power_control

v0.2.0

power_control v0.2.0 PowerControl

PowerControl is a library that enables runtime configuration of embedded linux for power conservation or performance via native elixir.

Getting Started

If using shoehorn, add :power_control to your shoehorn apps in your config.exs:

config :shoehorn,
  init: [:nerves_runtime, ..., :power_control],
  ...

It must come after :nerves_runtime but has no requirements other than that.

Once installed, startup configuration can be set in your config.exs like so:

config :power_control,
  cpu_governor: :powersave,
  disable_leds: true,
  disable_hdmi: true

The CPU Governor determines CPU clock speed behavior. Different devices can have different available governors. For more information about determining available governors, see the “Runtime Functionality” section below.

disable_leds disables all system leds. To selectively disable them, you may want to use the functions list_leds/0 and disable_led/1 manually. More complex LED management can be done with Nerves.Leds.

disable_hdmi disables the onboard hdmi of the pi. This may also disable other forms of video on hats or other peripherals, but this is not tested.

Runtime Functionality

PowerControl also allows you fetch information about your system and configure some settings during runtime, for example:

iex> list_cpus()
{:ok, [:cpu0]}
iex> cpu_info(:cpu0).speed
700000
iex> list_cpu_governors(:cpu0)
{:ok, [:ondemand, :userspace, :powersave, :conservative, :performance]}
iex> set_cpu_governor(:cpu0, :performance)
{:ok, :performance}
iex> cpu_info(:cpu0).speed
1000000

The functions list_cpus/0 and list_cpu_governors/1 can be used to determine what governors you have available on your device for configuration.

Other Errors and Advanced Configuration

All Led and CPU functions can return Elixir File error messages such as :enoent, which generally indicates you have configured the directories improperly for your specific device (unlikely) or that PowerControl does not support your device (most likely). In the unlikely case that your device can be supported by PowerControl but you must manually configure the directories, the config keys are cpu_dir and led_dir. I do not currently support configuring filenames.

Link to this section Summary

Functions

cpu_info(cpu)

Returns an info map for a CPU

disable_hdmi()

Disables the HDMI port

disable_led(led)

Disables an LED

list_cpu_governors(cpu)

Returns available governors for a CPU

list_cpus()

Lists system CPUS

list_leds()

Lists system LEDS

set_cpu_governor(cpu, governor)

Sets the governor for a CPU

Link to this section Functions

Link to this function cpu_info(cpu)

Returns an info map for a CPU.

iex> cpu_info(:cpu0)
%{max_speed: 1000000, min_speed: 700000, speed: 1000000}
Link to this function disable_hdmi()

Disables the HDMI port.

Returns :ok regardless of failure or success, but almost always succeeds.

iex> disable_hdmi()
:ok
Link to this function disable_led(led)

Disables an LED.

Uses a simple retry system if it fails, retries 2 times then gives up. To re-enable the LED or further configure LED settings, I reccomend Nerves.Leds.

iex> disable_led(:led0)
:ok

iex> disable_led(:invalid_led)
{:error, ...}
Link to this function list_cpu_governors(cpu)

Returns available governors for a CPU.

iex> list_cpu_governors(:cpu0)
{:ok, [:ondemand, :userspace, :powersave, :conservative, :performance]}
Link to this function list_cpus()

Lists system CPUS.

iex> list_cpus()
{:ok, [:cpu0]}
Link to this function list_leds()

Lists system LEDS.

iex> list_leds()
{:ok, [:led0]}
Link to this function set_cpu_governor(cpu, governor)

Sets the governor for a CPU.

iex> set_cpu_governor(:cpu0, :powersave)
{:ok, :powersave}

iex> set_cpu_governor(:cpu0, :invalid)
{:error, :invalid_governor}

# Running on non-nerves device or with bad governor file settings
iex> set_cpu_governor(:cpu0, :powersave)
{:error, :governor_file_not_found}