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Interrupt Driven Zero Copy Transfer Mode
[FreeRTOS+IO Transfer Modes]
Data DirectionThe interrupt driven zero copy transfer mode can be used with FreeRTOS_write().
DescriptionioconfigUSE_ZERO_COPY_TX must be set to 1 in FreeRTOSIOConfig.h for the zero copy transfer mode to be available. It must also be explicitly enabled for the peripheral being used within the same configuration file.
When the zero copy transfer mode is selected, FreeRTOS_write() does not write bytes directly to the peripheral. Instead the total number of bytes to write and a pointer to the first byte are stored inside the FreeRTOS+IO driver. The peripheral’s interrupt service routine then uses these values to perform the actual write operation. Bytes are copied directly from the supplied buffer to the peripheral’s registers, without being stored in any intermediary queue or buffer.
The FreeRTOS+IO driver creates and manages a mutex to ensure only one task can perform a zero copy write at a time. The mutex must be obtained before a task can start a zero copy write. The mutex is obtained using a FreeRTOS_ioctl() call with the ioctlOBTAIN_WRITE_MUTEX request code. The FreeRTOS_ioctl() return value must be checked to see if the mutex was obtained successfully before calling FreeRTOS_write(). Calls to FreeRTOS_write() will fail if they are made by a task that is not the mutex holder.
Zero copy writes are performed by the FreeRTOS+IO interrupt service routine. It is likely that the interrupt service routine will continue to access the buffer being written for a period of time after the call to FreeRTOS_write() that started the write operation has returned. The buffer must not be reused or in any way corrupted until the interrupt service routine has completed the write operation. The FreeRTOS_ioctl() ioctlOBTAIN_WRITE_MUTEX and ioctlWAIT_PREVIOUS_WRITE_COMPLETE request codes can be used for this purpose.
A task that holds the mutex, but does not perform a FreeRTOS_write(), must manually release the mutex by calling FreeRTOS_ioctl() with the request code set to ioctlRELEASE_WRITE_MUTEX.
The interrupt service routine and write mutex are implemented by the FreeRTOS+IO code, and do not need to be provided by the application writer.
|Interrupt Driven Zero Copy Transfer Mode|
The ioctlUSE_ZERO_COPY_TX request code is used in a call to FreeRTOS_ioctl() to configure a peripheral to use the interrupt driven zero copy transfer mode for writes. Note this request code will result in the peripheral’s interrupt being enabled, and the peripheral’s interrupt priority being set to the lowest possible. The ioctlSET_INTERRUPT_PRIORITY request code can be used to raise the peripheral’s priority if necessary.
Example UsageExamples are also provided on the FreeRTOS_write() API function documentation page.