Re: [PATCH v2 30/53] rtc: m48t35: support the SGI IP35 timekeeper's 1968 year base

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

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Hello,

On 13/10/2026 15:16:50+0200, Imre Kaloz wrote:
> The M48T35 of SGI IP35-family machines counts its year from 1968. Take
> the epoch from a platform id table, "rtc-m48t35" for IP27 and
> "rtc-m48t35-ip35" for IP35, and bound the year to the part's century: on
> IP27 a year past 2069 now fails to set.
> 
> Signed-off-by: Imre Kaloz <kaloz@kernel.org>
> ---
>  drivers/rtc/rtc-m48t35.c | 33 +++++++++++++++++++--------------
>  1 file changed, 19 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-m48t35.c b/drivers/rtc/rtc-m48t35.c
> index 92f19bf997b2..fa228a2bb527 100644
> --- a/drivers/rtc/rtc-m48t35.c
> +++ b/drivers/rtc/rtc-m48t35.c
> @@ -49,6 +49,7 @@ struct m48t35_priv {
>  	struct m48t35_rtc __iomem *reg;
>  	size_t size;
>  	unsigned long baseaddr;
> +	unsigned int epoch;
>  	spinlock_t lock;
>  };
>  
> @@ -86,10 +87,7 @@ static int m48t35_read_time(struct device *dev, struct rtc_time *tm)
>  	 * Account for differences between how the RTC uses the values
>  	 * and how they are defined in a struct rtc_time;
>  	 */
> -	tm->tm_year += 70;
> -	if (tm->tm_year <= 69)
> -		tm->tm_year += 100;
> -
> +	tm->tm_year += priv->epoch - 1900;
>  	tm->tm_mon--;
>  	return 0;
>  }
> @@ -108,18 +106,10 @@ static int m48t35_set_time(struct device *dev, struct rtc_time *tm)
>  	min = tm->tm_min;
>  	sec = tm->tm_sec;
>  
> -	if (yrs < 1970)
> -		return -EINVAL;
> -
> -	yrs -= 1970;
> -	if (yrs > 255)    /* They are unsigned */
> -		return -EINVAL;
> -
> -	if (yrs > 169)
> +	if (yrs < priv->epoch || yrs >= priv->epoch + 100)
>  		return -EINVAL;
>  
> -	if (yrs >= 100)
> -		yrs -= 100;
> +	yrs -= priv->epoch;
>  


Instead of open coding, you should use start_secs after setting the rtc
range in probe(). attached i a patch I refreshed for the
devm_rtc_allocate_device/devm_rtc_register_device conversion that you
can include in this series. drivers/rtc/rtc-mt6397.c is an example that
should be working for you.

I never sent the patch because it is not clear from the datasheet how
well leap year calculation works. My guess is that it worked in 2000 but
fails in 2100.

>  	sec = bin2bcd(sec);
>  	min = bin2bcd(min);
> @@ -149,9 +139,13 @@ static const struct rtc_class_ops m48t35_ops = {
>  
>  static int m48t35_probe(struct platform_device *pdev)
>  {
> +	const struct platform_device_id *id = platform_get_device_id(pdev);
>  	struct resource *res;
>  	struct m48t35_priv *priv;
>  
> +	if (!id)
> +		return -ENODEV;
> +
>  	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>  	if (!res)
>  		return -ENODEV;
> @@ -164,6 +158,7 @@ static int m48t35_probe(struct platform_device *pdev)
>  				     pdev->name))
>  		return -EBUSY;
>  
> +	priv->epoch = id->driver_data;
>  	priv->baseaddr = res->start;
>  	priv->reg = devm_ioremap(&pdev->dev, priv->baseaddr, priv->size);
>  	if (!priv->reg)
> @@ -178,11 +173,20 @@ static int m48t35_probe(struct platform_device *pdev)
>  	return PTR_ERR_OR_ZERO(priv->rtc);
>  }
>  
> +/* The year register on SGI IP35-family machines counts from 1968. */
> +static const struct platform_device_id m48t35_id_table[] = {
> +	{ "rtc-m48t35", 1970 },
> +	{ "rtc-m48t35-ip35", 1968 },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(platform, m48t35_id_table);
> +
>  static struct platform_driver m48t35_platform_driver = {
>  	.driver		= {
>  		.name	= "rtc-m48t35",
>  	},
>  	.probe		= m48t35_probe,
> +	.id_table	= m48t35_id_table,
>  };
>  
>  module_platform_driver(m48t35_platform_driver);
> @@ -191,3 +195,4 @@ MODULE_AUTHOR("Thomas Bogendoerfer <tsbogend@alpha.franken.de>");
>  MODULE_DESCRIPTION("M48T35 RTC driver");
>  MODULE_LICENSE("GPL");
>  MODULE_ALIAS("platform:rtc-m48t35");
> +MODULE_ALIAS("platform:rtc-m48t35-ip35");
> -- 
> 2.47.3
> 

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

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From 7aed40aa17edfdd132ad10f6fb1ca9f1f1ddac07 Mon Sep 17 00:00:00 2001
From: Alexandre Belloni <alexandre.belloni@bootlin.com>
Date: Tue, 13 Oct 2026 16:21:06 +0200
Subject: [PATCH] rtc: m48t35: switch to devm_rtc_allocate_device

Switch to devm_rtc_allocate_device/devm_rtc_register_device, this allows
for further improvement of the driver.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
---
 drivers/rtc/rtc-m48t35.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/rtc-m48t35.c b/drivers/rtc/rtc-m48t35.c
index 92f19bf997b2..0f4a3cb1e1f0 100644
--- a/drivers/rtc/rtc-m48t35.c
+++ b/drivers/rtc/rtc-m48t35.c
@@ -173,9 +173,13 @@ static int m48t35_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, priv);
 
-	priv->rtc = devm_rtc_device_register(&pdev->dev, "m48t35",
-				  &m48t35_ops, THIS_MODULE);
-	return PTR_ERR_OR_ZERO(priv->rtc);
+	priv->rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(priv->rtc))
+		return PTR_ERR(priv->rtc);
+
+	priv->rtc->ops = &m48t35_ops;
+
+	return devm_rtc_register_device(priv->rtc);
 }
 
 static struct platform_driver m48t35_platform_driver = {
-- 
2.56.0