linux-kernel - Re: [PATCH v2 2/2] rtc: pcf2127: Run a OTP refresh if not done before

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Date:   Thu, 14 Jan 2021 12:18:22 +0100
From:   Philipp Rosenberger <p.rosenberger@...bus.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     dan.carpenter@...cle.com, u.kleine-koenig@...gutronix.de,
        biwen.li@....com, lvb@...hos.com, bruno.thomsen@...il.com,
        l.sanfilippo@...bus.com, Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] rtc: pcf2127: Run a OTP refresh if not done before

On 14.01.21 12:11, Alexandre Belloni wrote:
> On 14/01/2021 11:30:37+0100, Philipp Rosenberger wrote:
>>>> +	ret = regmap_set_bits(pcf2127->regmap, PCF2127_REG_CLKOUT,
>>>> +			      PCF2127_BIT_CLKOUT_OTPR);
>>>> +	if (ret < 0) {
>>>> +		dev_err(dev, "%s: OTP refresh (clkout_ctrl) failed.\n", __func__);
>>>
>>> Please drop this error message.
>>
>> If I return from the probe with an error, shouldn't there be an error
>> message? Or should I ignore the problem at all and don't return from the
>> probe?
> 
> You can return from probe without an error message.
> 
>>
>>>
>>>> +		return ret;
>>>> +	}
>>>> +	msleep(100);
>>>
>>> Maybe this should be done just before setting the time. Or if you want
>>> to keep it in probe, then you could optimise by not waiting but ensuring
>>> the time between pcf2127_probe and the first pcf2127_rtc_set_time is
>>> more than 100ms.
>>>
>>
>> Doing it just before setting the time might be not the best way. The
>> watchdog might be used before the OTPR is done.
>>
>>  From the PCF2129 manual:
>> | The OTP refresh (see Section 8.3.2 on page 13) should ideally be
>> | executed as the first instruction after start-up and also after a
>> | reset due to an oscillator stop.
>>
>> As I see it this should be done before setting up the watchdog as well. So
>> sleeping if the OTPR wasn't done before might be the most viable solution.
>> So I would check the OTPR and only if the OTPR is not set starting an OTPR
>> and then sleep 100ms.
>>
> 
> Indeed, the remaining question is whether you should test OTPR or OSF.
> OSF states: "oscillator has stopped and chip reset has occurred since
> flag was last cleared" if OTPR is always 0 when OSF is 1, then OTPR is
> probably enough.

The datasheet is unclear about that. And I thought about that as well. 
The OSF flag is and should only reset when the time is set. But if I 
reboot or reload the driver without setting the time the OTRP would be 
rerun. So I thought it would be best to only rely on the OTPR bit. If 
someone has a better idea I'm open far that.

Best Regards,
Philipp