hwmon: (amd_energy) Add documentation

Document amd_energy driver with energy sensors supported by it.

Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Link: https://lore.kernel.org/r/20200519155011.56184-2-nchatrad@amd.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/amd_energy.rst

@@ -0,0 +1,109 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Kernel driver amd_energy
+==========================
+
+Supported chips:
+
+* AMD Family 17h Processors
+
+  Prefix: 'amd_energy'
+
+  Addresses used:  RAPL MSRs
+
+  Datasheets:
+
+  - Processor Programming Reference (PPR) for AMD Family 17h Model 01h, Revision B1 Processors
+
+	https://developer.amd.com/wp-content/resources/55570-B1_PUB.zip
+
+  - Preliminary Processor Programming Reference (PPR) for AMD Family 17h Model 31h, Revision B0 Processors
+
+	https://developer.amd.com/wp-content/resources/56176_ppr_Family_17h_Model_71h_B0_pub_Rev_3.06.zip
+
+Author: Naveen Krishna Chatradhi <nchatrad@amd.com>
+
+Description
+-----------
+
+The Energy driver exposes the energy counters that are
+reported via the Running Average Power Limit (RAPL)
+Model-specific Registers (MSRs) via the hardware monitor
+(HWMON) sysfs interface.
+
+1. Power, Energy and Time Units
+   MSR_RAPL_POWER_UNIT/ C001_0299:
+   shared with all cores in the socket
+
+2. Energy consumed by each Core
+   MSR_CORE_ENERGY_STATUS/ C001_029A:
+   32-bitRO, Accumulator, core-level power reporting
+
+3. Energy consumed by Socket
+   MSR_PACKAGE_ENERGY_STATUS/ C001_029B:
+   32-bitRO, Accumulator, socket-level power reporting,
+   shared with all cores in socket
+
+These registers are updated every 1ms and cleared on
+reset of the system.
+
+Note: If SMT is enabled, Linux enumerates all threads as cpus.
+Since, the energy status registers are accessed at core level,
+reading those registers from the sibling threads would result
+in duplicate values. Hence, energy counter entries are not
+populated for the siblings.
+
+Energy Caluclation
+------------------
+
+Energy information (in Joules) is based on the multiplier,
+1/2^ESU; where ESU is an unsigned integer read from
+MSR_RAPL_POWER_UNIT register. Default value is 10000b,
+indicating energy status unit is 15.3 micro-Joules increment.
+
+Reported values are scaled as per the formula
+
+scaled value = ((1/2^ESU) * (Raw value) * 1000000UL) in uJoules
+
+Users calculate power for a given domain by calculating
+	dEnergy/dTime for that domain.
+
+Energy accumulation
+--------------------------
+
+Current, Socket energy status register is 32bit, assuming a 240W
+2P system, the register would wrap around in
+
+	2^32*15.3 e-6/240 * 2 = 547.60833024 secs to wrap(~9 mins)
+
+The Core energy register may wrap around after several days.
+
+To improve the wrap around time, a kernel thread is implemented
+to accumulate the socket energy counters and one core energy counter
+per run to a respective 64-bit counter. The kernel thread starts
+running during probe, wakes up every 100secs and stops running
+when driver is removed.
+
+A socket and core energy read would return the current register
+value added to the respective energy accumulator.
+
+Sysfs attributes
+----------------
+
+=============== ========  =====================================
+Attribute	Label	  Description
+===============	========  =====================================
+
+* For index N between [1] and [nr_cpus]
+
+===============	========  ======================================
+energy[N]_input EcoreX	  Core Energy   X = [0] to [nr_cpus - 1]
+			  Measured input core energy
+===============	========  ======================================
+
+* For N between [nr_cpus] and [nr_cpus + nr_socks]
+
+===============	========  ======================================
+energy[N]_input EsocketX  Socket Energy X = [0] to [nr_socks -1]
+			  Measured input socket energy
+=============== ========  ======================================

Documentation/hwmon/index.rst

@@ -39,6 +39,7 @@ Hardware Monitoring Kernel Drivers
    adt7470
    adt7475
    amc6821
+   amd_energy
    asb100
    asc7621
    aspeed-pwm-tacho