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

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Message-ID: <20251219-ds1307-rx8901-add-v1-4-b13f346ebe93@axis.com>
Date: Fri, 19 Dec 2025 13:10:38 +0100
From: Fredrik M Olsson <fredrik.m.olsson@...s.com>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>, Rob Herring
	<robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Nobuhiro Iwamatsu <nobuhiro.iwamatsu.x90@...l.toshiba>
CC: <linux-rtc@...r.kernel.org>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Fredrik M Olsson <fredrik.m.olsson@...s.com>,
	<kernel@...s.com>
Subject: [PATCH 4/4] rtc: ds1307: Add support for reading RX8901CE battery
 VL status

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,
 };
 
 static const struct rtc_class_ops mcp794xx_rtc_ops = {

-- 
2.43.0