hwmon: (sbtsi) Add documentation

Document the SB-TSI sensor interface driver.
Signed-off-by: Kun Yi <kunyi@google.com>
Link: https://lore.kernel.org/r/20201211215427.3281681-3-kunyi@google.com
[groeck: Added SPDX license identifier, same as source]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -154,6 +154,7 @@ Hardware Monitoring Kernel Drivers
    pwm-fan
    q54sj108a2
    raspberrypi-hwmon
+   sbtsi_temp
    sch5627
    sch5636
    scpi-hwmon

Documentation/hwmon/sbtsi_temp.rst

+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver sbtsi_temp
+==================
+
+Supported hardware:
+
+  * Sideband interface (SBI) Temperature Sensor Interface (SB-TSI)
+    compliant AMD SoC temperature device.
+
+    Prefix: 'sbtsi_temp'
+
+    Addresses scanned: This driver doesn't support address scanning.
+
+    To instantiate this driver on an AMD CPU with SB-TSI
+    support, the i2c bus number would be the bus connected from the board
+    management controller (BMC) to the CPU. The i2c address is specified in
+    Section 6.3.1 of the SoC register reference: The SB-TSI address is normally
+    98h for socket 0 and 90h for socket 1, but it could vary based on hardware
+    address select pins.
+
+    Datasheet: The SB-TSI interface and protocol is available as part of
+               the open source SoC register reference at:
+
+	       https://www.amd.com/system/files/TechDocs/56255_OSRR.pdf
+
+               The Advanced Platform Management Link (APML) Specification is
+               available at:
+
+	       http://developer.amd.com/wordpress/media/2012/10/41918.pdf
+
+Author: Kun Yi <kunyi@google.com>
+
+Description
+-----------
+
+The SBI temperature sensor interface (SB-TSI) is an emulation of the software
+and physical interface of a typical 8-pin remote temperature sensor (RTS) on
+AMD SoCs. It implements one temperature sensor with readings and limit
+registers encode the temperature in increments of 0.125 from 0 to 255.875.
+Limits can be set through the writable thresholds, and if reached will trigger
+corresponding alert signals.