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

[Uwe Kleine-König]

On Sun, Jan 10, 2021 at 07:14:17PM +0200, Baruch Siach wrote:
> 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 at armlinux.org.uk>
> >> Fixes: 757642f9a584e ("gpio: mvebu: Add limited PWM support")
> >> Signed-off-by: Baruch Siach <baruch at tkos.co.il>
> >> ---
> >> 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?

It depends on how the hardware behaves in this case. One measure is to
use round-up (in .get_state) for the divisions -- as I already pointed
out. Then the problem only triggers if both on and off registers are
zero.

> > 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?

It would be good to understand and document this. Hardware PWMs not
supporting a zero duty cycle are always a surprise for me and probably
also others.

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

Yes, having this patch in a series fixing all the stuff I pointed out
would still be very welcome :-)

> 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?

Hmm, I thought keyservers are out of fashion. Anyhow, I updated my key
there. There is no WKD for @pengutronix.de addresses, but

	gpg --locate-external-keys uwe at kleine-koenig.org

should work just fine, too.

Best regards
Uwe

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