linux-kernel - Re: [PATCH v1 4/5] rtc: pcf8523: external capacitor configuration

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Date:   Thu, 13 Sep 2018 21:27:13 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Andrew Jeffery <andrew@...id.au>,
        Fabio Estevam <fabio.estevam@....com>,
        Joel Stanley <joel@....id.au>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
        Søren Andersen <san@...v.dk>
Subject: Re: [PATCH v1 4/5] rtc: pcf8523: external capacitor configuration

On 07/09/2018 21:35:07+0200, Sam Ravnborg wrote:
> From: Søren Andersen <san@...v.dk>
> 
> Add support for specifying the quartz load in the DT node.
> The pcf8523 may use either a 7 pF or an 12.5 pF xtal.
> If the rtc has the wrong configuration the time will
> drift several hours/week.
> 
> If nothing is specified in DT then the factory default of 7 pF is used.
> This is a change compared to before where the driver assumed 12.5 pF.
> 
> Note: all kernel dts trees are updated with the new property.
> 
> Signed-off-by: Søren Andersen <san@...v.dk>
> Signed-off-by: Sam Ravnborg <sam@...nborg.org>
> Cc: Alessandro Zummo <a.zummo@...ertech.it>
> Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
> ---
>  drivers/rtc/rtc-pcf8523.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-pcf8523.c b/drivers/rtc/rtc-pcf8523.c
> index 453615f8ac9a..3c19b4d21a2d 100644
> --- a/drivers/rtc/rtc-pcf8523.c
> +++ b/drivers/rtc/rtc-pcf8523.c
> @@ -85,7 +85,7 @@ static int pcf8523_write(struct i2c_client *client, u8 reg, u8 value)
>  	return 0;
>  }
>  
> -static int pcf8523_select_capacitance(struct i2c_client *client, bool high)
> +static int pcf8523_select_capacitance(struct i2c_client *client)
>  {
>  	u8 value;
>  	int err;
> @@ -94,7 +94,7 @@ static int pcf8523_select_capacitance(struct i2c_client *client, bool high)
>  	if (err < 0)
>  		return err;
>  
> -	if (!high)
> +	if (!device_property_present(&client->dev, "nxp,quartz_load_12.5pf"))

OK, this is super annoying we unfortunately can't change what was done
previously without breaking the DT ABI. Honestly, I suspect the 12.5pF
value is wrong for most boards anyway.

What I would do is use the value from DT (so either 7000fF or 12500fF)
and set it if present.
If not present, set 12.5pF and print a warning that this differs from
the RTC default.

Hopefully, at some point, we could leave it as it is as the value should
persist across reboot.

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