linux-kernel - Re: [PATCH v2 4/6] pwm: omap-dmtimer: Fix pwm disabling sequence

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Message-ID: <20200310155242.GT37466@atomide.com>
Date:   Tue, 10 Mar 2020 08:52:42 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Lokesh Vutla <lokeshvutla@...com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
        Sekhar Nori <nsekhar@...com>, Vignesh R <vigneshr@...com>,
        Sebastian Reichel <sre@...nel.org>
Subject: Re: [PATCH v2 4/6] pwm: omap-dmtimer: Fix pwm disabling sequence

* Lokesh Vutla <lokeshvutla@...com> [200310 07:06]:
> Hi Tony,
> 
> [...snip...]
> 
> >>>>  
> >>>> +	/*
> >>>> +	 * Disable auto reload so that the current cycle gets completed and
> >>>> +	 * then the counter stops.
> >>>> +	 */
> >>>>  	mutex_lock(&omap->mutex);
> >>>> -	omap->pdata->stop(omap->dm_timer);
> >>>> +	omap->pdata->set_pwm(omap->dm_timer,
> >>>> +			     pwm_get_polarity(pwm) == PWM_POLARITY_INVERSED,
> >>>> +			     true, OMAP_TIMER_TRIGGER_OVERFLOW_AND_COMPARE,
> >>>> +			     false);
> >>>> +
> >>>>  	mutex_unlock(&omap->mutex);
> >>>>  }
> >>>
> >>> I'm seeing an issue with this patch where after use something is
> >>> left on and power consumption stays higher by about 30 mW after
> >>> use.
> >>
> >> Interesting...What is the PWM period and duty cycle in the test case?
> >> Can you dump the following registers before and after disabling:
> >> - TLDR
> >> - TMAR
> >> - TCLR
> > 
> > Here's the state dumped before and after in omap_dm_timer_set_pwm():
> > 
> > omap_timer 4803e000.timer: XXX set_pwm before: tldr: fffffeb8 tmar: fffffffe tclr: 00000040
> > omap_timer 4803e000.timer: XXX set_pwm after: tldr: fffffeb8 tmar: fffffffe tclr: 00001842
> > omap_timer 4013e000.timer: XXX set_pwm before: tldr: fffffeb8 tmar: fffffffe tclr: 00000040
> > omap_timer 4013e000.timer: XXX set_pwm after: tldr: fffffeb8 tmar: fffffffe tclr: 00001842
> > omap_timer 4013e000.timer: XXX set_pwm before: tldr: fffffeb8 tmar: fffffffe tclr: 00001843
> > omap_timer 4013e000.timer: XXX set_pwm after: tldr: fffffeb8 tmar: fffffffe tclr: 00001841
> > omap_timer 4803e000.timer: XXX set_pwm before: tldr: fffffeb8 tmar: fffffffe tclr: 00001843
> > omap_timer 4803e000.timer: XXX set_pwm after: tldr: fffffeb8 tmar: fffffffe tclr: 00001841
> > 
> 
> Looking at the registers:
> period = 327 *(1000/clk_freq in MHz) ns
> duty_cycle =  perioid.
> 
> I did simulate this behavior on BeagleBoneBlack on timer7. PWM is going down
> after disabling.
> 
> > So looks like the start bit is still enabled after use?
> 
> Right, that is expected. The start bit gets disabled automatically once the pwm
> period completes. This is because auto reload bit is off. That's the main idea
> of this patch so that PWM period is completed after disabling, else PWM is
> stopped abruptly.

OK

> Not sure why it is not happening in your case. If you think it is not needed, I
> can drop this patch and add a limitation saying that PWM gets disabled
> immediately without completing the current cycle.

Could it be that we now have the cpu_pm notifier restore something
invalid after exiting idle that restarts the counter?

Regards,

Tony