w1: w1_therm: Add support for Maxim MAX31850 thermoelement IF.

MAX31850 shares family number 0x3B with DS1825. The device is generally
compatible with DS1825 but needs a different temperature readout.
It operates always in 14 bit mode and has all 4 higher bits of the
Config register set to 1. Conversion time is 100ms.

Signed-off-by: Markus Reichl <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/w1/slaves/w1_therm.c

···
 		return SLAVE_CONV_TIME_OVERRIDE(sl);
 }
 
+static inline int w1_DS1825_convert_time(struct w1_slave *sl)
+{
+	int ret;
+
+	if (!sl->family_data)
+		return -ENODEV;	/* device unknown */
+
+	if (SLAVE_CONV_TIME_OVERRIDE(sl) != CONV_TIME_DEFAULT)
+		return SLAVE_CONV_TIME_OVERRIDE(sl);
+
+	/* Return the conversion time, depending on resolution,
+	 * select maximum conversion time among all compatible devices
+	 */
+	switch (SLAVE_RESOLUTION(sl)) {
+	case 9:
+		ret = 95;
+		break;
+	case 10:
+		ret = 190;
+		break;
+	case 11:
+		ret = 375;
+		break;
+	case 12:
+		ret = 750;
+		break;
+	case 14:
+		ret = 100; /* MAX31850 only. Datasheet says 100ms  */
+		break;
+	default:
+		ret = 750;
+	}
+	return ret;
+}
+
 static inline int w1_DS18B20_write_data(struct w1_slave *sl,
 				const u8 *data)
 {
···
 
 	/* DS18B20 resolution is 9 to 12 bits */
 	/* GX20MH01 resolution is 9 to 14 bits */
+	/* MAX31850 resolution is fixed 14 bits */
 	if (val < W1_THERM_RESOLUTION_MIN || val > W1_THERM_RESOLUTION_MAX)
 		return -EINVAL;
 
···
 		+ W1_THERM_RESOLUTION_MIN;
 	/* GX20MH01 has one special case:
 	 *   >=14 means 14 bits when getting resolution from bit value.
+	 * MAX31850 delivers fixed 15 and has 14 bits.
 	 * Other devices have no more then 12 bits.
 	 */
 	if (resolution > W1_THERM_RESOLUTION_MAX)
···
 	return t;
 }
 
+/**
+ * w1_DS1825_convert_temp() - temperature computation for DS1825
+ * @rom: data read from device RAM (8 data bytes + 1 CRC byte)
+ *
+ * Can be called for any DS1825 compliant device.
+ * Is used by MAX31850, too
+ *
+ * Return: value in millidegrees Celsius.
+ */
+
+static inline int w1_DS1825_convert_temp(u8 rom[9])
+{
+	u16 bv;
+	s16 t;
+
+	/* Signed 16-bit value to unsigned, cpu order */
+	bv = le16_to_cpup((__le16 *)rom);
+
+	/* Config register bit 7 = 1 - MA31850 found, 14 bit resolution */
+	if (rom[4] & 0x80) {
+		/* Mask out bits 0 (Fault) and 1 (Reserved) */
+		/* Avoid arithmetic shift of signed value */
+		bv = (bv & 0xFFFC); /* Degrees, lowest 4 bits are 2^-1, 2^-2 and 2 zero bits */
+	}
+	t = (s16)bv;	/* Degrees, lowest bit is 2^-4 */
+	return (int)t * 1000 / 16;	/* Sign-extend to int; millidegrees */
+}
+
 /* Device capability description */
 /* GX20MH01 device shares family number and structure with DS18B20 */
 
···
 		.bulk_read			= false
 	},
 	{
+		/* Also used for MAX31850 */
 		.f				= &w1_therm_family_DS1825,
-		.convert			= w1_DS18B20_convert_temp,
-		.get_conversion_time	= w1_DS18B20_convert_time,
+		.convert			= w1_DS1825_convert_temp,
+		.get_conversion_time	= w1_DS1825_convert_time,
 		.set_resolution		= w1_DS18B20_set_resolution,
 		.get_resolution		= w1_DS18B20_get_resolution,
 		.write_data			= w1_DS18B20_write_data,