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

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Date:   Thu, 14 Jan 2021 12:11:59 +0100
From:   Philipp Rosenberger <p.rosenberger@...bus.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        dan.carpenter@...cle.com, 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 1/2] rtc: pcf2127: Disable Power-On Reset Override



On 14.01.21 11:53, Alexandre Belloni wrote:
> On 14/01/2021 11:43:22+0100, Philipp Rosenberger wrote:
>> On 14.01.21 10:33, Alexandre Belloni wrote:
>>> Hi,
>>>
>>> On 14/01/2021 10:10:32+0100, Philipp Rosenberger wrote:
>>>>
>>>>
>>>> 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.
>>>
>>> No one will ever read the message, the whole test is useless.
>>
>> Sorry, if I bother you with may questions. I'm unsure of why do you think
>> the test is useless. Is it because it is unlikely to happen? Or that it is
>> not relevant to report this?
> 
> It is not relevant because no action will be taken by the user following
> this message.

I can't really agree on that. As I consider myself a user. And I spend 
some time on debugging the watchdog of this chip as I didn't get any 
error or warning.
It is your subsystem, so you make the rules. But I don't like the idea 
of a watchdog which silently fails. But if you insist on removing this 
test I will do so.

Best Regards,
Philipp

>>
>>>>
>>>> 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.
>>>
>>> Don't, this is not an MFD. There is no issue with having the RTC driver
>>> being a clock provider.
>>
>> OK, this is a clear statement.
>>
>> Best Regards,
>> Philipp
>