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

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Message-ID: <20210114093325.GU3654@piout.net>
Date:   Thu, 14 Jan 2021 10:33:25 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Philipp Rosenberger <p.rosenberger@...bus.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

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.

> 
> 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.


-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com