The V4L2 async interface and the concept of graph binding

So far, with V4L2 driver development, we have not actually dealt with the probing order. That being said, we considered the synchronous approach, where bridge device drivers register devices for all sub-devices synchronously during their probing. However, this approach cannot be used with intrinsically asynchronous and unordered device registration systems, such as the flattened device tree. To address this, what we currently call the async interface has been introduced.

With this new approach, bridge drivers register lists of sub-device descriptors and notifier callbacks, and sub-device drivers register sub-devices that they are about to probe or have successfully probed. The async core will take care of matching sub-devices against hardware descriptors and calling bridge driver callbacks when matches are found. Another callback is called when the sub-device is unregistered. The async subsystem relies on device declaration...