The pwm interface
pwm interface allows access to a specific PWM channel on a device. The interface is restricted because it provides privileged access to PWM hardware.
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 [...]
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
channel(slot): PWM device channel number to export and expose to consuming snaps
chip-number(slot): chip base number to export
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.
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.
Last updated 2 years ago.