Re: [PATCH 5/5] pwm: rcar: add workaround to output "pseudo" low level

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

Hello,

On Thu, Dec 13, 2018 at 09:47:00AM +0000, Yoshihiro Shimoda wrote:
> > From: Uwe Kleine-Konig, Sent: Friday, December 7, 2018 6:14 PM
> > On Fri, Dec 07, 2018 at 05:29:33PM +0900, Yoshihiro Shimoda wrote:
> > > +static void rcar_pwm_workaround_output_low(struct rcar_pwm_chip *rp)
> > > +{
> > > +	/*
> > > +	 * This PWM Timer cannot output low because setting 0x000 is
> > > +	 * prohibited on PWMCNT.PH0 (High-Level Period) bitfields. So, avoiding
> > > +	 * the prohibited, this function changes the value from 0 to 1 as
> > > +	 * pseudo low level.
> > > +	 *
> > > +	 * TODO: Add GPIO handling to output low level.
> > > +	 */
> > > +	if ((rp->pwmcnt & RCAR_PWMCNT_PH0_MASK) == 0)
> > > +		rp->pwmcnt |= 1;
> > 
> > In my eyes this is too broken to do. Not sure I have the complete
> > picture, but given a small period (say 2) this 1 cycle might result in
> > 50 % duty cycle. Depending on how the hardware behaves if you disable
> > it, better do this instead.
> 
> My colleague suggests that this workaround code also changes the period
> as maximum (1023) to avoid 50 % duty cycle for such a case.

A negative side effect of that is that reenabling the pwm then takes
longer, right? For my mileage even a duty cycle of 1/1023 if 0 is
requested is rather unfortunate.

> What do you think that this idea is acceptable for upstream? Or, should
> I add gpio handling that Uwe suggested?

Given that it's impossible to implement a gpio handling that results in
well defined periods only I'm not a big fan of that either.

I let Thierry the joy of deciding here.

Best regards
Uwe

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