Kernel driver amd_energy

Supported chips:

Author: Naveen Krishna Chatradhi <>


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.

Frequency of the accumulator thread is set during the probe based on the chosen energy unit resolution. For example A. fine grain (1.625 micro J) B. course grain (0.125 milli J)

A socket and core energy read would return the current register value added to the respective energy accumulator.

Sysfs attributes




  • For index N between [1] and [nr_cpus]



Core Energy X = [0] to [nr_cpus - 1] Measured input core energy

  • For N between [nr_cpus] and [nr_cpus + nr_socks]



Socket Energy X = [0] to [nr_socks -1] Measured input socket energy

Note: To address CVE-2020-12912, the visibility of the energy[N]_input attributes is restricted to owner and groups only.