Asynchronous Notifications

Zephyr APIs often include async functions where an operation is initiated and the application needs to be informed when it completes, and whether it succeeded. Using k_poll() is often a good method, but some application architectures may be more suited to a callback notification, and operations like enabling clocks and power rails may need to be invoked before kernel functions are available so a busy-wait for completion may be needed.

This API is intended to be embedded within specific subsystems such as On-Off Manager and other APIs that support async transactions. The subsystem wrappers are responsible for extracting operation-specific data from requests that include a notification element, and for invoking callbacks with the parameters required by the API.

A limitation is that this API is not suitable for System Calls because:

  • sys_notify is not a kernel object;

  • copying the notification content from userspace will break use of CONTAINER_OF in the implementing function;

  • neither the spin-wait nor callback notification methods can be accepted from userspace callers.

Where a notification is required for an asynchronous operation invoked from a user mode thread the subsystem or driver should provide a syscall API that uses k_poll_signal for notification.

API Reference

group sys_notify_apis

Typedefs

typedef void (*sys_notify_generic_callback)()

Generic signature used to notify of result completion by callback.

Functions with this role may be invoked from any context including pre-kernel, ISR, or cooperative or pre-emptible threads. Compatible functions must be isr-ok and not sleep.

Parameters that should generally be passed to such functions include:

  • a pointer to a specific client request structure, i.e. the one that contains the sys_notify structure.

  • the result of the operation, either as passed to sys_notify_finalize() or extracted afterwards using sys_notify_fetch_result(). Expected values are service-specific, but the value shall be non-negative if the operation succeeded, and negative if the operation failed.

Functions

static inline uint32_t sys_notify_get_method(const struct sys_notify *notify)
int sys_notify_validate(struct sys_notify *notify)

Validate and initialize the notify structure.

This should be invoked at the start of any service-specific configuration validation. It ensures that the basic asynchronous notification configuration is consistent, and clears the result.

Note that this function does not validate extension bits (zeroed by async notify API init functions like sys_notify_init_callback()). It may fail to recognize that an uninitialized structure has been passed because only method bits of flags are tested against method settings. To reduce the chance of accepting an uninitialized operation service validation of structures that contain an sys_notify instance should confirm that the extension bits are set or cleared as expected.

Return values:

  • 0 – on successful validation and reinitialization

  • -EINVAL – if the configuration is not valid.

sys_notify_generic_callback sys_notify_finalize(struct sys_notify *notify, int res)

Record and signal the operation completion.

Parameters:

  • notify – pointer to the notification state structure.

  • res – the result of the operation. Expected values are service-specific, but the value shall be non-negative if the operation succeeded, and negative if the operation failed.

Returns:

If the notification is to be done by callback this returns the generic version of the function to be invoked. The caller must immediately invoke that function with whatever arguments are expected by the callback. If notification is by spin-wait or signal, the notification has been completed by the point this function returns, and a null pointer is returned.

static inline int sys_notify_fetch_result(const struct sys_notify *notify, int *result)

Check for and read the result of an asynchronous operation.

Parameters:

  • notify – pointer to the object used to specify asynchronous function behavior and store completion information.

  • result – pointer to storage for the result of the operation. The result is stored only if the operation has completed.

Return values:

  • 0 – if the operation has completed.

  • -EAGAIN – if the operation has not completed.

static inline void sys_notify_init_spinwait(struct sys_notify *notify)

Initialize a notify object for spin-wait notification.

Clients that use this initialization receive no asynchronous notification, and instead must periodically check for completion using sys_notify_fetch_result().

On completion of the operation the client object must be reinitialized before it can be re-used.

Parameters:

  • notify – pointer to the notification configuration object.

static inline void sys_notify_init_signal(struct sys_notify *notify, struct k_poll_signal *sigp)

Initialize a notify object for (k_poll) signal notification.

Clients that use this initialization will be notified of the completion of operations through the provided signal.

On completion of the operation the client object must be reinitialized before it can be re-used.

Note

This capability is available only when CONFIG_POLL is selected.

Parameters:

  • notify – pointer to the notification configuration object.

  • sigp – pointer to the signal to use for notification. The value must not be null. The signal must be reset before the client object is passed to the on-off service API.

static inline void sys_notify_init_callback(struct sys_notify *notify, sys_notify_generic_callback handler)

Initialize a notify object for callback notification.

Clients that use this initialization will be notified of the completion of operations through the provided callback. Note that callbacks may be invoked from various contexts depending on the specific service; see sys_notify_generic_callback.

On completion of the operation the client object must be reinitialized before it can be re-used.

Parameters:

  • notify – pointer to the notification configuration object.

  • handler – a function pointer to use for notification.

static inline bool sys_notify_uses_callback(const struct sys_notify *notify)

Detect whether a particular notification uses a callback.

The generic handler does not capture the signature expected by the callback, and the translation to a service-specific callback must be provided by the service. This check allows abstracted services to reject callback notification requests when the service doesn’t provide a translation function.

Returns:

true if and only if a callback is to be used for notification.

struct sys_notify
#include <notify.h>

State associated with notification for an asynchronous operation.

Objects of this type are allocated by a client, which must use an initialization function (e.g. sys_notify_init_signal()) to configure them. Generally the structure is a member of a service-specific client structure, such as onoff_client.

Control of the containing object transfers to the service provider when a pointer to the object is passed to a service function that is documented to take control of the object, such as onoff_service_request(). While the service provider controls the object the client must not change any object fields. Control reverts to the client:

  • if the call to the service API returns an error;

  • when operation completion is posted. This may occur before the call to the service API returns.

Operation completion is technically posted when the flags field is updated so that sys_notify_fetch_result() returns success. This will happen before the signal is posted or callback is invoked. Note that although the manager will no longer reference the sys_notify object past this point, the containing object may have state that will be referenced within the callback. Where callbacks are used control of the containing object does not revert to the client until the callback has been invoked. (Re-use within the callback is explicitly permitted.)

After control has reverted to the client the notify object must be reinitialized for the next operation.

The content of this structure is not public API to clients: all configuration and inspection should be done with functions like sys_notify_init_callback() and sys_notify_fetch_result(). However, services that use this structure may access certain fields directly.

union method
#include <notify.h>

Public Members

struct k_poll_signal *signal
sys_notify_generic_callback callback