Re: [PATCH v10 0/3] Change PWM-controlled LED pin active mode and algorithm

[Uwe Kleine-König]

Hello Nylon,

On Tue, Mar 04, 2025 at 05:02:13PM +0800, Nylon Chen wrote:
> Nylon Chen <nylon.chen@xxxxxxxxxx> 於 2025年1月23日 週四 上午8:20寫道：
> >
> > Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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));
> }

That sounds right.

Best regards
Uwe

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