The pwm interface

The pwm interface allows access to a specific PWM channel on a device. The interface is restricted because it provides privileged access to PWM hardware.

Use snap interface pwm to see which PWM devices are available on the system:

$ snap interface pwm
name: pwm
summary: allows access to specific PWM channel
slots:
- gadget:led-1
- gadget:led-2
- gadget:fan-1
- gadget:fan-2
[...]

Example

With an example snap application called app installed, the following command will connect its PWM interface to the specified PWM device channel:

$  snap connect app:activity-led gadget:led-1

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Developer details

Auto-connect: no
Attributes:

• channel (slot): PWM device channel number to export and expose to consuming snaps
• chip-number (slot): chip base number to export

To use a PWM device, the snap developer must add plugs:[pwm] to a snap’s snapcraft.yaml. The snap user can then access a specific pwm device with an interface connection.

Unless the snap is expected to actually use a set of PWM channels that is not predefined, it is recommended to define distinct plugs for each used pwm channel, like:

plugs:
  activity-led:
    interface: pwm
  warning-led:
    interface: pwm

This has the advantage of being self-documenting and 1-1 connections like these are easier to track and setup with auto-connections, if the latter is needed.

When the interface is connected, "echo (channel number) > /sys/class/pwm/pwmchipN/export" is run internally to enable access to the PWM channel.

Once connected, the consuming snap can use the device via /sys/class/pwm/pwmchipN/pwmX where N is the base of the PWM chip and X is channel number specified by the connected slot.

Finally, when the interface is disconnected, `“echo (channel number) > /sys/class/pwmchipN/unexport” is run internally to disable access to the PWM channel.

Code examples

The test code can be found in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/pwm_test.go

The source code for the pwm interface is in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/pwm.go.

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Last updated 3 years ago.