linux-kernel - [PATCH] rtc: pcf-8563: Report previously detected low-voltage via RTC_VL_BACKUP

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Date:   Fri, 9 Jun 2023 23:04:12 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        linux-rtc@...r.kernel.org
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] rtc: pcf-8563: Report previously detected low-voltage via
 RTC_VL_BACKUP_LOW

From: Jan Kiszka <jan.kiszka@...mens.com>

The VL bit in the seconds register remains set only until seconds are
written under main power. As this often happens during boot-up after
picking up a network time, make sure to preserve the low battery state
across this, caching it and returning it via the RTC_VL_BACKUP_LOW bit.

To permit userspace clearing this state during runtime, also implement
RTC_VL_CLR that works against the cached state.

This is emulating RTCs which have a battery voltage check that works
under main power as well.

Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
---
 drivers/rtc/rtc-pcf8563.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/rtc-pcf8563.c b/drivers/rtc/rtc-pcf8563.c
index 7e720472213c..f8c6cdb9a39d 100644
--- a/drivers/rtc/rtc-pcf8563.c
+++ b/drivers/rtc/rtc-pcf8563.c
@@ -81,6 +81,7 @@ struct pcf8563 {
 #ifdef CONFIG_COMMON_CLK
 	struct clk_hw		clkout_hw;
 #endif
+	bool low_bat;
 };
 
 static int pcf8563_read_block_data(struct i2c_client *client, unsigned char reg,
@@ -207,6 +208,7 @@ static int pcf8563_rtc_read_time(struct device *dev, struct rtc_time *tm)
 		return err;
 
 	if (buf[PCF8563_REG_SC] & PCF8563_SC_LV) {
+		pcf8563->low_bat = true;
 		dev_err(&client->dev,
 			"low voltage detected, date/time is not reliable.\n");
 		return -EINVAL;
@@ -277,6 +279,8 @@ static int pcf8563_rtc_set_time(struct device *dev, struct rtc_time *tm)
 static int pcf8563_rtc_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
 {
 	struct i2c_client *client = to_i2c_client(dev);
+	struct pcf8563 *pcf8563 = i2c_get_clientdata(client);
+	unsigned int state = 0;
 	int ret;
 
 	switch (cmd) {
@@ -284,9 +288,16 @@ static int pcf8563_rtc_ioctl(struct device *dev, unsigned int cmd, unsigned long
 		ret = i2c_smbus_read_byte_data(client, PCF8563_REG_SC);
 		if (ret < 0)
 			return ret;
-
-		return put_user(ret & PCF8563_SC_LV ? RTC_VL_DATA_INVALID : 0,
-				(unsigned int __user *)arg);
+		if (ret & PCF8563_SC_LV) {
+			state |= RTC_VL_DATA_INVALID;
+			pcf8563->low_bat = true;
+		}
+		if (pcf8563->low_bat)
+			state |= RTC_VL_BACKUP_LOW;
+		return put_user(state, (unsigned int __user *)arg);
+	case RTC_VL_CLR:
+		pcf8563->low_bat = false;
+		return 0;
 	default:
 		return -ENOIOCTLCMD;
 	}
-- 
2.35.3