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Message-ID: <20230111092407.j45jxtukctchto3q@pengutronix.de>
Date: Wed, 11 Jan 2023 10:24:07 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Conor Dooley <conor@...nel.org>
Cc: Thierry Reding <thierry.reding@...il.com>,
Conor Dooley <conor.dooley@...rochip.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
Hello,
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.
>
> > > 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.
>
> > > > + 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 ...
Just a small heads up:
I thought a bit about that (without pencil and paper on my way to work
today), and the optimal solution is non-trivial. In fact you have to
pick parameters A and B such that for a given C
(A + 1) * (B + 1) is maximal with
0 <= A < 0x100
0 <= B < 0xff
(A + 1) * (B + 1) <= C
A consistent non-optimal choice that is easier to calculate than a
complete search would be nice. But I think we're not there yet.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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