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

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Message-ID: <77d07f5e-2891-21d6-feee-19e841a8343e@kunbus.com>
Date:   Thu, 14 Jan 2021 11:30:37 +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 10:50, Alexandre Belloni wrote:
> On 13/01/2021 12:27:42+0100, Philipp Rosenberger wrote:
>> The datasheet of the PCF2127 states,it is recommended to process an OTP
>> refresh once the power is up and the oscillator is operating stable. The
>> OTP refresh takes less than 100 ms to complete.
>>
>> Signed-off-by: Philipp Rosenberger <p.rosenberger@...bus.com>
>> ---
>>   drivers/rtc/rtc-pcf2127.c | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/drivers/rtc/rtc-pcf2127.c b/drivers/rtc/rtc-pcf2127.c
>> index 378b1ce812d6..ca56dba64e79 100644
>> --- a/drivers/rtc/rtc-pcf2127.c
>> +++ b/drivers/rtc/rtc-pcf2127.c
>> @@ -58,6 +58,9 @@
>>   #define PCF2127_REG_ALARM_DM		0x0D
>>   #define PCF2127_REG_ALARM_DW		0x0E
>>   #define PCF2127_BIT_ALARM_AE			BIT(7)
>> +/* CLKOUT control register */
>> +#define PCF2127_REG_CLKOUT		0x0f
>> +#define PCF2127_BIT_CLKOUT_OTPR			BIT(5)
>>   /* Watchdog registers */
>>   #define PCF2127_REG_WD_CTL		0x10
>>   #define PCF2127_BIT_WD_CTL_TF0			BIT(0)
>> @@ -630,6 +633,19 @@ static int pcf2127_probe(struct device *dev, struct regmap *regmap,
>>   		dev_warn(dev, "Watchdog and alarm functions might not work properly\n");
>>   	}
>>   
>> +	/*
>> +	 * Set the OTP refresh bit unconditionally. If an OTP refresh was
>> +	 * already done the bit is already set and will not rerun the refresh
>> +	 * operation.
>> +	 */
>> +	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?

> 
>> +		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.

Best Regards
Philipp