hwmon: (lm75) Document why clones are not detected

Explain why clones of the LM75 are generally not detected by the
driver, and why this isn't going to change. Also update the
documentation to reflect the list of chip names currently supported by
the driver.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>

Documentation/hwmon/lm75

@@ -12,31 +12,46 @@ Supported chips:
     Addresses scanned: I2C 0x48 - 0x4f
     Datasheet: Publicly available at the National Semiconductor website
                http://www.national.com/
-  * Dallas Semiconductor DS75
-    Prefix: 'lm75'
-    Addresses scanned: I2C 0x48 - 0x4f
-    Datasheet: Publicly available at the Dallas Semiconductor website
-               http://www.maxim-ic.com/
-  * Dallas Semiconductor DS1775
-    Prefix: 'lm75'
-    Addresses scanned: I2C 0x48 - 0x4f
+  * Dallas Semiconductor DS75, DS1775
+    Prefixes: 'ds75', 'ds1775'
+    Addresses scanned: none
     Datasheet: Publicly available at the Dallas Semiconductor website
                http://www.maxim-ic.com/
   * Maxim MAX6625, MAX6626
-    Prefix: 'lm75'
-    Addresses scanned: I2C 0x48 - 0x4b
+    Prefixes: 'max6625', 'max6626'
+    Addresses scanned: none
     Datasheet: Publicly available at the Maxim website
                http://www.maxim-ic.com/
   * Microchip (TelCom) TCN75
     Prefix: 'lm75'
-    Addresses scanned: I2C 0x48 - 0x4f
+    Addresses scanned: none
+    Datasheet: Publicly available at the Microchip website
+               http://www.microchip.com/
+  * Microchip MCP9800, MCP9801, MCP9802, MCP9803
+    Prefix: 'mcp980x'
+    Addresses scanned: none
     Datasheet: Publicly available at the Microchip website
                http://www.microchip.com/
   * Analog Devices ADT75
     Prefix: 'adt75'
-    Addresses scanned: I2C 0x48 - 0x4f
+    Addresses scanned: none
     Datasheet: Publicly available at the Analog Devices website
                http://www.analog.com/adt75
+  * ST Microelectronics STDS75
+    Prefix: 'stds75'
+    Addresses scanned: none
+    Datasheet: Publicly available at the ST website
+               http://www.st.com/internet/analog/product/121769.jsp
+  * Texas Instruments TMP100, TMP101, TMP105, TMP75, TMP175, TMP275
+    Prefixes: 'tmp100', 'tmp101', 'tmp105', 'tmp175', 'tmp75', 'tmp275'
+    Addresses scanned: none
+    Datasheet: Publicly available at the Texas Instruments website
+               http://www.ti.com/product/tmp100
+               http://www.ti.com/product/tmp101
+               http://www.ti.com/product/tmp105
+               http://www.ti.com/product/tmp75
+               http://www.ti.com/product/tmp175
+               http://www.ti.com/product/tmp275
 
 Author: Frodo Looijaard <frodol@dds.nl>
 
@@ -55,21 +70,16 @@ range of -55 to +125 degrees.
 The LM75 only updates its values each 1.5 seconds; reading it more often
 will do no harm, but will return 'old' values.
 
-The LM75 is usually used in combination with LM78-like chips, to measure
-the temperature of the processor(s).
-
-The DS75, DS1775, MAX6625, and MAX6626 are supported as well.
-They are not distinguished from an LM75. While most of these chips
-have three additional bits of accuracy (12 vs. 9 for the LM75),
-the additional bits are not supported. Not only that, but these chips will
-not be detected if not in 9-bit precision mode (use the force parameter if
-needed).
-
-The TCN75 is supported as well, and is not distinguished from an LM75.
+The original LM75 was typically used in combination with LM78-like chips
+on PC motherboards, to measure the temperature of the processor(s). Clones
+are now used in various embedded designs.
 
 The LM75 is essentially an industry standard; there may be other
 LM75 clones not listed here, with or without various enhancements,
-that are supported.
+that are supported. The clones are not detected by the driver, unless
+they reproduce the exact register tricks of the original LM75, and must
+therefore be instantiated explicitly. The specific enhancements (such as
+higher resolution) are not currently supported by the driver.
 
 The LM77 is not supported, contrary to what we pretended for a long time.
 Both chips are simply not compatible, value encoding differs.

drivers/hwmon/lm75.c

@@ -249,19 +249,30 @@ static int lm75_detect(struct i2c_client *new_client,
 				     I2C_FUNC_SMBUS_WORD_DATA))
 		return -ENODEV;
 
-	/* Now, we do the remaining detection. There is no identification-
-	   dedicated register so we have to rely on several tricks:
-	   unused bits, registers cycling over 8-address boundaries,
-	   addresses 0x04-0x07 returning the last read value.
-	   The cycling+unused addresses combination is not tested,
-	   since it would significantly slow the detection down and would
-	   hardly add any value.
-
-	   The National Semiconductor LM75A is different than earlier
-	   LM75s.  It has an ID byte of 0xaX (where X is the chip
-	   revision, with 1 being the only revision in existence) in
-	   register 7, and unused registers return 0xff rather than the
-	   last read value. */
+	/*
+	 * Now, we do the remaining detection. There is no identification-
+	 * dedicated register so we have to rely on several tricks:
+	 * unused bits, registers cycling over 8-address boundaries,
+	 * addresses 0x04-0x07 returning the last read value.
+	 * The cycling+unused addresses combination is not tested,
+	 * since it would significantly slow the detection down and would
+	 * hardly add any value.
+	 *
+	 * The National Semiconductor LM75A is different than earlier
+	 * LM75s.  It has an ID byte of 0xaX (where X is the chip
+	 * revision, with 1 being the only revision in existence) in
+	 * register 7, and unused registers return 0xff rather than the
+	 * last read value.
+	 *
+	 * Note that this function only detects the original National
+	 * Semiconductor LM75 and the LM75A. Clones from other vendors
+	 * aren't detected, on purpose, because they are typically never
+	 * found on PC hardware. They are found on embedded designs where
+	 * they can be instantiated explicitly so detection is not needed.
+	 * The absence of identification registers on all these clones
+	 * would make their exhaustive detection very difficult and weak,
+	 * and odds are that the driver would bind to unsupported devices.
+	 */
 
 	/* Unused bits */
 	conf = i2c_smbus_read_byte_data(new_client, 1);