Resource Management
There are various situations where it’s necessary to coordinate resource use at runtime among multiple clients. These include power rails, clocks, other peripherals, and binary device power management. The complexity of properly managing multiple consumers of a device in a multithreaded system, especially when transitions may be asynchronous, suggests that a shared implementation is desirable.
Zephyr provides managers for several coordination policies. These managers are embedded into services that use them for specific functions.
On-Off Manager
An on-off manager supports an arbitrary number of clients of a service which has a binary state. Example applications are power rails, clocks, and binary device power management.
The manager has the following properties:
The stable states are off, on, and error. The service always begins in the off state. The service may also be in a transition to a given state.
The core operations are request (add a dependency) and release (remove a dependency). Supporting operations are reset (to clear an error state) and cancel (to reclaim client data from an in-progress transition). The service manages the state based on calls to functions that initiate these operations.
The service transitions from off to on when first client request is received.
The service transitions from on to off when last client release is received.
Each service configuration provides functions that implement the transition from off to on, from on to off, and optionally from an error state to off. Transitions must be invokable from both thread and interrupt context.
The request and reset operations are asynchronous using Asynchronous Notifications. Both operations may be cancelled, but cancellation has no effect on the in-progress transition.
Requests to turn on may be queued while a transition to off is in progress: when the service has turned off successfully it will be immediately turned on again (where context allows) and waiting clients notified when the start completes.
Requests are reference counted, but not tracked. That means clients are responsible for recording whether their requests were accepted, and for initiating a release only if they have previously successfully completed a request. Improper use of the API can cause an active client to be shut out, and the manager does not maintain a record of specific clients that have been granted a request.
Failures in executing a transition are recorded and inhibit further requests or releases until the manager is reset. Pending requests are notified (and cancelled) when errors are discovered.
Transition operation completion notifications are provided through Asynchronous Notifications.
Clients and other components interested in tracking all service state changes, including when a service begins turning off or enters an error state, can be informed of state transitions by registering a monitor with onoff_monitor_register(). Notification of changes are provided before issuing completion notifications associated with the new state.
Note
A generic API may be implemented by multiple drivers where the common
case is asynchronous. The on-off client structure may be an
appropriate solution for the generic API. Where drivers that can
guarantee synchronous context-independent transitions a driver may
use onoff_sync_service and its supporting API rather than
onoff_manager, with only a small reduction in functionality
(primarily no support for the monitor API).