CMSIS-Driver  Version 2.8.0
Peripheral Interface for Middleware and Application Code
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Use of Storage APIs

Function pointers within ARM_DRIVER_STORAGE form the set of operations constituting the Storage driver. Their implementation is platform-specific, and needs to be supplied by the porting effort.

Some of these APIs will always operate synchronously:

This means that control returns to the caller with a relevant status code only after the completion of the operation (or the discovery of a failure condition).

The remainder of the APIs:

can function asynchronously if the underlying controller supports it; that is if ARM_STORAGE_CAPABILITIES::asynchronous_ops is set. In the case of asynchronous operation, the invocation returns early (with ARM_DRIVER_OK) and results in a completion callback later. If ARM_STORAGE_CAPABILITIES::asynchronous_ops is not set, then all such APIs execute synchronously, and control returns to the caller with a status code only after the completion of the operation (or the discovery of a failure condition).

If ARM_STORAGE_CAPABILITIES::asynchronous_ops is set, a storage driver may still choose to execute asynchronous operations in a synchronous manner. If so, the driver returns a positive value to indicate successful synchronous completion (or an error code in case of failure) and no further invocation of completion callback should be expected. The expected return value for synchronous completion of such asynchronous operations varies depending on the operation. For operations involving data access, it often equals the amount of data transferred or affected. For non data-transfer operations, such as EraseAll or Initialize, it is usually 1.

Here's a code snippet to suggest how asynchronous APIs might be used by callers to handle both synchronous and asynchronous execution by the underlying storage driver:

ASSERT(ARM_DRIVER_OK == 0); // this is a precondition; it doesn't need to be put in code
int32_t returnValue = drv->asynchronousAPI(...);
if (returnValue < ARM_DRIVER_OK) {
// handle error.
} else if (returnValue == ARM_DRIVER_OK) {
ASSERT(drv->GetCapabilities().asynchronous_ops == 1);
// handle early return from asynchronous execution; remainder of the work is done in the callback handler.
} else {
// handle synchronous completion.

THis example is mixing synchronous and asynchronous APIs: Sample Use of Storage Driver