hwmon: (lm90) Fix/Add detection of G781-1

When support for G781 was added, chips with ID 0x01 were found at I2C
addresses 0x4c and 0x4d. The G781 datasheet (version 1.3 from October 2003)
says that the device ID for G781-1 is 0x03, not 0x01. Also, the datasheet
states that the chip at I2C address is G781 and the chip at I2C address
0x4d is G781-1.

A G781-1 at I2C address 0x4d was now found to have a chip ID of 0x03
as suggested by the datasheet. Accept both 0x01 and 0x03 chip IDs at both
addresses to ensure that all variants of G781 are detected properly.

While at it, improve chip detection accuracy by reading two additional
registers and ensuring that only expected bits are set in those registers.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/lm90.c

@@ -1730,13 +1730,42 @@ static const char *lm90_detect_gmt(struct i2c_client *client, int chip_id,
 				   int config1, int convrate)
 {
 	int address = client->addr;
-	const char *name = NULL;
 
-	if ((address == 0x4c || address == 0x4d) && chip_id == 0x01 &&
-	    !(config1 & 0x3f) && convrate <= 0x08)
-		name = "g781";
+	/*
+	 * According to the datasheet, G781 is supposed to be at I2C Address
+	 * 0x4c and have a chip ID of 0x01. G781-1 is supposed to be at I2C
+	 * address 0x4d and have a chip ID of 0x03. However, when support
+	 * for G781 was added, chips at 0x4c and 0x4d were found to have a
+	 * chip ID of 0x01. A G781-1 at I2C address 0x4d was now found with
+	 * chip ID 0x03.
+	 * To avoid detection failures, accept chip ID 0x01 and 0x03 at both
+	 * addresses.
+	 * G784 reports manufacturer ID 0x47 and chip ID 0x01. A public
+	 * datasheet is not available. Extensive testing suggests that
+	 * the chip appears to be fully compatible with G781.
+	 * Available register dumps show that G751 also reports manufacturer
+	 * ID 0x47 and chip ID 0x01 even though that chip does not officially
+	 * support those registers. This makes chip detection somewhat
+	 * vulnerable. To improve detection quality, read the offset low byte
+	 * and alert fault queue registers and verify that only expected bits
+	 * are set.
+	 */
+	if ((chip_id == 0x01 || chip_id == 0x03) &&
+	    (address == 0x4c || address == 0x4d) &&
+	    !(config1 & 0x3f) && convrate <= 0x08) {
+		int reg;
 
-	return name;
+		reg = i2c_smbus_read_byte_data(client, LM90_REG_REMOTE_OFFSL);
+		if (reg < 0 || reg & 0x1f)
+			return NULL;
+		reg = i2c_smbus_read_byte_data(client, TMP451_REG_CONALERT);
+		if (reg < 0 || reg & 0xf1)
+			return NULL;
+
+		return "g781";
+	}
+
+	return NULL;
 }
 
 static const char *lm90_detect_ti(struct i2c_client *client, int chip_id,