Re: [PATCH v4 06/16] pwm: lpss: Correct get_state result for base_unit == 0

Uwe Kleine-König
2020-07-11T06:11:44Z

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On Thu, Jul 09, 2020 at 05:47:59PM +0200, Hans de Goede wrote:
> Hi,
>
> On 7/9/20 4:50 PM, Andy Shevchenko wrote:
> > On Wed, Jul 08, 2020 at 11:14:22PM +0200, Hans de Goede wrote:
> > > The datasheet specifies that programming the base_unit part of the
> > > ctrl register to 0 results in a contineous low signal.
> > >
> > > Adjust the get_state method to reflect this by setting pwm_state.period
> > > to 1 and duty_cycle to 0.
> >
> > ...
> >
> > > + if (freq == 0) {
> > > + /* In this case the PWM outputs a continous low signal */
> >
> > > + state->period = 1;
> >
> > I guess this should be something like half of the range (so base unit calc
> > will give 128). Because with period = 1 (too small) it will give too small
> > base unit (if apply) and as a result we get high frequency pulses.
>
> You are right, that if after this the user only changes the duty-cycle
> things will work very poorly, we will end up with a base_unit value of
> e.g 65535 and then have almost no duty-cycle resolution at all.

Is this a problem of the consumer that we don't need to solve? Are there
known consumers running into this problem?

pwm_lpss_prepare() is buggy here, a request for a too low period should be
refused.

Best regards
Uwe

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

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