linux-kernel - Re: [PATCH 4/4] rtc: ds1307: Add support for reading RX8901CE battery VL status

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Message-ID: <20260131000549071abafa@mail.local>
Date: Sat, 31 Jan 2026 01:05:49 +0100
From: Alexandre Belloni <alexandre.belloni@...tlin.com>
To: Fredrik M Olsson <fredrik.m.olsson@...s.com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Nobuhiro Iwamatsu <nobuhiro.iwamatsu.x90@...l.toshiba>,
	linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...s.com
Subject: Re: [PATCH 4/4] rtc: ds1307: Add support for reading RX8901CE
 battery VL status

On 19/12/2025 13:10:38+0100, Fredrik M Olsson wrote:
> Adds support for:
> - Reading the battery voltage low status using the RTC_VL_READ ioctl,
>   which also reports invalid time information if the VLF flag is set.
> 
> Signed-off-by: Fredrik M Olsson <fredrik.m.olsson@...s.com>
> ---
>  drivers/rtc/rtc-ds1307.c | 46 +++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 43 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-ds1307.c b/drivers/rtc/rtc-ds1307.c
> index 99d95e520108..ca062ed0c867 100644
> --- a/drivers/rtc/rtc-ds1307.c
> +++ b/drivers/rtc/rtc-ds1307.c
> @@ -133,8 +133,11 @@ enum ds_type {
>  #define RX8901_REG_INTF			0x0e
>  #define RX8901_REG_INTF_VLF		BIT(1)
>  #define RX8901_REG_PWSW_CFG		0x37
> +#define RX8901_REG_PWSW_CFG_VBATLDETEN	BIT(4)
>  #define RX8901_REG_PWSW_CFG_INIEN	BIT(6)
>  #define RX8901_REG_PWSW_CFG_CHGEN	BIT(7)
> +#define RX8901_REG_BUF_INTF		0x46
> +#define RX8901_REG_BUF_INTF_VBATLF	BIT(3)
>  
>  #define MCP794XX_REG_CONTROL		0x07
>  #	define MCP794XX_BIT_ALM0_EN	0x10
> @@ -458,6 +461,39 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_RTC_INTF_DEV
> +static int rx8901_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
> +{
> +	struct ds1307 *ds1307 = dev_get_drvdata(dev);
> +	unsigned int regflag, tmp = 0;
> +	int ret = 0;
> +
> +	switch (cmd) {
> +	case RTC_VL_READ:
> +		ret = regmap_read(ds1307->regmap, RX8901_REG_INTF, &regflag);
> +		if (ret)
> +			return ret;
> +
> +		if (regflag & RX8901_REG_INTF_VLF)
> +			tmp |= RTC_VL_DATA_INVALID;
> +
> +		ret = regmap_read(ds1307->regmap, RX8901_REG_BUF_INTF, &regflag);
> +		if (ret)
> +			return ret;
> +
> +		if (regflag & RX8901_REG_BUF_INTF_VBATLF)
> +			tmp |= RTC_VL_BACKUP_LOW;
> +
> +		return put_user(tmp, (unsigned int __user *)arg);
> +	default:
> +		return -ENOIOCTLCMD;
> +	}
> +	return ret;
> +}
> +#else
> +#define rx8901_ioctl NULL
> +#endif
> +
>  static int ds1337_read_alarm(struct device *dev, struct rtc_wkalrm *t)
>  {
>  	struct ds1307		*ds1307 = dev_get_drvdata(dev);
> @@ -599,10 +635,13 @@ static u8 do_trickle_setup_rx8130(struct ds1307 *ds1307, u32 ohms, bool diode)
>  	return setup;
>  }
>  
> -static u8 do_trickle_setup_rx8901(struct ds1307 *ds1307, u32 ohms, bool diode)
> +static u8 do_trickle_setup_rx8901(struct ds1307 *ds1307, u32 ohms __always_unused, bool diode)
>  {
> -	/* make sure that the backup battery is enabled */
> -	u8 setup = RX8901_REG_PWSW_CFG_INIEN;
> +	/*
> +	 * make sure that the backup battery is enabled and that battery
> +	 * voltage detection is performed
> +	 */
> +	u8 setup = RX8901_REG_PWSW_CFG_INIEN | RX8901_REG_PWSW_CFG_VBATLDETEN;
>  
>  	if (diode)
>  		setup |= RX8901_REG_PWSW_CFG_CHGEN;
> @@ -1005,6 +1044,7 @@ static const struct rtc_class_ops rx8130_rtc_ops = {
>  static const struct rtc_class_ops rx8901_rtc_ops = {
>  	.read_time      = ds1307_get_time,
>  	.set_time       = ds1307_set_time,
> +	.ioctl          = rx8901_ioctl,
>  };
>  

This seems to be an unrelated changed that hasn't been squashed in the
proper patch.


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