Re: [PATCH v2 3/6] pwm: jz4740: Apply configuration atomically

Paul Cercueil
2019-07-29T21:19:42Z

Hi Uwe,

Le mer. 24 juil. 2019 à 2:47, Uwe =?iso-8859-1?q?Kleine-K=F6nig?=
<u.kleine-koenig@pengutronix.de> a écrit :
> Hello Paul,
>
> On Tue, Jul 23, 2019 at 04:46:40PM -0400, Paul Cercueil wrote:
>> Le lun. 22 juil. 2019 à 15:34, Uwe =?iso-8859-1?q?Kleine-K=F6nig?=
>> <u.kleine-koenig@pengutronix.de> a écrit :
>> > On Fri, Jun 07, 2019 at 05:44:07PM +0200, Paul Cercueil wrote:
>> > > - is_enabled = jz4740_timer_is_enabled(pwm->hwpwm);
>> > > - if (is_enabled)
>> > > - jz4740_pwm_disable(chip, pwm);
>> > > + jz4740_pwm_disable(chip, pwm);
>> >
>> > I assume this stops the PWM. Does this complete the currently
>> running
>> > period? How does the PWM behave then? (Does it still drive the
>> output?
>> > If so, on which level?)
>>
>> Some PWM channels work in one mode "TCU1" and others work in
>> "TCU2". The
>> mode in which channels work depends on the version of the SoC.
>>
>> When stopped, the pins of TCU1 channels will be driven to the
>> inactive
>> level (which depends on the polarity). It is unknown whether or not
>> the
>> currently running period is completed. We set a bit to configure for
>> "abrupt shutdown", so I expect that it's not, but somebody would
>> need
>> to hook up a logic analyzer to see what's the exact behaviour with
>> and without that bit.
>
> This might be done even without a logic analyzer. Just do something
> like:
>
> pwm_apply_state(pwm, { .enabled = 1, .period = 5s })
> pwm_apply_state(pwm, { .enabled = 1, .period = 5s, .duty = 5s })
>
> and if that takes less then 5s the period is not completed.
>
> And note that "abrupt shutdown" is a bug.

I remember you asked that already in an older patchset.
The result of this test is that the period is never completed,
independently of the "abrupt shutdown" bit.

Cheers,
-Paul

>> TCU2 channels on the other hand will stop in the middle of a period,
>> leaving the pin hanging at whatever level it was before the stop.
>> That's the rationale behind the trick in commit 6580fd173070 ("pwm:
>> jz4740: Force TCU2 channels to return to their init level").
>
> Strange, but ok.
>
> Best regards
> Uwe
>
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