521 lines
23 KiB
ReStructuredText
521 lines
23 KiB
ReStructuredText
FreeRTOS Additions
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==================
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Overview
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--------
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ESP-IDF FreeRTOS is based on the Xtensa port of FreeRTOS v8.2.0 with significant modifications
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for SMP compatibility (see :doc:`ESP-IDF FreeRTOS SMP Changes<../../api-guides/freertos-smp>`).
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However various features specific to ESP-IDF FreeRTOS have been added. The features are as follows:
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:ref:`ring-buffers`: Ring buffers were added to provide a form of buffer that could accept
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entries of arbitrary lengths.
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:ref:`hooks`: ESP-IDF FreeRTOS hooks provides support for registering extra Idle and
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Tick hooks at run time. Moreover, the hooks can be asymmetric amongst both CPUs.
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.. _ring-buffers:
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Ring Buffers
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------------
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The ESP-IDF FreeRTOS ring buffer is a strictly FIFO buffer that supports arbitrarily sized items.
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Ring buffers are a more memory efficient alternative to FreeRTOS queues in situations where the
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size of items is variable. The capacity of a ring buffer is not measured by the number of items
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it can store, but rather by the amount of memory used for storing items. You may apply for a
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piece of memory on the ring buffer to send an item, or just use the API to copy your data and send
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(according to the send API you call). For efficiency reasons,
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**items are always retrieved from the ring buffer by reference**. As a result, all retrieved
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items *must also be returned* in order for them to be removed from the ring buffer completely.
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The ring buffers are split into the three following types:
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**No-Split** buffers will guarantee that an item is stored in contiguous memory and will not
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attempt to split an item under any circumstances. Use no-split buffers when items must occupy
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contiguous memory. *Only this buffer type allows you getting the data item address and writting
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to the item by yourself.*
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**Allow-Split** buffers will allow an item to be split when wrapping around if doing so will allow
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the item to be stored. Allow-split buffers are more memory efficient than no-split buffers but
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can return an item in two parts when retrieving.
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**Byte buffers** do not store data as separate items. All data is stored as a sequence of bytes,
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and any number of bytes and be sent or retrieved each time. Use byte buffers when separate items
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do not need to be maintained (e.g. a byte stream).
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.. note::
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No-split/allow-split buffers will always store items at 32-bit aligned addresses. Therefore when
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retrieving an item, the item pointer is guaranteed to be 32-bit aligned. This is useful
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especially when you need to send some data to the DMA.
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.. note::
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Each item stored in no-split/allow-split buffers will **require an additional 8 bytes for a header**.
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Item sizes will also be rounded up to a 32-bit aligned size (multiple of 4 bytes), however the true
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item size is recorded within the header. The sizes of no-split/allow-split buffers will also
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be rounded up when created.
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Usage
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^^^^^
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The following example demonstrates the usage of :cpp:func:`xRingbufferCreate`
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and :cpp:func:`xRingbufferSend` to create a ring buffer then send an item to it.
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.. code-block:: c
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#include "freertos/ringbuf.h"
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static char tx_item[] = "test_item";
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...
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//Create ring buffer
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RingbufHandle_t buf_handle;
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buf_handle = xRingbufferCreate(1028, RINGBUF_TYPE_NOSPLIT);
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if (buf_handle == NULL) {
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printf("Failed to create ring buffer\n");
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}
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//Send an item
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UBaseType_t res = xRingbufferSend(buf_handle, tx_item, sizeof(tx_item), pdMS_TO_TICKS(1000));
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if (res != pdTRUE) {
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printf("Failed to send item\n");
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}
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The following example demonstrates the usage of :cpp:func:`xRingbufferSendAcquire` and
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:cpp:func:`xRingbufferSendComplete` instead of :cpp:func:`xRingbufferSend` to apply for the
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memory on the ring buffer (of type `RINGBUF_TYPE_NOSPLIT`) and then send an item to it. This way
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adds one more step, but allows getting the address of the memory to write to, and writing to the
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memory yourself.
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.. code-block:: c
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#include "freertos/ringbuf.h"
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#include "soc/lldesc.h"
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typedef struct {
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lldesc_t dma_desc;
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uint8_t buf[1];
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} dma_item_t;
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#define DMA_ITEM_SIZE(N) (sizeof(lldesc_t)+(((N)+3)&(~3)))
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...
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//Retrieve space for DMA descriptor and corresponding data buffer
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//This has to be done with SendAcquire, or the address may be different when copy
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dma_item_t item;
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UBaseType_t res = xRingbufferSendAcquire(buf_handle,
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&item, DMA_ITEM_SIZE(buffer_size), pdMS_TO_TICKS(1000));
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if (res != pdTRUE) {
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printf("Failed to acquire memory for item\n");
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}
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item->dma_desc = (lldesc_t) {
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.size = buffer_size,
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.length = buffer_size,
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.eof = 0,
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.owner = 1,
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.buf = &item->buf,
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};
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//Actually send to the ring buffer for consumer to use
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res = xRingbufferSendComplete(buf_handle, &item);
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if (res != pdTRUE) {
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printf("Failed to send item\n");
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}
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The following example demonstrates retrieving and returning an item from a **no-split ring buffer**
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using :cpp:func:`xRingbufferReceive` and :cpp:func:`vRingbufferReturnItem`
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.. code-block:: c
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...
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//Receive an item from no-split ring buffer
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size_t item_size;
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char *item = (char *)xRingbufferReceive(buf_handle, &item_size, pdMS_TO_TICKS(1000));
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//Check received item
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if (item != NULL) {
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//Print item
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for (int i = 0; i < item_size; i++) {
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printf("%c", item[i]);
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}
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printf("\n");
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//Return Item
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vRingbufferReturnItem(buf_handle, (void *)item);
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} else {
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//Failed to receive item
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printf("Failed to receive item\n");
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}
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The following example demonstrates retrieving and returning an item from an **allow-split ring buffer**
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using :cpp:func:`xRingbufferReceiveSplit` and :cpp:func:`vRingbufferReturnItem`
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.. code-block:: c
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...
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//Receive an item from allow-split ring buffer
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size_t item_size1, item_size2;
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char *item1, *item2;
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BaseType_t ret = xRingbufferReceiveSplit(buf_handle, (void **)&item1, (void **)&item2, &item_size1, &item_size2, pdMS_TO_TICKS(1000));
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//Check received item
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if (ret == pdTRUE && item1 != NULL) {
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for (int i = 0; i < item_size1; i++) {
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printf("%c", item1[i]);
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}
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vRingbufferReturnItem(buf_handle, (void *)item1);
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//Check if item was split
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if (item2 != NULL) {
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for (int i = 0; i < item_size2; i++) {
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printf("%c", item2[i]);
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}
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vRingbufferReturnItem(buf_handle, (void *)item2);
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}
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printf("\n");
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} else {
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//Failed to receive item
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printf("Failed to receive item\n");
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}
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The following example demonstrates retrieving and returning an item from a **byte buffer**
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using :cpp:func:`xRingbufferReceiveUpTo` and :cpp:func:`vRingbufferReturnItem`
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.. code-block:: c
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...
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//Receive data from byte buffer
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size_t item_size;
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char *item = (char *)xRingbufferReceiveUpTo(buf_handle, &item_size, pdMS_TO_TICKS(1000), sizeof(tx_item));
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//Check received data
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if (item != NULL) {
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//Print item
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for (int i = 0; i < item_size; i++) {
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printf("%c", item[i]);
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}
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printf("\n");
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//Return Item
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vRingbufferReturnItem(buf_handle, (void *)item);
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} else {
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//Failed to receive item
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printf("Failed to receive item\n");
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}
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For ISR safe versions of the functions used above, call :cpp:func:`xRingbufferSendFromISR`, :cpp:func:`xRingbufferReceiveFromISR`,
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:cpp:func:`xRingbufferReceiveSplitFromISR`, :cpp:func:`xRingbufferReceiveUpToFromISR`, and :cpp:func:`vRingbufferReturnItemFromISR`
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Sending to Ring Buffer
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^^^^^^^^^^^^^^^^^^^^^^
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The following diagrams illustrate the differences between no-split/allow-split buffers
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and byte buffers with regards to sending items/data. The diagrams assume that three
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items of sizes **18, 3, and 27 bytes** are sent respectively to a **buffer of 128 bytes**.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_send_non_byte_buf.diag
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:caption: Sending items to no-split/allow-split ring buffers
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:align: center
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For no-split/allow-split buffers, a header of 8 bytes precedes every data item. Furthermore, the space
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occupied by each item is **rounded up to the nearest 32-bit aligned size** in order to maintain overall
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32-bit alignment. However the true size of the item is recorded inside the header which will be
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returned when the item is retrieved.
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Referring to the diagram above, the 18, 3, and 27 byte items are **rounded up to 20, 4, and 28 bytes**
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respectively. An 8 byte header is then added in front of each item.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_send_byte_buf.diag
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:caption: Sending items to byte buffers
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:align: center
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Byte buffers treat data as a sequence of bytes and does not incur any overhead
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(no headers). As a result, all data sent to a byte buffer is merged into a single item.
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Referring to the diagram above, the 18, 3, and 27 byte items are sequentially written to the
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byte buffer and **merged into a single item of 48 bytes**.
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Using SendAcquire and SendComplete
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Items in no-split buffers are acquired (by SendAcquire) in strict FIFO order and must be sent to
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the buffer by SendComplete for the data to be accessible by the consumer. Multiple items can be
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sent or acquired without calling SendComplete, and the items do not necessarily need to be
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completed in the order they were acquired. However the receiving of data items must occur in FIFO
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order, therefore not calling SendComplete the earliest acquired item will prevent the subsequent
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items from being received.
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The following diagrams illustrate what will happen when SendAcquire/SendComplete don't happen in
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the same order. At the beginning, there is already an data item of 16 bytes sent to the ring
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buffer. Then SendAcquire is called to acquire space of 20, 8, 24 bytes on the ring buffer.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_send_acquire_complete.diag
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:caption: SendAcquire/SendComplete items in no-split ring buffers
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:align: center
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After that, we fill (use) the buffers, and send them to the ring buffer by SendComplete in the
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order of 8, 24, 20. When 8 bytes and 24 bytes data are sent, the consumer still can only get the
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16 bytes data item. Due to the usage if 20 bytes item is not complete, it's not available, nor
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the following data items.
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When the 20 bytes item is finally completed, all the 3 data items can be received now, in the
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order of 20, 8, 24 bytes, right after the 16 bytes item existing in the buffer at the beginning.
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Allow-split/byte buffers do not allow using SendAcquire/SendComplete since acquired buffers are
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required to be complete (not wrapped).
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Wrap around
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^^^^^^^^^^^
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The following diagrams illustrate the differences between no-split, allow-split, and byte
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buffers when a sent item requires a wrap around. The diagrams assumes a buffer of **128 bytes**
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with **56 bytes of free space that wraps around** and a sent item of **28 bytes**.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_wrap_no_split.diag
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:caption: Wrap around in no-split buffers
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:align: center
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No-split buffers will **only store an item in continuous free space and will not split
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an item under any circumstances**. When the free space at the tail of the buffer is insufficient
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to completely store the item and its header, the free space at the tail will be **marked as dummy data**.
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The buffer will then wrap around and store the item in the free space at the head of the buffer.
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Referring to the diagram above, the 16 bytes of free space at the tail of the buffer is
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insufficient to store the 28 byte item. Therefore the 16 bytes is marked as dummy data and
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the item is written to the free space at the head of the buffer instead.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_wrap_allow_split.diag
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:caption: Wrap around in allow-split buffers
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:align: center
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Allow-split buffers will attempt to **split the item into two parts** when the free space at the tail
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of the buffer is insufficient to store the item data and its header. Both parts of the
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split item will have their own headers (therefore incurring an extra 8 bytes of overhead).
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Referring to the diagram above, the 16 bytes of free space at the tail of the buffer is insufficient
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to store the 28 byte item. Therefore the item is split into two parts (8 and 20 bytes) and written
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as two parts to the buffer.
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.. note::
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Allow-split buffers treats the both parts of the split item as two separate items, therefore call
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:cpp:func:`xRingbufferReceiveSplit` instead of :cpp:func:`xRingbufferReceive` to receive both
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parts of a split item in a thread safe manner.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_wrap_byte_buf.diag
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:caption: Wrap around in byte buffers
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:align: center
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Byte buffers will **store as much data as possible into the free space at the tail of buffer**. The remaining
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data will then be stored in the free space at the head of the buffer. No overhead is incurred when wrapping
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around in byte buffers.
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Referring to the diagram above, the 16 bytes of free space at the tail of the buffer is insufficient to
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completely store the 28 bytes of data. Therefore the 16 bytes of free space is filled with data, and the
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remaining 12 bytes are written to the free space at the head of the buffer. The buffer now contains
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data in two separate continuous parts, and each part continuous will be treated as a separate item by the
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byte buffer.
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Retrieving/Returning
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^^^^^^^^^^^^^^^^^^^^
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The following diagrams illustrates the differences between no-split/allow-split and
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byte buffers in retrieving and returning data.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_read_ret_non_byte_buf.diag
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:caption: Retrieving/Returning items in no-split/allow-split ring buffers
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:align: center
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Items in no-split/allow-split buffers are **retrieved in strict FIFO order** and **must be returned**
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for the occupied space to be freed. Multiple items can be retrieved before returning, and the items
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do not necessarily need to be returned in the order they were retrieved. However the freeing of space
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must occur in FIFO order, therefore not returning the earliest retrieved item will prevent the space
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of subsequent items from being freed.
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Referring to the diagram above, the **16, 20, and 8 byte items are retrieved in FIFO order**. However the items
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are not returned in they were retrieved (20, 8, 16). As such, the space is not freed until the first item
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(16 byte) is returned.
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.. packetdiag:: ../../../_static/diagrams/ring-buffer/ring_buffer_read_ret_byte_buf.diag
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:caption: Retrieving/Returning data in byte buffers
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:align: center
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Byte buffers **do not allow multiple retrievals before returning** (every retrieval must be followed by a return
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before another retrieval is permitted). When using :cpp:func:`xRingbufferReceive` or
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:cpp:func:`xRingbufferReceiveFromISR`, all continuous stored data will be retrieved. :cpp:func:`xRingbufferReceiveUpTo`
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or :cpp:func:`xRingbufferReceiveUpToFromISR` can be used to restrict the maximum number of bytes retrieved. Since
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every retrieval must be followed by a return, the space will be freed as soon as the data is returned.
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Referring to the diagram above, the 38 bytes of continuous stored data at the tail of the buffer is retrieved,
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returned, and freed. The next call to :cpp:func:`xRingbufferReceive` or :cpp:func:`xRingbufferReceiveFromISR`
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then wraps around and does the same to the 30 bytes of continuous stored data at the head of the buffer.
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Ring Buffers with Queue Sets
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Ring buffers can be added to FreeRTOS queue sets using :cpp:func:`xRingbufferAddToQueueSetRead` such that every
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time a ring buffer receives an item or data, the queue set is notified. Once added to a queue set, every
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attempt to retrieve an item from a ring buffer should be preceded by a call to :cpp:func:`xQueueSelectFromSet`.
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To check whether the selected queue set member is the ring buffer, call :cpp:func:`xRingbufferCanRead`.
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The following example demonstrates queue set usage with ring buffers.
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.. code-block:: c
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#include "freertos/queue.h"
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#include "freertos/ringbuf.h"
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...
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//Create ring buffer and queue set
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RingbufHandle_t buf_handle = xRingbufferCreate(1028, RINGBUF_TYPE_NOSPLIT);
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QueueSetHandle_t queue_set = xQueueCreateSet(3);
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//Add ring buffer to queue set
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if (xRingbufferAddToQueueSetRead(buf_handle, queue_set) != pdTRUE) {
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printf("Failed to add to queue set\n");
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}
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...
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//Block on queue set
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xQueueSetMemberHandle member = xQueueSelectFromSet(queue_set, pdMS_TO_TICKS(1000));
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//Check if member is ring buffer
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if (member != NULL && xRingbufferCanRead(buf_handle, member) == pdTRUE) {
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//Member is ring buffer, receive item from ring buffer
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size_t item_size;
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char *item = (char *)xRingbufferReceive(buf_handle, &item_size, 0);
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//Handle item
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...
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} else {
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...
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}
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Ring Buffers with Static Allocation
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The :cpp:func:`xRingbufferCreateStatic` can be used to create ring buffers with specific memory requirements (such as a ring buffer being allocated in external RAM). All blocks of memory used by a ring buffer must be manually allocated beforehand then passed to the :cpp:func:`xRingbufferCreateStatic` to be initialized as a ring buffer. These blocks include the following:
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- The ring buffer's data structure of type :cpp:type:`StaticRingbuffer_t`
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- The ring buffer's storage area of size ``xBufferSize``. Note that ``xBufferSize`` must be 32-bit aligned for no-split/allow-split buffers.
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The manner in which these blocks are allocated will depend on the users requirements (e.g. all blocks being statically declared, or dynamically allocated with specific capabilities such as external RAM).
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.. note::
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The :ref:`CONFIG_FREERTOS_SUPPORT_STATIC_ALLOCATION` option must be enabled in `menuconfig` for statically allocated ring buffers to be available.
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.. note::
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When deleting a ring buffer created via :cpp:func:`xRingbufferCreateStatic`,
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the function :cpp:func:`vRingbufferDelete` will not free any of the memory blocks. This must be done manually by the user after :cpp:func:`vRingbufferDelete` is called.
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The code snippet below demonstrates a ring buffer being allocated entirely in external RAM.
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.. code-block:: c
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#include "freertos/ringbuf.h"
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#include "freertos/semphr.h"
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#include "esp_heap_caps.h"
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#define BUFFER_SIZE 400 //32-bit aligned size
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#define BUFFER_TYPE RINGBUF_TYPE_NOSPLIT
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...
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//Allocate ring buffer data structure and storage area into external RAM
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StaticRingbuffer_t *buffer_struct = (StaticRingbuffer_t *)heap_caps_malloc(sizeof(StaticRingbuffer_t), MALLOC_CAP_SPIRAM);
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uint8_t *buffer_storage = (uint8_t *)heap_caps_malloc(sizeof(uint8_t)*BUFFER_SIZE, MALLOC_CAP_SPIRAM);
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//Create a ring buffer with manually allocated memory
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RingbufHandle_t handle = xRingbufferCreateStatic(BUFFER_SIZE, BUFFER_TYPE, buffer_storage, buffer_struct);
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...
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//Delete the ring buffer after used
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vRingbufferDelete(handle);
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//Manually free all blocks of memory
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free(buffer_struct);
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free(buffer_storage);
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Ring Buffer API Reference
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-------------------------
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.. note::
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Ideally, ring buffers can be used with multiple tasks in an SMP fashion where the **highest
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priority task will always be serviced first.** However due to the usage of binary semaphores
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in the ring buffer's underlying implementation, priority inversion may occur under very
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specific circumstances.
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The ring buffer governs sending by a binary semaphore which is given whenever space is
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freed on the ring buffer. The highest priority task waiting to send will repeatedly take
|
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the semaphore until sufficient free space becomes available or until it times out. Ideally
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this should prevent any lower priority tasks from being serviced as the semaphore should
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always be given to the highest priority task.
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However in between iterations of acquiring the semaphore, there is a **gap in the critical
|
|
section** which may permit another task (on the other core or with an even higher priority) to
|
|
free some space on the ring buffer and as a result give the semaphore. Therefore the semaphore
|
|
will be given before the highest priority task can re-acquire the semaphore. This will result
|
|
in the **semaphore being acquired by the second highest priority task** waiting to send, hence
|
|
causing priority inversion.
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This side effect will not affect ring buffer performance drastically given if the number
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|
of tasks using the ring buffer simultaneously is low, and the ring buffer is not operating
|
|
near maximum capacity.
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.. include-build-file:: inc/ringbuf.inc
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.. _hooks:
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Hooks
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|
-----
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FreeRTOS consists of Idle Hooks and Tick Hooks which allow for application
|
|
specific functionality to be added to the Idle Task and Tick Interrupt.
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|
ESP-IDF provides its own Idle and Tick Hook API in addition to the hooks
|
|
provided by Vanilla FreeRTOS. ESP-IDF hooks have the added benefit of
|
|
being run time configurable and asymmetrical.
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|
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|
Vanilla FreeRTOS Hooks
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|
^^^^^^^^^^^^^^^^^^^^^^
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|
|
|
Idle and Tick Hooks in vanilla FreeRTOS are implemented by the user
|
|
defining the functions ``vApplicationIdleHook()`` and ``vApplicationTickHook()``
|
|
respectively somewhere in the application. Vanilla FreeRTOS will run the user
|
|
defined Idle Hook and Tick Hook on every iteration of the Idle Task and Tick
|
|
Interrupt respectively.
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|
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|
Vanilla FreeRTOS hooks are referred to as **Legacy Hooks** in ESP-IDF FreeRTOS.
|
|
To enable legacy hooks, :ref:`CONFIG_FREERTOS_LEGACY_HOOKS` should be enabled
|
|
in :doc:`project configuration menu </api-reference/kconfig>`.
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|
|
|
Due to vanilla FreeRTOS being designed for single core, ``vApplicationIdleHook()``
|
|
and ``vApplicationTickHook()`` can only be defined once. However, the ESP32 is dual core
|
|
in nature, therefore same Idle Hook and Tick Hook are used for both cores (in other words,
|
|
the hooks are symmetrical for both cores).
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|
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|
ESP-IDF Idle and Tick Hooks
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
Due to the the dual core nature of the ESP32, it may be necessary for some
|
|
applications to have separate hooks for each core. Furthermore, it may
|
|
be necessary for the Idle Tasks or Tick Interrupts to execute multiple hooks
|
|
that are configurable at run time. Therefore the ESP-IDF provides it's own hooks
|
|
API in addition to the legacy hooks provided by Vanilla FreeRTOS.
|
|
|
|
The ESP-IDF tick/idle hooks are registered at run time, and each tick/idle hook
|
|
must be registered to a specific CPU. When the idle task runs/tick Interrupt
|
|
occurs on a particular CPU, the CPU will run each of its registered idle/tick hooks
|
|
in turn.
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|
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|
Hooks API Reference
|
|
-------------------
|
|
|
|
.. include-build-file:: inc/esp_freertos_hooks.inc
|