Re: [PATCH] pwm: pwm-samsung: Trigger manual update when disabling PWM

[Marten Lindahl <martenli@axis.com>]

On Wed, Sep 08, 2021 at 04:54:04PM +0200, Krzysztof Kozlowski wrote:
> On 08/09/2021 16:27, Mårten Lindahl wrote:
> > When duty-cycle is at full level (100%), the TCNTn and TCMPn registers
> > needs to be flushed in order to disable the signal. The PWM manual does
> > not say anything about this, but states that only clearing the TCON
> > auto-reload bit should be needed, and this seems to be true when the PWM
> > duty-cycle is not at full level. This can be observed on an Axis
> > ARTPEC-8, by running:
> > 
> >   echo <period> > pwm/period
> >   echo <period> > pwm/duty_cycle
> >   echo 1 > pwm/enable
> >   echo 0 > pwm/enable
> > 
> > Since the TCNTn and TCMPn registers are activated when enabling the PWM
> > (setting TCON auto-reload bit), and are not touched when disabling the
> > PWM, the double buffered auto-reload function seems to be still active.
> > Lowering duty-cycle, and restoring it again in between the enabling and
> > disabling, makes the disable work since it triggers a reload of the
> > TCNTn and TCMPn registers.
> > 
> > Fix this by securing a reload of the TCNTn and TCMPn registers when
> > disabling the PWM and having a full duty-cycle.
> > 
> > Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
> > ---
> >  drivers/pwm/pwm-samsung.c | 25 +++++++++++++++++++++----
> >  1 file changed, 21 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
> > index f6c528f02d43..7d90652402a0 100644
> > --- a/drivers/pwm/pwm-samsung.c
> > +++ b/drivers/pwm/pwm-samsung.c
> > @@ -105,6 +105,9 @@ struct samsung_pwm_chip {
> >  static DEFINE_SPINLOCK(samsung_pwm_lock);
> >  #endif
> >  
> > +static void __pwm_samsung_manual_update(struct samsung_pwm_chip *chip,
> > +				      struct pwm_device *pwm);
> > +
> >  static inline
> >  struct samsung_pwm_chip *to_samsung_pwm_chip(struct pwm_chip *chip)
> >  {
> > @@ -278,17 +281,21 @@ static void pwm_samsung_disable(struct pwm_chip *chip, struct pwm_device *pwm)
> >  
> >  	our_chip->disabled_mask |= BIT(pwm->hwpwm);
> >  
> > +	/*
> > +	 * In case the PWM is at 100% duty cycle, force a manual
> > +	 * update to prevent the signal from staying high.
> > +	 */
> > +	if (readl(our_chip->base + REG_TCMPB(pwm->hwpwm)) == (u32)-1U)
> > +		__pwm_samsung_manual_update(our_chip, pwm);
> 
> I did not test it but looks reasonable. Just please move it above
> setting "our_chip->disabled_mask", so the code will be close to
> pwm_samsung_enable.
> 

Hi Krzysztof!

Thank you for the quick reply! I will do so.

Kind regards
Mårten

> 
> Best regards,
> Krzysztof