eeprom: at24: Probe for DDR3 thermal sensor in the SPD case

The DDR3 SPD data structure advertises the presence of a thermal
sensor on a DDR3 module in byte 32, bit 7. Let's use this information
to explicitly instantiate the thermal sensor I2C client instead of
having to rely on class-based I2C probing.

The temp sensor i2c address can be derived from the SPD i2c address,
so we can directly instantiate the device and don't have to probe
for it. If the temp sensor has been instantiated already by other
means (e.g. class-based auto-detection), then the busy-check in
i2c_new_client_device will detect this.

Note: Thermal sensors on DDR4 DIMM's are instantiated from the
      ee1004 driver.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/68113672-3724-44d5-9ff8-313dd6628f8c@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/eeprom/at24.c

@@ -563,6 +563,31 @@ static unsigned int at24_get_offset_adj(u8 flags, unsigned int byte_len)
 	}
 }
 
+static void at24_probe_temp_sensor(struct i2c_client *client)
+{
+	struct at24_data *at24 = i2c_get_clientdata(client);
+	struct i2c_board_info info = { .type = "jc42" };
+	int ret;
+	u8 val;
+
+	/*
+	 * Byte 2 has value 11 for DDR3, earlier versions don't
+	 * support the thermal sensor present flag
+	 */
+	ret = at24_read(at24, 2, &val, 1);
+	if (ret || val != 11)
+		return;
+
+	/* Byte 32, bit 7 is set if temp sensor is present */
+	ret = at24_read(at24, 32, &val, 1);
+	if (ret || !(val & BIT(7)))
+		return;
+
+	info.addr = 0x18 | (client->addr & 7);
+
+	i2c_new_client_device(client->adapter, &info);
+}
+
 static int at24_probe(struct i2c_client *client)
 {
 	struct regmap_config regmap_config = { };
@@ -762,6 +787,10 @@ static int at24_probe(struct i2c_client *client)
 		}
 	}
 
+	/* If this a SPD EEPROM, probe for DDR3 thermal sensor */
+	if (cdata == &at24_data_spd)
+		at24_probe_temp_sensor(client);
+
 	pm_runtime_idle(dev);
 
 	if (writable)