hwmon: (sbrmi) Add Documentation

- Add documentation for SB-RMI module

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Akshay Gupta <Akshay.Gupta@amd.com>
Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Link: https://lore.kernel.org/r/20210726133615.9709-2-nchatrad@amd.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -160,6 +160,7 @@ Hardware Monitoring Kernel Drivers
    pwm-fan
    q54sj108a2
    raspberrypi-hwmon
+   sbrmi
    sbtsi_temp
    sch5627
    sch5636

Documentation/hwmon/sbrmi.rst

@@ -0,0 +1,79 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver sbrmi
+===================
+
+Supported hardware:
+
+  * Sideband Remote Management Interface (SB-RMI) compliant AMD SoC
+    device connected to the BMC via the APML.
+
+    Prefix: 'sbrmi'
+
+    Addresses scanned: This driver doesn't support address scanning.
+
+    To instantiate this driver on an AMD CPU with SB-RMI
+    support, the i2c bus number would be the bus connected from the board
+    management controller (BMC) to the CPU.
+    The SMBus address is really 7 bits. Some vendors and the SMBus
+    specification show the address as 8 bits, left justified with the R/W
+    bit as a write (0) making bit 0. Some vendors use only the 7 bits
+    to describe the address.
+    As mentioned in AMD's APML specification, The SB-RMI address is
+    normally 78h(0111 100W) or 3Ch(011 1100) for socket 0 and 70h(0111 000W)
+    or 38h(011 1000) for socket 1, but it could vary based on hardware
+    address select pins.
+
+    Datasheet: The SB-RMI interface and protocol along with the Advanced
+               Platform Management Link (APML) Specification is available
+               as part of the open source SoC register reference at:
+
+               https://www.amd.com/en/support/tech-docs?keyword=55898
+
+Author: Akshay Gupta <akshay.gupta@amd.com>
+
+Description
+-----------
+
+The APML provides a way to communicate with the SB Remote Management interface
+(SB-RMI) module from the external SMBus master that can be used to report socket
+power on AMD platforms using mailbox command and resembles a typical 8-pin remote
+power sensor's I2C interface to BMC.
+
+This driver implements current power with power cap and power cap max.
+
+sysfs-Interface
+---------------
+Power sensors can be queried and set via the standard ``hwmon`` interface
+on ``sysfs``, under the directory ``/sys/class/hwmon/hwmonX`` for some value
+of ``X`` (search for the ``X`` such that ``/sys/class/hwmon/hwmonX/name`` has
+content ``sbrmi``)
+
+================ ===== ========================================================
+Name             Perm   Description
+================ ===== ========================================================
+power1_input     RO    Current Power consumed
+power1_cap       RW    Power limit can be set between 0 and power1_cap_max
+power1_cap_max   RO    Maximum powerlimit calculated and reported by the SMU FW
+================ ===== ========================================================
+
+The following example show how the 'Power' attribute from the i2c-addresses
+can be monitored using the userspace utilities like ``sensors`` binary::
+
+  # sensors
+  sbrmi-i2c-1-38
+  Adapter: bcm2835 I2C adapter
+  power1:       61.00 W (cap = 225.00 W)
+
+  sbrmi-i2c-1-3c
+  Adapter: bcm2835 I2C adapter
+  power1:       28.39 W (cap = 224.77 W)
+  #
+
+Also, Below shows how get and set the values from sysfs entries individually::
+  # cat /sys/class/hwmon/hwmon1/power1_cap_max
+  225000000
+
+  # echo 180000000 > /sys/class/hwmon/hwmon1/power1_cap
+  # cat /sys/class/hwmon/hwmon1/power1_cap
+  180000000