linux-kernel - Re: [PATCH v7 06/13] pwm: add support for sl28cpld PWM controller

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Message-ID: <92116be9aa56250becc4019c6c7a1538@walle.cc>
Date:   Fri, 07 Aug 2020 09:55:19 +0200
From:   Michael Walle <michael@...le.cc>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Lee Jones <lee.jones@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>, Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v7 06/13] pwm: add support for sl28cpld PWM controller

Am 2020-08-07 09:45, schrieb Uwe Kleine-König:
> On Fri, Aug 07, 2020 at 09:28:31AM +0200, Michael Walle wrote:
>> Hi Uwe, Hi Lee,
>> 
>> Am 2020-08-06 10:40, schrieb Uwe Kleine-König:
>> > On Mon, Aug 03, 2020 at 11:35:52AM +0200, Michael Walle wrote:
>> > > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
>> > > index 7dbcf6973d33..a0d50d70c3b9 100644
>> > > --- a/drivers/pwm/Kconfig
>> > > +++ b/drivers/pwm/Kconfig
>> > > @@ -428,6 +428,16 @@ config PWM_SIFIVE
>> > >  	  To compile this driver as a module, choose M here: the module
>> > >  	  will be called pwm-sifive.
>> > >
>> > > +config PWM_SL28CPLD
>> > > +	tristate "Kontron sl28cpld PWM support"
>> > > +	select MFD_SIMPLE_MFD_I2C
>> >
>> > Is it sensible to present this option to everyone? Maybe
>> >
>> > 	depends on SOME_SYMBOL_ONLY_TRUE_ON_SL28CPLD || COMPILE_TEST
>> 
>> Because there is now no real MFD driver anymore, there is also
>> no symbol for that. The closest would be ARCH_ARM64 but I don't
>> think that is a good idea.
>> 
>> Lee, what do you think about adding a symbol to the MFD, which
>> selects MFD_SIMPLE_MFD_I2C but doesn't enable any C modules?
>> 
>> I.e.
>> config MFD_SL28CPLD
>>     tristate "Kontron sl28cpld"
>>     select MFD_SIMPLE_MFD_I2C
>>     help
>>       Say yes here to add support for the Kontron sl28cpld board
>>       management controller.
>> 
>> Then all the other device driver could depend on the MFD_SL28CPLD
>> symbol.
>> 
>> [..]
>> 
>> > > +static void sl28cpld_pwm_get_state(struct pwm_chip *chip,
>> > > +				   struct pwm_device *pwm,
>> > > +				   struct pwm_state *state)
>> > > +{
>> > > +	struct sl28cpld_pwm *priv = dev_get_drvdata(chip->dev);
>> > > +	unsigned int reg;
>> > > +	int prescaler;
>> > > +
>> > > +	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CTRL, &reg);
>> > > +
>> > > +	state->enabled = reg & SL28CPLD_PWM_CTRL_ENABLE;
>> > > +
>> > > +	prescaler = FIELD_GET(SL28CPLD_PWM_CTRL_PRESCALER_MASK, reg);
>> > > +	state->period = SL28CPLD_PWM_PERIOD(prescaler);
>> > > +
>> > > +	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CYCLE, &reg);
>> > > +	state->duty_cycle = SL28CPLD_PWM_TO_DUTY_CYCLE(reg);
>> >
>> > Should reg be masked to SL28CPLD_PWM_CYCLE_MAX, or is it guaranteed that
>> > the upper bits are zero?
>> 
>> Mh, the hardware guarantees that bit7 is zero. So masking with
>> SL28CPLD_PWM_CYCLE_MAX won't buy us much. But what I could think
>> could go wrong is this: someone set the prescaler to != 0 and the
>> duty cycle to a value greater than the max value for this particular
>> prescaler mode. For the above calculations this would result in a
>> duty_cycle greater than the period, if I'm not mistaken.
>> 
>> The behavior of the hardware is undefined in that case (at the moment
>> it will be always on, I guess). So this isn't a valid setting.
>> Nevertheless it might happen. So what about the following:
>> 
>> state->duty_cycle = min(state->duty_cycle, state->period);
> 
> If you care about this: This can also happen (at least shortly) in
> sl28cpld_pwm_apply() as you write SL28CPLD_PWM_CTRL before
> SL28CPLD_PWM_CYCLE there.

It could also happen if it was the other way around, couldn't it?
Changing modes might glitch.

I care more about returning valid values to the PWM core ;)

-michael