000000000000, the previous calculation of
duty_scale overflows and yields a duty cycle ratio of 0% instead of
100%. Fix this by clamping the requested duty cycle to the maximal
possible duty cycle first. This way it is possible to use a native
integer division instead of a (depending on the architecture) more
expensive 64bit division.

With this change in place duty_scale cannot be bigger than 256 which
allows to simplify the calculation of duty_val.

Fixes: 6f0841a8197b ("pwm: Add support for Azoteq IQS620A PWM generator")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
 drivers/pwm/pwm-iqs620a.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Hi Uwe,

Thank you for your work here; it's coming together quite nicely.

On Thu, Jan 14, 2021 at 10:50:26PM +0100, Uwe Kleine-König wrote:
> If state->duty_cycle is 0x100000000000000, the previous calculation of
> duty_scale overflows and yields a duty cycle ratio of 0% instead of
> 100%. Fix this by clamping the requested duty cycle to the maximal
> possible duty cycle first. This way it is possible to use a native
> integer division instead of a (depending on the architecture) more
> expensive 64bit division.
> 
> With this change in place duty_scale cannot be bigger than 256 which
> allows to simplify the calculation of duty_val.
> 
> Fixes: 6f0841a8197b ("pwm: Add support for Azoteq IQS620A PWM generator")
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> ---
>  drivers/pwm/pwm-iqs620a.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-iqs620a.c b/drivers/pwm/pwm-iqs620a.c
> index 5ede8255926e..eb03f60c5db8 100644
> --- a/drivers/pwm/pwm-iqs620a.c
> +++ b/drivers/pwm/pwm-iqs620a.c
> @@ -46,7 +46,9 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  {
>  	struct iqs620_pwm_private *iqs620_pwm;
>  	struct iqs62x_core *iqs62x;
> -	u64 duty_scale;
> +	unsigned duty_cycle;
> +	unsigned duty_scale;
> +
>  	int ret;
>  

Nit: there is a rogue newline here. I'm also not such a fan of mixing
shorthand 'unsigned' with existing 'unsigned int' (as in probe).

>  	if (state->polarity != PWM_POLARITY_NORMAL)
> @@ -70,7 +72,8 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	 * For lower duty cycles (e.g. 0), the PWM output is simply disabled to
>  	 * allow an external pull-down resistor to hold the GPIO3/LTX pin low.
>  	 */
> -	duty_scale = div_u64(state->duty_cycle * 256, IQS620_PWM_PERIOD_NS);
> +	duty_cycle = min_t(u64, state->duty_cycle, IQS620_PWM_PERIOD_NS);
> +	duty_scale = duty_cycle * 256 / IQS620_PWM_PERIOD_NS;
>  
>  	mutex_lock(&iqs620_pwm->lock);
>  
> @@ -82,7 +85,7 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	}
>  
>  	if (duty_scale) {
> -		u8 duty_val = min_t(u64, duty_scale - 1, 0xff);
> +		u8 duty_val = duty_scale - 1;
>  
>  		ret = regmap_write(iqs62x->regmap, IQS620_PWM_DUTY_CYCLE,
>  				   duty_val);
> -- 
> 2.29.2
> 

That being said, the patch is functionally correct and I validated all
corner cases with actual hardware. Feel free to add to future cosmetic
revisions:

Tested-by: Jeff LaBundy <jeff@labundy.com>

Kind regards,
Jeff LaBundy