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

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Message-ID: <6584a59b-657d-adc9-fab2-eb1a9baba05d@st.com>
Date:   Wed, 6 Feb 2019 09:42:48 +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 2/5/19 11:25 PM, Thierry Reding wrote:
> On Tue, Feb 05, 2019 at 09:47:32PM +0100, Uwe Kleine-König wrote:
>> Hello,
>>
>> On Tue, Feb 05, 2019 at 01:40:27PM +0100, Fabrice Gasnier wrote:
>>> Add suspend/resume PM sleep ops. When going to low power, disable
>>> active PWM channel. Active PWM channel is resumed, by calling
>>> pwm_apply_state(). This is inspired by Thierry's comment in [1].
>>> Don't touch inactive channels, as it may be used by other LPTimer MFD
>>> child driver.
>>> [1]https://lkml.org/lkml/2017/12/5/175
>>>
>>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...com>
>>> ---
>>>  drivers/pwm/pwm-stm32-lp.c | 38 ++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 38 insertions(+)
>>>
>>> diff --git a/drivers/pwm/pwm-stm32-lp.c b/drivers/pwm/pwm-stm32-lp.c
>>> index 0059b24c..0c40d48 100644
>>> --- a/drivers/pwm/pwm-stm32-lp.c
>>> +++ b/drivers/pwm/pwm-stm32-lp.c
>>> @@ -13,6 +13,7 @@
>>>  #include <linux/mfd/stm32-lptimer.h>
>>>  #include <linux/module.h>
>>>  #include <linux/of.h>
>>> +#include <linux/pinctrl/consumer.h>
>>>  #include <linux/platform_device.h>
>>>  #include <linux/pwm.h>
>>>  
>>> @@ -20,6 +21,8 @@ struct stm32_pwm_lp {
>>>  	struct pwm_chip chip;
>>>  	struct clk *clk;
>>>  	struct regmap *regmap;
>>> +	struct pwm_state suspend;
>>> +	bool suspended;
>>>  };
>>>  
>>>  static inline struct stm32_pwm_lp *to_stm32_pwm_lp(struct pwm_chip *chip)
>>> @@ -223,6 +226,40 @@ static int stm32_pwm_lp_remove(struct platform_device *pdev)
>>>  	return pwmchip_remove(&priv->chip);
>>>  }
>>>  
>>> +#ifdef CONFIG_PM_SLEEP
>>> +static int stm32_pwm_lp_suspend(struct device *dev)
>>> +{
>>> +	struct stm32_pwm_lp *priv = dev_get_drvdata(dev);
>>> +
>>> +	pwm_get_state(&priv->chip.pwms[0], &priv->suspend);
>>> +	priv->suspended = priv->suspend.enabled;
>>> +
>>> +	/* safe to call pwm_disable() for already disabled pwm */
>>> +	pwm_disable(&priv->chip.pwms[0]);
>>> +
>>> +	return pinctrl_pm_select_sleep_state(dev);
>>
>> IMHO a PWM should not stop if the PWM user didn't call pwm_disable() (or
>> pwm_apply_state() with .enabled = false).
>>
>> I don't understand all the PM details, but I think there is no defined
>> order in which devices are signalled to suspend. If so the PWM might be
>> stopped before its consumer. Then the PWM changes state without the
>> consumer being aware of that.
>>
>> I understand Thierry's position in the link you provided in the commit
>> log consistant with my position here.
> 
> Agreed, we should let users of the PWM take care of resuming the PWM in
> the state and at the point in time that they require so. PWM users will
> also likely do a pwm_disable() during their suspend implementation, so
> doing this again in a PWM ->suspend() is not necessary, even if perhaps
> harmless.
> 
> So this leaves only the pinctrl_pm_select_sleep_state() call. That seems
> fine, but I'm not sure that that's currently guaranteed to work. I think
> we may need to implement device link support in the PWM framework to
> guarantee the proper suspend/resume sequencing. Otherwise you may end up
> in a situation where the PWM is actually suspended before the user and
> glitch the pins before the user has a chance to disable the PWM.

Hi Uwe, Thierry,

I agree with both of you on the analysis.

> 
> I think it'd be fine to merge the driver change here first before we add
> device link support if this works for you. Just mentioning the issue
> here in case you ever run into it.

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.

Thanks for reviewing,
Best Regards,
Fabrice
> 
> Thierry
>