rtc: ds1307: add support for watchdog timer on ds1388

The DS1388 variant has watchdog timer capabilities. When using a DS1388
and having enabled CONFIG_WATCHDOG_CORE register a watchdog device for
the DS1388.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200330025500.6991-1-chris.packham@alliedtelesis.co.nz
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -238,6 +238,7 @@ config RTC_DRV_AS3722
 config RTC_DRV_DS1307
 	tristate "Dallas/Maxim DS1307/37/38/39/40/41, ST M41T00, EPSON RX-8025, ISL12057"
 	select REGMAP_I2C
+	select WATCHDOG_CORE if WATCHDOG
 	help
 	  If you say yes here you get support for various compatible RTC
 	  chips (often with battery backup) connected with I2C. This driver

drivers/rtc/rtc-ds1307.c

@@ -22,6 +22,7 @@
 #include <linux/hwmon-sysfs.h>
 #include <linux/clk-provider.h>
 #include <linux/regmap.h>
+#include <linux/watchdog.h>
 
 /*
  * We can't determine type by probing, but if we expect pre-Linux code
@@ -144,8 +145,15 @@ enum ds_type {
 #	define M41TXX_BIT_CALIB_SIGN	BIT(5)
 #	define M41TXX_M_CALIBRATION	GENMASK(4, 0)
 
+#define DS1388_REG_WDOG_HUN_SECS	0x08
+#define DS1388_REG_WDOG_SECS		0x09
 #define DS1388_REG_FLAG			0x0b
+#	define DS1388_BIT_WF		BIT(6)
 #	define DS1388_BIT_OSF		BIT(7)
+#define DS1388_REG_CONTROL		0x0c
+#	define DS1388_BIT_RST		BIT(0)
+#	define DS1388_BIT_WDE		BIT(1)
+
 /* negative offset step is -2.034ppm */
 #define M41TXX_NEG_OFFSET_STEP_PPB	2034
 /* positive offset step is +4.068ppm */
@@ -854,6 +862,72 @@ static int m41txx_rtc_set_offset(struct device *dev, long offset)
 				  ctrl_reg);
 }
 
+#ifdef CONFIG_WATCHDOG_CORE
+static int ds1388_wdt_start(struct watchdog_device *wdt_dev)
+{
+	struct ds1307 *ds1307 = watchdog_get_drvdata(wdt_dev);
+	u8 regs[2];
+	int ret;
+
+	ret = regmap_update_bits(ds1307->regmap, DS1388_REG_FLAG,
+				 DS1388_BIT_WF, 0);
+	if (ret)
+		return ret;
+
+	ret = regmap_update_bits(ds1307->regmap, DS1388_REG_CONTROL,
+				 DS1388_BIT_WDE | DS1388_BIT_RST, 0);
+	if (ret)
+		return ret;
+
+	/*
+	 * watchdog timeouts are measured in seconds. So ignore hundredths of
+	 * seconds field.
+	 */
+	regs[0] = 0;
+	regs[1] = bin2bcd(wdt_dev->timeout);
+
+	ret = regmap_bulk_write(ds1307->regmap, DS1388_REG_WDOG_HUN_SECS, regs,
+				sizeof(regs));
+	if (ret)
+		return ret;
+
+	return regmap_update_bits(ds1307->regmap, DS1388_REG_CONTROL,
+				  DS1388_BIT_WDE | DS1388_BIT_RST,
+				  DS1388_BIT_WDE | DS1388_BIT_RST);
+}
+
+static int ds1388_wdt_stop(struct watchdog_device *wdt_dev)
+{
+	struct ds1307 *ds1307 = watchdog_get_drvdata(wdt_dev);
+
+	return regmap_update_bits(ds1307->regmap, DS1388_REG_CONTROL,
+				  DS1388_BIT_WDE | DS1388_BIT_RST, 0);
+}
+
+static int ds1388_wdt_ping(struct watchdog_device *wdt_dev)
+{
+	struct ds1307 *ds1307 = watchdog_get_drvdata(wdt_dev);
+	u8 regs[2];
+
+	return regmap_bulk_read(ds1307->regmap, DS1388_REG_WDOG_HUN_SECS, regs,
+				sizeof(regs));
+}
+
+static int ds1388_wdt_set_timeout(struct watchdog_device *wdt_dev,
+				  unsigned int val)
+{
+	struct ds1307 *ds1307 = watchdog_get_drvdata(wdt_dev);
+	u8 regs[2];
+
+	wdt_dev->timeout = val;
+	regs[0] = 0;
+	regs[1] = bin2bcd(wdt_dev->timeout);
+
+	return regmap_bulk_write(ds1307->regmap, DS1388_REG_WDOG_HUN_SECS, regs,
+				 sizeof(regs));
+}
+#endif
+
 static const struct rtc_class_ops rx8130_rtc_ops = {
 	.read_time      = ds1307_get_time,
 	.set_time       = ds1307_set_time,
@@ -1576,6 +1650,46 @@ static void ds1307_clks_register(struct ds1307 *ds1307)
 
 #endif /* CONFIG_COMMON_CLK */
 
+#ifdef CONFIG_WATCHDOG_CORE
+static const struct watchdog_info ds1388_wdt_info = {
+	.options = WDIOF_SETTIMEOUT | WDIOF_KEEPALIVEPING | WDIOF_MAGICCLOSE,
+	.identity = "DS1388 watchdog",
+};
+
+static const struct watchdog_ops ds1388_wdt_ops = {
+	.owner = THIS_MODULE,
+	.start = ds1388_wdt_start,
+	.stop = ds1388_wdt_stop,
+	.ping = ds1388_wdt_ping,
+	.set_timeout = ds1388_wdt_set_timeout,
+
+};
+
+static void ds1307_wdt_register(struct ds1307 *ds1307)
+{
+	struct watchdog_device	*wdt;
+
+	if (ds1307->type != ds_1388)
+		return;
+
+	wdt = devm_kzalloc(ds1307->dev, sizeof(*wdt), GFP_KERNEL);
+
+	wdt->info = &ds1388_wdt_info;
+	wdt->ops = &ds1388_wdt_ops;
+	wdt->timeout = 99;
+	wdt->max_timeout = 99;
+	wdt->min_timeout = 1;
+
+	watchdog_init_timeout(wdt, 0, ds1307->dev);
+	watchdog_set_drvdata(wdt, ds1307);
+	devm_watchdog_register_device(ds1307->dev, wdt);
+}
+#else
+static void ds1307_wdt_register(struct ds1307 *ds1307)
+{
+}
+#endif /* CONFIG_WATCHDOG_CORE */
+
 static const struct regmap_config regmap_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
@@ -1865,6 +1979,7 @@ static int ds1307_probe(struct i2c_client *client,
 
 	ds1307_hwmon_register(ds1307);
 	ds1307_clks_register(ds1307);
+	ds1307_wdt_register(ds1307);
 
 	return 0;