The Async memcpy API
====================

Overview
--------

{IDF_TARGET_NAME} has a DMA engine which can help to offload internal memory copy operations from the CPU in a asynchronous way.

The async memcpy API wraps all DMA configurations and operations, the signature of :cpp:func:`esp_async_memcpy` is almost the same to the standard libc one.

Thanks to the benefit of the DMA, we don't have to wait for each memory copy to be done before we issue another memcpy request. By the way, it's still possible to know when memcpy is finished by listening in the memcpy callback function.

.. only:: esp32s2

    .. note::
        Memory copy from/to external PSRAM is not supported on ESP32-S2, :cpp:func:`esp_async_memcpy` will abort returning an error if buffer address is not in SRAM.

Configure and Install driver
----------------------------

:cpp:func:`esp_async_memcpy_install` is used to install the driver with user's configuration. Please note that async memcpy has to be called with the handle returned from :cpp:func:`esp_async_memcpy_install`.

Driver configuration is described in :cpp:type:`async_memcpy_config_t`:
:cpp:member:`backlog`: This is used to configured the maximum number of DMA operation that can be working at the background at the same time.
:cpp:member:`flags`: This is used to enable some special driver features.

:c:macro:`ASYNC_MEMCPY_DEFAULT_CONFIG` provides a default configuration, which specifies the backlog to 8.

.. highlight:: c

::

    async_memcpy_config_t config = ASYNC_MEMCPY_DEFAULT_CONFIG();
    // update the maximum data stream supported by underlying DMA engine
    config.backlog = 16;
    async_memcpy_t driver = NULL;
    ESP_ERROR_CHECK(esp_async_memcpy_install(&config, &driver)); // install driver, return driver handle

Send memory copy request
------------------------

:cpp:func:`esp_async_memcpy` is the API to send memory copy request to DMA engine. It must be called after driver is installed successfully. This API is thread safe, so it can be called from different tasks.

Different from the libc version of `memcpy`, user should also pass a callback to :cpp:func:`esp_async_memcpy`, if it's necessary to be notified when the memory copy is done. The callback is executed in the ISR context, make sure you won't violate the the restriction applied to ISR handler.

Besides that, the callback function should reside in IRAM space by applying `IRAM_ATTR` attribute. The prototype of the callback function is :cpp:type:`async_memcpy_isr_cb_t`, please note that, the callback function should return true if it wakes up a high priority task by some API like :cpp:func:`xSemaphoreGiveFromISR`.

.. highlight:: c

::

    Semphr_Handle_t semphr; //already initialized in somewhere

    // Callback implementation, running in ISR context
    static IRAM_ATTR bool my_async_memcpy_cb(async_memcpy_t mcp_hdl, async_memcpy_event_t *event, void *cb_args)
    {
        SemaphoreHandle_t sem = (SemaphoreHandle_t)cb_args;
        BaseType_t high_task_wakeup = pdFALSE;
        SemphrGiveInISR(semphr, &high_task_wakeup); // high_task_wakeup set to pdTRUE if some high priority task unblocked
        return high_task_wakeup == pdTRUE;
    }

    // Called from user's context
    ESP_ERROR_CHECK(esp_async_memcpy(driver_handle, to, from, copy_len, my_async_memcpy_cb, my_semaphore));
    //Do something else here
    SemphrTake(my_semaphore, ...); //wait until the buffer copy is done

Uninstall driver (optional)
---------------------------

:cpp:func:`esp_async_memcpy_uninstall` is used to uninstall asynchronous memcpy driver. It's not necessary to uninstall the driver after each memcpy operation. If you know your application won't use this driver anymore, then this API can recycle the memory for you.

API Reference
-------------

.. include-build-file:: inc/esp_async_memcpy.inc
