linux-kernel - Re: [PATCH 2/4] pwm: stm32-lp: Add power management support

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Message-ID: <ddb93754-2734-e4c2-a209-517984c6d814@st.com>
Date:   Wed, 6 Feb 2019 15:54:50 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
CC:     <jic23@...nel.org>, <robh+dt@...nel.org>, <mark.rutland@....com>,
        <alexandre.torgue@...com>, <mcoquelin.stm32@...il.com>,
        <linux-iio@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-pwm@...r.kernel.org>,
        <vilhelm.gray@...il.com>,
        <linux-stm32@...md-mailman.stormreply.com>,
        Tomasz Duszynski <tduszyns@...il.com>
Subject: Re: [PATCH 2/4] pwm: stm32-lp: Add power management support

On 2/6/19 1:55 PM, Thierry Reding wrote:
> On Wed, Feb 06, 2019 at 09:54:05AM +0100, Uwe Kleine-König wrote:
>> On Wed, Feb 06, 2019 at 09:42:48AM +0100, Fabrice Gasnier wrote:
>>> If you agree with the current approach, I can send a V2 with Tomasz's
>>> suggestion to remove the ifdefs and use __maybe_unused instead.
>>
>> I think the suspend callback should have something like:
>>
>> 	if (is_still_enabled) {
>> 		/*
>> 		 * The consumer didn't stop us, so refuse to suspend.
>> 		 */
>> 		dev_err(dev, "The consumer didn't stop us, so refuse to suspend.\n");
>> 		return -EBUSY;
>> 	}
>>
>> This way there are no bad surprises if the pwm is suspended before its
>> consumer and it's obvious what is missing.

Thierry, Uwe,

When the pwm is suspended before its consumer, the bad surprise is the
suspend request will fail... I'm not sure a new attempt may be better.
So, it looks like the only way to have this clean is by implementing the
device link e.g. via pwm_get() ?

> 
> Something that just occurred to me: perhaps as part of pwm_get() we
> should track where we were being requested from so that we could give
> hints in situations like this as to where the consumer is that forgot
> to suspend the PWM.

The current approach handles the situation where the consumer forgot to
suspend the PWM... I can add some warning about that in the suspend
routine, incl the label.

What do you think? What's the best approach ?

Please advise,
BR,
Fabrice
> 
> I suppose we already have pwm_device.label to help with this, but
> perhaps we could improve things if we stored __builtin_return_address
> during pwm_get() to help users pinpoint where they need to look.
> 
> Thierry
>