linux-kernel - Re: [PATCH v2 1/2] rtc: pcf2127: Disable Power-On Reset Override

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Message-ID: <39a69676-83f2-07f4-99ae-ce4f89d5314c@kunbus.com>
Date:   Thu, 14 Jan 2021 10:10:32 +0100
From:   Philipp Rosenberger <p.rosenberger@...bus.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     dan.carpenter@...cle.com, biwen.li@....com, lvb@...hos.com,
        bruno.thomsen@...il.com, l.sanfilippo@...bus.com,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] rtc: pcf2127: Disable Power-On Reset Override



On 14.01.21 09:05, Uwe Kleine-König wrote:
> On Wed, Jan 13, 2021 at 12:27:41PM +0100, Philipp Rosenberger wrote:
>> To resume normal operation after a total power loss (no or empty
>> battery) the "Power-On Reset Override (PORO)" facility needs to be
>> disabled.
>>
>> As the oscillator may take a long time (200 ms to 2 s) to resume normal
>> operation. The default behaviour is to use the PORO facility.
> 
> I'd write instead: The register reset value sets PORO enabled and the
> data sheet recommends setting it to disabled for normal operation.

Sounds good, I will rephrase it.

> In my eyes having a reset default value that is unsuitable for
> production use is just another bad design choice of this chip. At least
> now this is known and can be somewhat fixed in software. :-\

Yes, had my fair share of WTF moments with this chip.

>> But with the PORO active no interrupts are generated on the interrupt
>> pin (INT).
> 
> This sentence about no interrupts is your observation, or does this base
> on some authoritative source (datasheet, FAE or similar)?
>

Yes this is only may observation. I tested this with the OM13513 
demoboard with PCF2127 and pcf2129. So I should rephrase it to something 
like this:

Some testes suggests that no interrupts are generated on the interrupt 
pin if the PORP is active.

>> Signed-off-by: Philipp Rosenberger <p.rosenberger@...bus.com>
>> ---
>>   drivers/rtc/rtc-pcf2127.c | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/drivers/rtc/rtc-pcf2127.c b/drivers/rtc/rtc-pcf2127.c
>> index 39a7b5116aa4..378b1ce812d6 100644
>> --- a/drivers/rtc/rtc-pcf2127.c
>> +++ b/drivers/rtc/rtc-pcf2127.c
>> @@ -26,6 +26,7 @@
>>   
>>   /* Control register 1 */
>>   #define PCF2127_REG_CTRL1		0x00
>> +#define PCF2127_BIT_CTRL1_POR_OVRD		BIT(3)
>>   #define PCF2127_BIT_CTRL1_TSF1			BIT(4)
>>   /* Control register 2 */
>>   #define PCF2127_REG_CTRL2		0x01
>> @@ -612,6 +613,23 @@ static int pcf2127_probe(struct device *dev, struct regmap *regmap,
>>   		ret = devm_rtc_nvmem_register(pcf2127->rtc, &nvmem_cfg);
>>   	}
>>   
>> +	/*
>> +	 * The "Power-On Reset Override" facility prevents the RTC to do a reset
>> +	 * after power on. For normal operation the PORO must be disabled.
>> +	 */
>> +	regmap_clear_bits(pcf2127->regmap, PCF2127_REG_CTRL1,
>> +				PCF2127_BIT_CTRL1_POR_OVRD);
>> +	/*
>> +	 * If the PORO can't be disabled, just move on. The RTC should
>> +	 * work fine, but functions like watchdog and alarm interrupts might
>> +	 * not work. There will be no interrupt generated on the interrupt pin.
>> +	 */
>> +	ret = regmap_test_bits(pcf2127->regmap, PCF2127_REG_CTRL1, PCF2127_BIT_CTRL1_POR_OVRD);
>> +	if (ret <= 0) {
>> +		dev_err(dev, "%s: can't disable PORO (ctrl1).\n", __func__);
>> +		dev_warn(dev, "Watchdog and alarm functions might not work properly\n");
> 
> I would not emit two messages here. Also including __func__ isn't so
> nice IMHO. (Great for debugging, but not in production code IMHO.)

Yes, I dislike the style of the messages in this module. I just thought 
to keep it consistent.

I'm thinking of rewriting this driver as MFD driver. We use the CLKOUT 
for some products. So maybe a RTC, watchdog and clock driver on top of 
an MFD. But I'm not sure if it is really a good idea. The behavior of 
the chip to disable the watchdog when reading ctrl2 (i think it was) 
giving me a headache.

> We should consider a Cc: to stable.

Yes, this is a good idea. I need to apply this to 5.4 anyway, as we 
develop a product with 5.4.

> Best regards
> Uwe
> 

Thanks and Best Regards,
Philipp