[PATCH v4] pwm: iqs620a: Fix overflow and optimize calculations

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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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")
Tested-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---

Hi Jeff,

On Thu, Jan 14, 2021 at 10:07:20PM -0600, Jeff LaBundy wrote:
> Thank you for your work here; it's coming together quite nicely.

Thank you for your prompt feedback, it adds much to the fun to be able
to rely on someone who cares and looks into patches.

> On Thu, Jan 14, 2021 at 10:50:26PM +0100, Uwe Kleine-König wrote:
> >  	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).

full ack. checkpatch even warns about unsigned without int. *sigh*

> > [...]
> 
> 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>

done, thanks

These are the only changes since v3 (i.e.

 - use unsigned int instead of unsigned
 - remove empty line
 - add Tested-by

)

Best regards
Uwe

 drivers/pwm/pwm-iqs620a.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-iqs620a.c b/drivers/pwm/pwm-iqs620a.c
index 5ede8255926e..14b18fb4f527 100644
--- a/drivers/pwm/pwm-iqs620a.c
+++ b/drivers/pwm/pwm-iqs620a.c
@@ -46,7 +46,8 @@ 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 int duty_cycle;
+	unsigned int duty_scale;
 	int ret;
 
 	if (state->polarity != PWM_POLARITY_NORMAL)
@@ -70,7 +71,8 @@ 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 +84,7 @@ 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

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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