linux-kernel - Re: [PATCH v8 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

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Date:   Tue, 12 Mar 2019 12:22:38 +0530
From:   Yash Shah <yash.shah@...ive.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     mark.rutland@....com, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, Palmer Dabbelt <palmer@...ive.com>,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        Sachin Ghadi <sachin.ghadi@...ive.com>,
        Thierry Reding <thierry.reding@...il.com>,
        kernel@...gutronix.de, Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v8 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

On Mon, Mar 11, 2019 at 6:59 PM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> Hello,
>
> On Mon, Mar 11, 2019 at 05:10:17PM +0530, Yash Shah wrote:
> > On Fri, Mar 8, 2019 at 5:27 PM Uwe Kleine-König
> > <u.kleine-koenig@...gutronix.de> wrote:
> > > On Fri, Mar 08, 2019 at 04:59:36PM +0530, Yash Shah wrote:
> > > > On Thu, Mar 7, 2019 at 8:57 PM Uwe Kleine-König
> > > > <u.kleine-koenig@...gutronix.de> wrote:
> > > > > > +     if (state->period != cur_state.period) {
> > > > >
> > > > > Did you test this with more than one consumer? For sure the following
> > > > > should work:
> > > > >
> > > > >         pwm1 = pwm_get(.. the first ..);
> > > > >         pwm_apply_state(pwm1, { .enabled = true, .period = 10000000, .... });
> > > > >
> > > > >         pwm2 = pwm_get(.. the second ..);
> > > > >         pwm_apply_state(pwm2, { .enabled = true, .period = 10000000, .... });
> > > > >
> > > > > but for the second pwm_apply_state() run state->period is likely not
> > > > > exactly 10000000.
> > > >
> > > > Yes, I have tested multiple consumers using sysfs interface. It is working.
> > >
> > > Can you provide details about your testing here? What is the parent clk
> > > rate? Which settings did you test? Can you confirm my claim that the
> > > above sequence would fail or point out my error in reasoning?
> > >
> >
> > I have tested on HiFive Unleashed board using sysfs interface.
> > Parent clk rate is around 512 Mhz.
> > I have tested scenarios as you mentioned above with various period and
> > duty_cycle values.
> >
> > After considering your below suggestion,
> >  | To get the result independent of the prior configuration you better use
> >  | the real targeted period length as input instead of the last configured
> >  | approximation
> > I will introduce approx_period feild, which will be used as the
> > targeted period length.
> > Also, in pwm_sifive_get_state, I will make below change
> > - state->period = pwm->real_period;
> > + state->period = pwm->approx_period.
> > So with this change in place, I believe the cur_state.period for the
> > second pwm_apply_state() above (pwm2) will be exactly 10000000
>
> I don't understand your intention completely. Just send a new patch
> round, then I will gladly take another look.

I was planning to go with above-mentioned change but then I realized
that pwm state should always reflect the current hardware state, so I
have dropped the above idea.
Coming back to your concern on

 |     if (state->period != cur_state.period) {

It is not failing for the scenario you mentioned but failing for
another way around.

         pwm1 = pwm_get(.. the first ..);
         pwm_apply_state(pwm1, { .enabled = true, .period = 10000000, .... });

         pwm2 = pwm_get(.. the second ..);
         pwm_apply_state(pwm2, { .enabled = true, .period = 20000000, .... });

The pwm2 should get an error for period mismatch but with v8 patch, it
is not getting any error.

I am sending a v9 patch which has the fix for this.
With v9 changes, I have tested all scenarios and it is working fine.

>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |