linux-kernel - Re: [PATCH v23] pwm: airoha: Add support for EN7581 SoC

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Message-ID: <68bf2509.050a0220.702b3.c003@mx.google.com>
Date: Mon, 8 Sep 2025 20:48:38 +0200
From: Christian Marangi <ansuelsmth@...il.com>
To: Uwe Kleine-König <ukleinek@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
	Benjamin Larsson <benjamin.larsson@...exis.eu>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v23] pwm: airoha: Add support for EN7581 SoC

On Fri, Aug 01, 2025 at 11:15:41AM +0200, Uwe Kleine-König wrote:
> Hello Christian,
> 
> On Tue, Jul 08, 2025 at 04:50:52PM +0200, Christian Marangi wrote:
> > +static u32 airoha_pwm_get_period_ticks_from_ns(u32 period_ns)
> > +{
> > +	return period_ns / AIROHA_PWM_PERIOD_TICK_NS;
> > +}
> > +
> > +static u32 airoha_pwm_get_duty_ticks_from_ns(u32 period_ns, u32 duty_ns)
> > +{
> > +	return mul_u64_u32_div(duty_ns, AIROHA_PWM_DUTY_FULL, period_ns);
> > +}
> > +
> > [...]
> > +static int airoha_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > +			    const struct pwm_state *state)
> > +{
> > [...]
> > +	/*
> > +	 * Period goes at 4ns step, normalize it to check if we can
> 
> 4 ms or 4 ns?
>

4ms you are right

> > +	 * share a generator.
> > +	 */
> > +	period_ns = rounddown(period_ns, AIROHA_PWM_PERIOD_TICK_NS);
> > +
> > +	/* Convert ns to ticks */
> > +	period_ticks = airoha_pwm_get_period_ticks_from_ns(period_ns);
> 
> Rounding down to the next multiple of 4ns isn't needed for
> airoha_pwm_get_period_ticks_from_ns() which is just a division by
> AIROHA_PWM_PERIOD_TICK_NS.
> 

Ok will drop.

> > +	duty_ticks = airoha_pwm_get_duty_ticks_from_ns(period_ns, duty_ns);
> 
> As duty_ticks depends on the selected period_ticks, I think the bucket
> selection algorithm is still wrong.
> 
> Consider a request to implement
> 
> 	period_ns = 256 ms
> 	duty_ns = 128 ms
> 
> which at first correctly results in
> 
> 	period_ticks = 64
> 	duty_ticks = 127
> 
> If however all buckets are used and we only find one with say 62 period
> ticks we get period_ns = 248 and with that duty_ticks should better be
> 131 and not 127.
> 

Hi Uwe,

sorry for checking this only now and maybe we need to catch this again.

Maybe we are getting confused here but itsn't this already handled by
the upper condition?

		/* Ignore bucket with invalid configs */
		if (bucket_period_ticks > period_ticks ||
		    bucket_duty_ticks > duty_ticks)
			continue;

		/*
		 * Search for a bucket closer to the requested period/duty
		 * that has the maximal possible period that isn't bigger
		 * than the requested period. For that period pick the maximal
		 * duty cycle that isn't bigger than the requested duty_cycle.
		 */
		if (bucket_period_ticks > best_period_ticks ||
		    (bucket_period_ticks == best_period_ticks &&
		     bucket_duty_ticks > best_duty_ticks)) {
			best_period_ticks = bucket_period_ticks;
			best_duty_ticks = bucket_duty_ticks;
			best = i;
		}

We first limit for a bucket that doesn't got over both period and duty
and then we search for period and best duty. This should account for
never exceeding a duty since both period and duty are precalculated for
the current bucket and even if duty depends on period, again it's
precalculated. Am I missing something?

> > +
> > +	return airoha_pwm_config(pc, pwm, period_ticks, duty_ticks);
> > +}
> 
> Best regards
> Uwe



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