linux-kernel - Re: [PATCH v2 2/2] pwm: sprd: Add Spreadtrum PWM support

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Message-ID: <20190814133043.peh63lwngujjuo5d@pengutronix.de>
Date:   Wed, 14 Aug 2019 15:30:43 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Orson Zhai <orsonzhai@...il.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-pwm@...r.kernel.org, DTML <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel@...gutronix.de
Subject: Re: [PATCH v2 2/2] pwm: sprd: Add Spreadtrum PWM support

Hello,

On Wed, Aug 14, 2019 at 08:23:37PM +0800, Baolin Wang wrote:
> On Wed, 14 Aug 2019 at 18:55, Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > On Wed, Aug 14, 2019 at 06:01:50PM +0800, Baolin Wang wrote:
> > > On Wed, 14 Aug 2019 at 17:23, Uwe Kleine-König
> > > <u.kleine-koenig@...gutronix.de> wrote:
> > > > On Wed, Aug 14, 2019 at 04:42:28PM +0800, Baolin Wang wrote:
> > > > > On Tue, 13 Aug 2019 at 23:16, Uwe Kleine-König
> > > > > <u.kleine-koenig@...gutronix.de> wrote:
> > > > > > On Tue, Aug 13, 2019 at 09:46:41PM +0800, Baolin Wang wrote:
> > > > > > [...]
> > > > > Not really, our hardware's method is, when you changed a new
> > > > > configuration (MOD or duty is changed) , the hardware will wait for a
> > > > > while to complete current period, then change to the new period.
> > > >
> > > > Can you describe that in more detail? This doesn't explain why MOD must be
> > > > configured before DUTY. Is there another reason for that?
> > >
> > > Sorry, I did not explain this explicitly. When we change a new PWM
> > > configuration, the PWM controller will make sure the current period is
> > > completed before changing to a new period. Once setting the MOD
> > > register (since we always set MOD firstly), that will tell the
> > > hardware that a new period need to change.
> >
> > So if the current period just ended after you reconfigured MOD but
> > before you wrote to DUTY we'll see a bogus period, right? I assume the
> > same holds true for writing the prescale value?
> 
> I confirmed again, I am sorry I missed something before. Yes, like you
> said before, writing DUTY triggers the hardware to actually apply the
> values written to MOD and DUTY to the output. So write DUTY last. I
> will update the comments and change the PWM configure like:
> 
> sprd_pwm_write(spc, pwm->hwpwm, SPRD_PWM_PRESCALE, prescale);
> sprd_pwm_write(spc, pwm->hwpwm, SPRD_PWM_MOD, SPRD_PWM_MOD_MAX);
> sprd_pwm_write(spc, pwm->hwpwm, SPRD_PWM_DUTY, duty);

So PRESCALE is independent and it can still happen that writing PRESCALE
affects the output before MOD and DUTY do?

Best regards
Uwe

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