The i2c interface

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The i2c interface permits access to a specific I2C controller. It’s intended to be used with Ubuntu Core and its scope and specification are defined as part of the gadget snap for the deployed Ubuntu Core image.

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

To permit access, the application snaps define an i2c plug which is associated with a corresponding and identically-named i2c slot in the gadget snap.

Using the i2c interface does not require Store approval and permission, but usage is restricted because it provides privileged access to hardware devices. Enabling the i2c interface to auto-connect can be managed in the dashboard. The snap user can also access a specific i2c device with a manual interface connection.

Interface documentation:

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

Developer details

Auto-connect: no

Attributes:

  • path (plug, slot): path to i2c device node.
    Example: path: /dev/i2c-0
  • sysfs-name (plug, slot): a proper name found in /sys/bus/i2c/devices
    Example: sysfs-name: 1-0050

Only one of path or sysfs-name should be used in either the plug or slot definition, but both must match. That is to say, if the gadget declares the slot-side using sysfs-name, the snap declaring the plug-side must also use sysfs-name.

The slot-side of the interface is used to derive which i2c node in /dev/ is provided to the plug-side of the connection:

slots:
  i2c-1:
    interface: i2c
    path: /dev/i2c-1

To prevent connection to arbitrary i2c slots, the plug and slot must share the same i2c attribute, including the name of the plug and slot:

plugs:
  i2c-1:
    interface: i2c
    path: /dev/i2c-1

When the slot and plug are connected, a udev rule is automatically generated and tagged for the plug side for the device path, such as:

KERNEL=="i2c-1", TAG+="snap_client-snap_app-accessing-1-port"

The test code for the interface is in the snapd repository: https://github.com/canonical/snapd/blob/master/interfaces/builtin/i2c_test.go

The source code for the interface is in the snapd repository: https://github.com/canonical/snapd/blob/master/interfaces/builtin/i2c.go

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