rtc: ds1307: add temperature sensor support for ds3231

DS3231 has the temperature registers with a resolution of 0.25
degree celsius.  This enables to get the value through hwmon.

	# cat /sys/class/i2c-adapter/i2c-2/2-0068/hwmon/hwmon0/temp1_input
	21000

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/Kconfig

@@ -212,6 +212,15 @@ config RTC_DRV_DS1307
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-ds1307.
 
+config RTC_DRV_DS1307_HWMON
+	bool "HWMON support for rtc-ds1307"
+	depends on RTC_DRV_DS1307 && HWMON
+	depends on !(RTC_DRV_DS1307=y && HWMON=m)
+	default y
+	help
+	  Say Y here if you want to expose temperature sensor data on
+	  rtc-ds1307 (only DS3231)
+
 config RTC_DRV_DS1374
 	tristate "Dallas/Maxim DS1374"
 	help

drivers/rtc/rtc-ds1307.c

@@ -19,6 +19,8 @@
 #include <linux/rtc.h>
 #include <linux/slab.h>
 #include <linux/string.h>
+#include <linux/hwmon.h>
+#include <linux/hwmon-sysfs.h>
 
 /*
  * We can't determine type by probing, but if we expect pre-Linux code
@@ -842,6 +844,90 @@ out:
 	return;
 }
 
+/*----------------------------------------------------------------------*/
+
+#ifdef CONFIG_RTC_DRV_DS1307_HWMON
+
+/*
+ * Temperature sensor support for ds3231 devices.
+ */
+
+#define DS3231_REG_TEMPERATURE	0x11
+
+/*
+ * A user-initiated temperature conversion is not started by this function,
+ * so the temperature is updated once every 64 seconds.
+ */
+static int ds3231_hwmon_read_temp(struct device *dev, s16 *mC)
+{
+	struct ds1307 *ds1307 = dev_get_drvdata(dev);
+	u8 temp_buf[2];
+	s16 temp;
+	int ret;
+
+	ret = ds1307->read_block_data(ds1307->client, DS3231_REG_TEMPERATURE,
+					sizeof(temp_buf), temp_buf);
+	if (ret < 0)
+		return ret;
+	if (ret != sizeof(temp_buf))
+		return -EIO;
+
+	/*
+	 * Temperature is represented as a 10-bit code with a resolution of
+	 * 0.25 degree celsius and encoded in two's complement format.
+	 */
+	temp = (temp_buf[0] << 8) | temp_buf[1];
+	temp >>= 6;
+	*mC = temp * 250;
+
+	return 0;
+}
+
+static ssize_t ds3231_hwmon_show_temp(struct device *dev,
+				struct device_attribute *attr, char *buf)
+{
+	int ret;
+	s16 temp;
+
+	ret = ds3231_hwmon_read_temp(dev, &temp);
+	if (ret)
+		return ret;
+
+	return sprintf(buf, "%d\n", temp);
+}
+static SENSOR_DEVICE_ATTR(temp1_input, S_IRUGO, ds3231_hwmon_show_temp,
+			NULL, 0);
+
+static struct attribute *ds3231_hwmon_attrs[] = {
+	&sensor_dev_attr_temp1_input.dev_attr.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(ds3231_hwmon);
+
+static void ds1307_hwmon_register(struct ds1307 *ds1307)
+{
+	struct device *dev;
+
+	if (ds1307->type != ds_3231)
+		return;
+
+	dev = devm_hwmon_device_register_with_groups(&ds1307->client->dev,
+						ds1307->client->name,
+						ds1307, ds3231_hwmon_groups);
+	if (IS_ERR(dev)) {
+		dev_warn(&ds1307->client->dev,
+			"unable to register hwmon device %ld\n", PTR_ERR(dev));
+	}
+}
+
+#else
+
+static void ds1307_hwmon_register(struct ds1307 *ds1307)
+{
+}
+
+#endif
+
 static int ds1307_probe(struct i2c_client *client,
 			const struct i2c_device_id *id)
 {
@@ -1207,6 +1293,8 @@ read_rtc:
 		}
 	}
 
+	ds1307_hwmon_register(ds1307);
+
 	return 0;
 
 exit: