Message-ID: <Y75ss+aSCjx7E9SZ@wendy>
Date:   Wed, 11 Jan 2023 08:00:51 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
CC:     Conor Dooley <conor@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        <linux-kernel@...r.kernel.org>, <linux-pwm@...r.kernel.org>,
        <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v13 1/2] pwm: add microchip soft ip corePWM driver

Hey Uwe,

On Wed, Jan 11, 2023 at 08:02:50AM +0100, Uwe Kleine-König wrote:
> On Wed, Jan 11, 2023 at 12:15:29AM +0000, Conor Dooley wrote:
> > On Tue, Jan 10, 2023 at 11:48:05PM +0100, Uwe Kleine-König wrote:
> > > On Wed, Dec 21, 2022 at 11:29:12AM +0000, Conor Dooley wrote:
> > > > From: Conor Dooley <conor.dooley@...rochip.com>
> > 
> > > > +		delay_us = DIV_ROUND_UP_ULL(remaining_ns, NSEC_PER_USEC);
> > > > +		if ((delay_us / 1000) > MAX_UDELAY_MS)
> > > > +			msleep(delay_us / 1000 + 1);
> > > 
> > > Is this better than
> > > 
> > > 	msleep(DIV_ROUND_UP(delay_us, 1000);
> > > 
> > > ? Also I wonder about your usage of MAX_UDELAY_MS. This is about
> > 
> > I probably started hacking on the example you gave and didn't notice
> > the U. What I have here is ~what you suggested last time.
> 
> A series with (up to now) 13 revisions and long delays between the
> review rounds (which are mostly attributed to my time schedule) is
> difficult to handle on both sides. Some repetition isn't easy to prevent
> in such a case. Sorry for that.

It is what it is, you've only got so much time :)

> > > udelay() but you're using usleep_range()?
> > > 
> > > > +		else
> > > > +			usleep_range(delay_us, delay_us * 2);
> > > 
> > > I wonder if there isn't a function that implements something like
> > > 
> > > 	wait_until(mchp_core_pwm->update_timestamp);
> > > 
> > > which would be a bit nicer than doing this by hand. Maybe fsleep()?
> > 
> > That'd be fsleep(delay_us), but does at least clean up some of the
> > messing.
> > 
> > > > +static void mchp_core_pwm_apply_duty(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > +				     const struct pwm_state *state, u64 duty_steps,
> > > > +				     u8 period_steps)
> > > > +{
> > > > +	struct mchp_core_pwm_chip *mchp_core_pwm = to_mchp_core_pwm(chip);
> > > > +	u8 posedge, negedge;
> > > > +	u8 period_steps_val = PREG_TO_VAL(period_steps);
> > > > +
> > > > +	/*
> > > > +	 * Setting posedge == negedge doesn't yield a constant output,
> > > > +	 * so that's an unsuitable setting to model duty_steps = 0.
> > > > +	 * In that case set the unwanted edge to a value that never
> > > > +	 * triggers.
> > > > +	 */
> > > > +	if (state->polarity == PWM_POLARITY_INVERSED) {
> > > > +		negedge = !duty_steps ? period_steps_val : 0u;
> > > 
> > > IMHO
> > > 
> > > 		negedge = duty_steps ? 0 : period_steps_val;
> > > 
> > > is a bit easier to parse.
> > > 
> > > > +		posedge = duty_steps;
> > > > +	} else {
> > > > +		posedge = !duty_steps ? period_steps_val : 0u;
> > > > +		negedge = duty_steps;
> > > > +	}
> > > 
> > > The following code is equivalent:
> > > 
> > > 	u8 first_edge = 0, second_edge = duty_steps;
> > > 
> > > 	/*
> > > 	 * Setting posedge == negedge doesn't yield a constant output,
> > > 	 * so that's an unsuitable setting to model duty_steps = 0.
> > > 	 * In that case set the unwanted edge to a value that never
> > > 	 * triggers.
> > > 	 */
> > > 	if (duty_steps == 0)
> > > 		first_edge = period_steps_val;
> > > 
> > > 	if (state->polarity == PWM_POLARITY_INVERSED) {
> > > 		negedge = first_edge;
> > > 		posedge = second_edge;
> > > 	} else {
> > > 		posedge = first_edge;
> > > 		negedge = second_edge;
> > > 	}
> > > 
> > > I'm not sure if it's easier to understand. What do you think?
> > 
> > Despite having used them, I dislike ternary statements.
> 
> My variant is a bit longer and uses more variables, but has less
> repetition. I don't expect a relevant change on the generated code. I
> slightly prefer my variant, but I let you choose which one you prefer.

Yah, I prefer anything that doesn't have ternarys in it.

> > > > +	writel_relaxed(posedge, mchp_core_pwm->base + MCHPCOREPWM_POSEDGE(pwm->hwpwm));
> > > > +	writel_relaxed(negedge, mchp_core_pwm->base + MCHPCOREPWM_NEGEDGE(pwm->hwpwm));
> > > > +}
> > > > +
> > > > +static void mchp_core_pwm_calc_period(const struct pwm_state *state, unsigned long clk_rate,
> > > > +				      u16 *prescale, u8 *period_steps)
> > > > +{
> > > > +	u64 tmp;
> > > > +
> > > > +	/*
> > > > +	 * Calculate the period cycles and prescale values.
> > > > +	 * The registers are each 8 bits wide & multiplied to compute the period
> > > > +	 * using the formula:
> > > > +	 * (clock_period) * (prescale + 1) * (period_steps + 1)
> > > > +	 * so the maximum period that can be generated is 0x10000 times the
> > > > +	 * period of the input clock.
> > > > +	 * However, due to the design of the "hardware", it is not possible to
> > > > +	 * attain a 100% duty cycle if the full range of period_steps is used.
> > > > +	 * Therefore period_steps is restricted to 0xFE and the maximum multiple
> > > > +	 * of the clock period attainable is 0xFF00.
> > > > +	 */
> > > > +	tmp = mul_u64_u64_div_u64(state->period, clk_rate, NSEC_PER_SEC);
> > > > +
> > > > +	/*
> > > > +	 * The hardware adds one to the register value, so decrement by one to
> > > > +	 * account for the offset
> > > > +	 */
> > > > +	if (tmp >= MCHPCOREPWM_PERIOD_MAX) {
> > > > +		*prescale = MCHPCOREPWM_PRESCALE_MAX - 1;
> > > > +		*period_steps = MCHPCOREPWM_PERIOD_STEPS_MAX - 1;
> > > > +
> > > > +		return;
> > > > +	}
> > > > +
> > > > +	*prescale = div_u64(tmp, MCHPCOREPWM_PERIOD_STEPS_MAX);
> > > > +	/* PREG_TO_VAL() can produce a value larger than UINT8_MAX */
> > > > +	*period_steps = div_u64(tmp, PREG_TO_VAL(*prescale)) - 1;
> > > 
> > > This looks wrong, but I didn't think long about that. Did we discuss
> > > this already and/or are you sure this is correct?
> > 
> > We did discuss it previously AFAICT;
> > https://lore.kernel.org/linux-pwm/896d73ac-05af-8673-8379-29011800be83@microchip.com/
> > 
> > [...]
> > Unfortunately, I don't think I am seeing what you're seeing.
> 
> Well, the calculation lands in the right ballpark for sure, but if my
> intuition is right, it's not as exact as it could be. I need some time
> with pencil and paper ...
> 
> > [...]
> > Perhaps I need to watch a lecture on how to write a PWM driver since I
> > am clearly no good at it, given the 15 revisions. Do you know of any?
> 
> I'm not aware of such a lecture.

I thought you were doing one at FOSDEM!

> I'm willing to take the blame for some
> of the revisions because I'm very picky and the math involved here isn't
> trivial.

I'd rather the maths was right. Fixing it up front is better than trying
to debug it from a customer complaint - so thanks for that.
And the maths was very naive to begin with, although I only submitted it
in the summer, I think I wrote the driver prior to having upstreamed a
single patch, and I think that showed in the maths.

> And I sometimes wonder about myself pointing out an issue in
> (say) v5 which was there unnoticed already in v1.

I dunno. I feel bad if I do that too - but if the problem you didn't
notice earlier on is a bug, rather than some sort of style comment, I
don't see why you wouldn't call it out.

> In sum a patch series going through such a high number of revisions is
> mostly a good sign.

Or that it was poor to begin with & barely improved over time...

> In the end we can be sure that the merged code is
> checked deep-rootedly and that both you and me have a certain amount of
> endurance. :-)

I wasn't really blaming you for the number of revisions, the number of
silly mistakes I've made really irritates me.

Thanks,
Conor.


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