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

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Date:   Thu, 14 Jan 2021 09:05:33 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Philipp Rosenberger <p.rosenberger@...bus.com>
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 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.
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. :-\

> 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)?

> 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.)

We should consider a Cc: to stable.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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