The gpio-chardev interface

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The gpio-chardev interface allows access to specific GPIO chardev lines.

Interface documentation:

See Interface management and Supported interfaces for further details on how interfaces are used.

Use snap interface gpio-chardev to see which gpio chardev devices are available on the system:

$ snap interface gpio-chardev
name:          gpio-chardev
summary:       allows access to specific GPIO chardev lines
documentation: https://snapcraft.io/docs/gpio-chardev-interface
slots:
  - pi:bcm-gpio-0
  - pi:bcm-gpio-1
  - pi:bcm-gpio-10
[...]

Developer details

Auto-connect: no
Attributes:

  • source-chip (slot, mandatory): list of alternative labels of the source GPIO chip
  • lines (slot, mandatory): GPIO lines present in the source chip

Requires snapd 2.72+.

Hardware IO interfaces covers some general considerations common to these kinds of devices.

When the interface is connected:

  • snap-gpio-helper sets up a virtual GPIO device exposing the specified lines defined in the slot as a character device node at /dev/snap/gpio-chardev/<slot-snap>/<slot-name>.
  • snapd sets up a symlink at /dev/snap/gpio-chardev/<plug-snap>/<plug-name> pointing at the virtual slot device mentioned above.

Once connected, the consuming snap can use the device via /dev/snap/gpio-chardev/<plug-snap>/<plug-name>.

Important notes:
  • Slot definitions are only allowed for gadget snaps.
  • This interface cannot be used if there is an active connection to the older gpio interface.
  • source-chip being a list enables sharing of a gadget snap between a number of devices, for which the same lines are exposed by differently labeled GPIO controllers.
  • lines can be expressed by joining them with a comma: n[,m] or as a range: n-m (inclusive) or a combination of both, assuming the ranges do not overlap, e.g.: lines: 0,2,4-8.
  • lines are exported in the order of presence in the source GPIO chip device, and not in the order in which they are declared in the attribute.
  • Any given line can only be exported by one slot.
  • gpio-aggregator kernel module with configfs support is required (i.e. UC24+).
  • gpio interface will not work on UC26+ due to missing kernel support.

Migration from gpio to gpio-chardev

Since gpio and gpio-chardev interfaces cannot be connected at the same time, all existing gpio interface connections must be disconnected first before connecting to gpio-chardev.

This is only required if the gadget snap has both gpio and gpio-chardev slots declared since performing a gadget refresh to a revision that only declares gpio-chardev slots will automatically disconnect gpio connections due to missing slot.

It is recommended to only have only one slot type on the gadget, either gpio or gpio-chardev.

Code examples

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

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