Message-ID: <CAHh=Yk8kC1+D4dPQ7iAtn1WSPSk+OU6vwEyGF9VZAS5o5gPHmA@mail.gmail.com>
Date: Tue, 4 Mar 2025 17:02:13 +0800
From: Nylon Chen <nylon.chen@...ive.com>
To: Uwe Kleine-König <u.kleine-koenig@...libre.com>
Cc: linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org, 
	devicetree@...r.kernel.org, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Samuel Holland <samuel.holland@...ive.com>, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 0/3] Change PWM-controlled LED pin active mode and algorithm

Nylon Chen <nylon.chen@...ive.com> 於 2025年1月23日 週四 上午8:20寫道：
>
> Uwe Kleine-König <u.kleine-koenig@...libre.com> 於 2025年1月22日 週三 下午7:44寫道：
> >
> > Hello Nylon,
> >
> > I took another look in the driver and found another problem:
> Hi Uwe, Thank you for the information.
>
> I'll need some time to verify and understand these details, as well as
> conduct the relevant tests
> >
> > On Tue, Jan 21, 2025 at 07:12:10PM +0100, Uwe Kleine-König wrote:
> > > On Tue, Jan 21, 2025 at 04:47:46PM +0800, Nylon Chen wrote:
> > > > Uwe Kleine-König <u.kleine-koenig@...libre.com> 於 2025年1月21日 週二 下午3:47寫道：
> > > > >
> > > > > Hello,
> > > > >
> > > > > On Sun, Jan 19, 2025 at 03:03:16PM +0800, Nylon Chen wrote:
> > > > > > I ran some basic tests by changing the period and duty cycle in both
> > > > > > decreasing and increasing sequences (see the script below).
> > > > >
> > > > > What is clk_get_rate(ddata->clk) for you?
> > > > 130 MHz
> > >
> > > OK, so the possible period lengths are
> > >
> > >       (1 << (16 + scale)) / (130 MHz)
> > >
> > > for scale in [0, .. 15], right? That's
> > >
> > >       2^scale * 504123.07692307694 ns
> > >
> > > So testing period in the range between 5000 ns and 15000 ns isn't
> > > sensible? Did I get something wrong? If the above is right, switching
> > > between period=1008246 ns and 1008247 ns is likely to trigger a
> > > warning.
> >
> > The possible periods are of the form
> >
> >         2^scale * A
> >
> > where A is constant and only depends on the input clock rate. scale
> > ranges over [0, ... 15]. (If I got it right in my last mail, we have A =
> > 504123.07692307694 ns.)
> >
> > If you request say:
> >
> >         .period = 3.9 * A
> >         .duty_cycle = 1.9 * A
> >
> > the period actually emitted by the PWM will be 2 * A. But to calculate
> > frac the originally requested period (i.e. 3.9 * A) is used. So frac
> > becomes 31927 resulting in .duty_cycle being ~0.974 A. A better value
> > would be frac = 62259 which results in .duty_cycle ≅ 1.9 * A.
> > (Depending on A the values for frac might be off by one due to rounding
> > differences.)
> >
> > So I would expect that PWM_DEBUG is angry with you if you go from
> >
> >         .period = 2 * A
> >         .duty_cycle = 1.9 * A
> >
> > to
> >
> >         .period = 3.9 * A
> >         .duty_cycle = 1.9 * A
> >
> > .
> >
> > Best regards
> > Uwe

Hi Uwe, Based on your suggestions, I conducted relevant tests and
corrected algorithm errors.

According to this test program, I can make the system generate related
error messages on v10's patchset.

e.g.
[ 75.043652] pwm-sifive 10021000.pwm: .apply is supposed to round down
duty_cycle (requested: 360/504000, applied: 361/504124)
[ 75.055042] pwm-sifive 10021000.pwm: .apply is supposed to round down
period (requested: 504000, applied: 504124)

PWMCHIP=1
PWMCHANNEL=0
PERIOD=504000
STEP=1
MAX_DUTY=504000

echo 0 > /sys/class/pwm/pwmchip${PWMCHIP}/export

echo $PERIOD > /sys/class/pwm/pwmchip${PWMCHIP}/pwm${PWMCHANNEL}/period
echo "Set period to $PERIOD ns (scale=0 region)"

COUNT=$((MAX_DUTY / STEP))
echo "Testing duty-cycle from 0 to $MAX_DUTY in step of $STEP..."
echo "Total steps (forward): $((COUNT+1))"


CURRENT=0
while [ $CURRENT -le $MAX_DUTY ]; do
    echo $CURRENT > /sys/class/pwm/pwmchip${PWMCHIP}/pwm${PWMCHANNEL}/duty_cycle
    CURRENT=$((CURRENT + STEP))
done

echo "Now do a backward test from $MAX_DUTY down to 0 in step of $STEP..."
echo "Total steps (backward): $((COUNT+1))"


CURRENT=$MAX_DUTY
while [ $CURRENT -ge 0 ]; do
    echo $CURRENT > /sys/class/pwm/pwmchip${PWMCHIP}/pwm${PWMCHANNEL}/duty_cycle
    CURRENT=$((CURRENT - STEP))
done


echo 0 > /sys/class/pwm/pwmchip${PWMCHIP}/pwm${PWMCHANNEL}/enable
echo ${PWMCHANNEL} > /sys/class/pwm/pwmchip${PWMCHIP}/unexport

echo "Done!"

Based on your previous suggestions, I have made the following related
modifications, and now I'm able to fix the relevant errors.

But I want to make sure my understanding aligns with your suggestions,
so I'd like to discuss with you first before sending the patch.

- In .apply, use "round down" for calculations to ensure the value
doesn't exceed what the user requested. (Never set a duty cycle higher
than what the user requested.)
pwm_sifive_apply() {
    - frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
    +frac = num / state->period;
}
- When converting hardware values back to duty_cycle in .get_state,
use "round up" to compensate for the errors introduced by .apply.()
pwm_sifive_get_state() {
    - state->duty_cycle = (u64)duty * ddata->real_period >> PWM_SIFIVE_CMPWIDTH;
    + state->duty_cycle = DIV_ROUND_UP_ULL((u64)duty *
ddata->real_period, (1U << PWM_SIFIVE_CMPWIDTH));
}

Powered by blists - more mailing lists