Re: [PATCH v3 2/2] pwm: sprd: Add Spreadtrum PWM support

[Uwe Kleine-König]

Hello Baolin,

On Thu, Aug 15, 2019 at 11:34:27AM +0800, Baolin Wang wrote:
> On Wed, 14 Aug 2019 at 23:03, Uwe Kleine-König
> <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> >
> > On Wed, Aug 14, 2019 at 08:46:11PM +0800, Baolin Wang wrote:
> 
> > > +
> > > +     /*
> > > +      * The hardware provides a counter that is feed by the source clock.
> > > +      * The period length is (PRESCALE + 1) * MOD counter steps.
> > > +      * The duty cycle length is (PRESCALE + 1) * DUTY counter steps.
> > > +      * Thus the period_ns and duty_ns calculation formula should be:
> > > +      * period_ns = NSEC_PER_SEC * (prescale + 1) * mod / clk_rate
> > > +      * duty_ns = NSEC_PER_SEC * (prescale + 1) * duty / clk_rate
> > > +      */
> > > +     val = sprd_pwm_read(spc, pwm->hwpwm, SPRD_PWM_PRESCALE);
> > > +     prescale = val & SPRD_PWM_PRESCALE_MSK;
> > > +     tmp = (prescale + 1) * NSEC_PER_SEC * SPRD_PWM_MOD_MAX;
> > > +     state->period = DIV_ROUND_CLOSEST_ULL(tmp, chn->clk_rate);
> > > +
> > > +     val = sprd_pwm_read(spc, pwm->hwpwm, SPRD_PWM_DUTY);
> > > +     duty = val & SPRD_PWM_DUTY_MSK;
> > > +     tmp = (prescale + 1) * NSEC_PER_SEC * duty;
> > > +     state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, chn->clk_rate);
> > > +
> > > +     /* Disable PWM clocks if the PWM channel is not in enable state. */
> > > +     if (!state->enabled)
> > > +             clk_bulk_disable_unprepare(SPRD_PWM_CHN_CLKS_NUM, chn->clks);
> > > +}
> > > +
> > > +static int sprd_pwm_config(struct sprd_pwm_chip *spc, struct pwm_device *pwm,
> > > +                        int duty_ns, int period_ns)
> > > +{
> > > +     struct sprd_pwm_chn *chn = &spc->chn[pwm->hwpwm];
> > > +     u32 prescale, duty;
> > > +     u64 tmp;
> > > +
> > > +     /*
> > > +      * The hardware provides a counter that is feed by the source clock.
> > > +      * The period length is (PRESCALE + 1) * MOD counter steps.
> > > +      * The duty cycle length is (PRESCALE + 1) * DUTY counter steps.
> > > +      *
> > > +      * To keep the maths simple we're always using MOD = SPRD_PWM_MOD_MAX.
> >
> > Did you spend some thoughts about how wrong your period can get because
> > of that "lazyness"?
> >
> > Let's assume a clk rate of 100/3 MHz. Then the available period lengths
> > are:
> >
> >         PRESCALE =  0  ->  period =   7.65 µs
> >         PRESCALE =  1  ->  period =  15.30 µs
> >         ...
> >         PRESCALE = 17  ->  period = 137.70 µs
> >         PRESCALE = 18  ->  period = 145.35 µs
> >
> > So the error can be up to (nearly) 7.65 µs (or in general
> 
> Yes, but for our use case (pwm backlight), the precision can meet our
> requirement. Moreover, we usually do not change the period, just
> adjust the duty to change the back light.

Is this a license requirement for you SoC to only drive a backlight with
the PWM? The idea of having a PWM driver on your platform is that it can
also be used to control a step motor or a laser.

> > 255 / clk_rate) because if 145.34 µs is requested you configure
> > PRESCALE = 17 and so yield a period of 137.70 µs. If however you'd pick
> 
> I did not get you here, if period is 145.34, we still get the
> corresponding PRESCALE = 18 by below formula:
> 
> tmp = (u64)chn->clk_rate * period_ns;
> do_div(tmp, NSEC_PER_SEC);
> prescale = DIV_ROUND_CLOSEST_ULL(tmp, SPRD_PWM_MOD_MAX) - 1;

I assumed you switch back to rounding down to match your comment and
which is how I think a pwm should behave. With rounding down we get
PRESCALE = 17 as I claimed. 

> > PRESCALE = 18 and MOD = 254 you get a period of 144.78 µs and so the
> > error is only 0.56 µs which is a factor of 13 better.
> >
> > Hmm.
> >
> > > +      * The value for PRESCALE is selected such that the resulting period
> > > +      * gets the maximal length not bigger than the requested one with the
> > > +      * given settings (MOD = SPRD_PWM_MOD_MAX and input clock).
> > > +      */
> > > +     duty = duty_ns * SPRD_PWM_MOD_MAX / period_ns;
> >
> > I wonder if you loose some precision here as you use period_ns but might
> > actually implement a shorter period.
> >
> > Quick example, again consider clk_rate = 100 / 3 MHz,
> > period_ns = 145340, duty_ns = 72670. Then you end up with
> >
> >         PRESCALE = 17
> >         MOD = 255
> >         DUTY = 127
> 
> Incorrect, we will get PRESCALE = 18,  MOD = 255, DUTY = 127.
> 
> > That corresponds to period_ns = 137700, duty_ns = 68580. With DUTY = 134
> > you get 72360 ns which is still smaller than the requested 72670 ns.
> 
> Incorrect, with DUTY = 134 (PRESCALE = 18  ->  period = 145.35 µs),
> duty_ns = 76380ns

Yes, as above. When using rounding-closest your error is not in [0, 7.65
µs] but in [-3.825 µs, 3.825 µs]. Doesn't make it better.
 
> > (But then again it is not obvious which of the two is the "better"
> > approximation because Thierry doesn't seem to see the necessity to
> > discuss or define a policy here.)
> 
> Like I said, this is the simple calculation formula which can meet our
> requirement (we limit our DUTY value can only be 0 - 254).
> duty = duty_ns * SPRD_PWM_MOD_MAX / period_ns;

simple is often good but sometimes different from correct. And even with
rounding closest instead of rounding down you're giving away precision
here and the size of the error interval is the same, it is just centered
around 0 instead of only positive. If I hadn't spend so much time with
pwm reviews this week I'd try to come up with an example.

> > (And to pick up the thoughts about not using SPRD_PWM_MOD_MAX
> > unconditionally, you could also use
> >
> >         PRESCALE = 18
> >         MOD = 254
> >         DUTY = 127
> >
> > yielding period_ns = 144780 and duty_ns = 72390. Summary:
> >
> >         Request:                 72670 / 145340
> >         your result:             68580 / 137700
> >         also possible:           72390 / 144780
> >
> > Judge yourself.)
> >
> > > +     tmp = (u64)chn->clk_rate * period_ns;
> > > +     do_div(tmp, NSEC_PER_SEC);
> > > +     prescale = DIV_ROUND_CLOSEST_ULL(tmp, SPRD_PWM_MOD_MAX) - 1;
> >
> > Now that you use DIV_ROUND_CLOSEST_ULL the comment is wrong because you
> > might provide a period bigger than the requested one. Also you divide
> 
> Again, that's the precision can meet our requirement.

If you go back to rounding down, use the matching rounding up in
.get_state and adapt your comment describing you're sticking to MOD=255
to say explicitly that you're loosing precision I can live with that. I
even did the math for .get_state in my previous mail for you.

The idea of the requirement to round down is that I want to introduce
this as policy for the PWM framework to get some uniform behaviour from
all lowlevel drivers. If you do this now I won't bother you later when
the requirement is implemented in your driver. And the comment helps
someone who evaluates your SoC to judge if there is still work to do if
they have higher requirements for the PWM.

> > twice instead of once before. (I don't know what architecture your SoC
> > uses, but compared to a multiplication a division is usually expensive.)
> > Also the math is more complicated now as you have a round-down div and a
> > round-closest div.
> >
> > My preference for how to fix that is to restore the behaviour of v2 that
> > matches the comment and adapt .get_state() instead.
> 
> Using DIV_ROUND_CLOSEST_ULL can get a same prescale which matches with
> .get_state().

I don't get you here. Do you say that with DIV_ROUND_CLOSEST_ULL you get
the same result but DIV_ROUND_CLOSEST_ULL matches .get_state while
rounding down doesn't? I cannot follow.

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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