Asynchronous Memory Copy

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Overview

ESP32-S2 has a DMA engine which can help to offload internal memory copy operations from the CPU in an asynchronous way.

The async memcpy API wraps all DMA configurations and operations. The signature of esp_async_memcpy() is almost the same as the standard libc memcpy function.

The DMA allows multiple memory copy requests to be queued up before the first one is completed, which allows overlap of computation and memory copy. Moreover, it is still possible to know the exact time when a memory copy request is completed by registering an event callback.

Configure and Install Driver

There are several ways to install the async memcpy driver, depending on the underlying DMA engine:

  • esp_async_memcpy_install_cpdma() is used to install the async memcpy driver based on the CP DMA engine.

  • esp_async_memcpy_install() is a generic API to install the async memcpy driver with a default DMA engine. If the SoC has the CP DMA engine, the default DMA engine is CP DMA. Otherwise, the default DMA engine is AHB GDMA.

Driver configuration is described in async_memcpy_config_t:

  • backlog: This is used to configure the maximum number of memory copy transactions that can be queued up before the first one is completed. If this field is set to zero, then the default value 4 will be applied.

  • sram_trans_align: Declare SRAM alignment for both data address and copy size, set to zero if the data has no restriction in alignment. If set to a quadruple value (i.e., 4X), the driver will enable the burst mode internally, which is helpful for some performance related application.

  • psram_trans_align: Declare PSRAM alignment for both data address and copy size. User has to give it a valid value (only 16, 32, 64 are supported) if the destination of memcpy is located in PSRAM. The default alignment (i.e., 16) will be applied if it is set to zero. Internally, the driver configures the size of block used by DMA to access PSRAM, according to the alignment.

  • flags: This is used to enable some special driver features.

async_memcpy_config_t config = ASYNC_MEMCPY_DEFAULT_CONFIG();
// update the maximum data stream supported by underlying DMA engine
config.backlog = 8;
async_memcpy_handle_t driver = NULL;
ESP_ERROR_CHECK(esp_async_memcpy_install(&config, &driver)); // install driver with default DMA engine

Send Memory Copy Request

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, you can optionally pass a callback to esp_async_memcpy(), so that you can be notified when the memory copy is finished. Note that the callback is executed in the ISR context, please make sure you will not call any blocking functions in the callback.

The prototype of the callback function is async_memcpy_isr_cb_t. The callback function should only return true if it wakes up a high priority task by RTOS APIs like xSemaphoreGiveFromISR().

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

// Create a semaphore used to report the completion of async memcpy
SemaphoreHandle_t semphr = xSemaphoreCreateBinary();

// 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
xSemaphoreTake(my_semaphore, portMAX_DELAY); // Wait until the buffer copy is done

Uninstall Driver

esp_async_memcpy_uninstall() is used to uninstall asynchronous memcpy driver. It is not necessary to uninstall the driver after each memcpy operation. If you know your application will not use this driver anymore, then this API can recycle the memory and other hardware resources for you.

API Reference

Header File

Functions

esp_err_t esp_async_memcpy_install_cpdma(const async_memcpy_config_t *config, async_memcpy_handle_t *mcp)

Install async memcpy driver, with CPDMA as the backend.

Note

CPDMA is a CPU peripheral, aiming for memory copy.

Parameters

  • config -- [in] Configuration of async memcpy

  • mcp -- [out] Returned driver handle

Returns

  • ESP_OK: Install async memcpy driver successfully

  • ESP_ERR_INVALID_ARG: Install async memcpy driver failed because of invalid argument

  • ESP_ERR_NO_MEM: Install async memcpy driver failed because out of memory

  • ESP_FAIL: Install async memcpy driver failed because of other error

esp_err_t esp_async_memcpy_install(const async_memcpy_config_t *config, async_memcpy_handle_t *mcp)

Install async memcpy driver with the default DMA backend.

Note

On chip with CPDMA support, CPDMA is the default choice. On chip with AHB-GDMA support, AHB-GDMA is the default choice.

Parameters

  • config -- [in] Configuration of async memcpy

  • mcp -- [out] Returned driver handle

Returns

  • ESP_OK: Install async memcpy driver successfully

  • ESP_ERR_INVALID_ARG: Install async memcpy driver failed because of invalid argument

  • ESP_ERR_NO_MEM: Install async memcpy driver failed because out of memory

  • ESP_FAIL: Install async memcpy driver failed because of other error

esp_err_t esp_async_memcpy_uninstall(async_memcpy_handle_t mcp)

Uninstall async memcpy driver.

Parameters

mcp -- [in] Handle of async memcpy driver that returned from esp_async_memcpy_install

Returns

  • ESP_OK: Uninstall async memcpy driver successfully

  • ESP_ERR_INVALID_ARG: Uninstall async memcpy driver failed because of invalid argument

  • ESP_FAIL: Uninstall async memcpy driver failed because of other error

esp_err_t esp_async_memcpy(async_memcpy_handle_t mcp, void *dst, void *src, size_t n, async_memcpy_isr_cb_t cb_isr, void *cb_args)

Send an asynchronous memory copy request.

Note

The callback function is invoked in interrupt context, never do blocking jobs in the callback.

Parameters

  • mcp -- [in] Handle of async memcpy driver that returned from esp_async_memcpy_install

  • dst -- [in] Destination address (copy to)

  • src -- [in] Source address (copy from)

  • n -- [in] Number of bytes to copy

  • cb_isr -- [in] Callback function, which got invoked in interrupt context. Set to NULL can bypass the callback.

  • cb_args -- [in] User defined argument to be passed to the callback function

Returns

  • ESP_OK: Send memory copy request successfully

  • ESP_ERR_INVALID_ARG: Send memory copy request failed because of invalid argument

  • ESP_FAIL: Send memory copy request failed because of other error

Structures

struct async_memcpy_event_t

Async memory copy event data.

Public Members

void *data

Event data

struct async_memcpy_config_t

Type of async memcpy configuration.

Public Members

uint32_t backlog

Maximum number of transactions that can be prepared in the background

size_t sram_trans_align

DMA transfer alignment (both in size and address) for SRAM memory

size_t psram_trans_align

DMA transfer alignment (both in size and address) for PSRAM memory

uint32_t flags

Extra flags to control async memcpy feature

Macros

ASYNC_MEMCPY_DEFAULT_CONFIG()

Default configuration for async memcpy.

Type Definitions

typedef struct async_memcpy_context_t *async_memcpy_handle_t

Async memory copy driver handle.

typedef bool (*async_memcpy_isr_cb_t)(async_memcpy_handle_t mcp_hdl, async_memcpy_event_t *event, void *cb_args)

Type of async memcpy interrupt callback function.

Note

User can call OS primitives (semaphore, mutex, etc) in the callback function. Keep in mind, if any OS primitive wakes high priority task up, the callback should return true.

Param mcp_hdl

Handle of async memcpy

Param event

Event object, which contains related data, reserved for future

Param cb_args

User defined arguments, passed from esp_async_memcpy function

Return

Whether a high priority task is woken up by the callback function

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