141 lines
4.1 KiB
ReStructuredText
141 lines
4.1 KiB
ReStructuredText
.. _nanokernel_ring_buffers:
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Nanokernel Ring Buffers
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#######################
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Definition
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**********
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The ring buffer is defined in :file:`include/misc/ring_buffer.h` and
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:file:`kernel/nanokernel/ring_buffer.c`. This is an array-based
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circular buffer, stored in first-in-first-out order. The APIs allow
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for enqueueing and retrieval of chunks of data up to 1024 bytes in size,
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along with two metadata values (type ID and an app-specific integer).
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Unlike nanokernel FIFOs, storage of enqueued items and their metadata
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is managed in a fixed buffer and there are no preconditions on the data
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enqueued (other than the size limit). Since the size annotation is only
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an 8-bit value, sizes are expressed in terms of 32-bit chunks.
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Internally, the ring buffer always maintains an empty 32-bit block in the
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buffer to distinguish between empty and full buffers. Any given entry
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in the buffer will use a 32-bit block for metadata plus any data attached.
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If the size of the buffer array is a power of two, the ring buffer will
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use more efficient masking instead of expensive modulo operations to
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maintain itself.
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Concurrency
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***********
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Concurrency control of ring buffers is not implemented at this level.
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Depending on usage (particularly with respect to number of concurrent
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readers/writers) applications may need to protect the ring buffer with
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mutexes and/or use semaphores to notify consumers that there is data to
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read.
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For the trivial case of one producer and one consumer, concurrency
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shouldn't be needed.
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Example: Initializing a Ring Buffer
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===================================
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There are three ways to initialize a ring buffer. The first two are through use
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of macros which defines one (and an associated private buffer) in file scope.
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You can declare a fast ring buffer that uses mask operations by declaring
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a power-of-two sized buffer:
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.. code-block:: c
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/* Buffer with 2^8 or 256 elements */
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SYS_RING_BUF_DECLARE_POW2(my_ring_buf, 8);
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Arbitrary-sized buffers may also be declared with a different macro, but
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these will always be slower due to use of modulo operations:
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.. code-block:: c
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#define MY_RING_BUF_SIZE 93
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SYS_RING_BUF_DECLARE_SIZE(my_ring_buf, MY_RING_BUF_SIZE);
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Alternatively, a ring buffer may be initialized manually. Whether the buffer
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will use modulo or mask operations will be detected automatically:
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.. code-block:: c
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#define MY_RING_BUF_SIZE 64
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struct my_struct {
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struct ring_buffer rb;
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uint32_t buffer[MY_RING_BUF_SIZE];
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...
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};
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struct my_struct ms;
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void init_my_struct {
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sys_ring_buf_init(&ms.rb, sizeof(ms.buffer), ms.buffer);
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...
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}
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Example: Enqueuing data
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=======================
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.. code-block:: c
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int ret;
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ret = sys_ring_buf_put(&ring_buf, TYPE_FOO, 0, &my_foo, SIZE32_OF(my_foo));
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if (ret == -EMSGSIZE) {
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... not enough room for the message ..
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}
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If the type or value fields are sufficient, the data pointer and size may be 0.
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.. code-block:: c
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int ret;
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ret = sys_ring_buf_put(&ring_buf, TYPE_BAR, 17, NULL, 0);
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if (ret == -EMSGSIZE) {
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... not enough room for the message ..
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}
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Example: Retrieving data
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========================
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.. code-block:: c
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int ret;
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uint32_t data[6];
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size = SIZE32_OF(data);
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ret = sys_ring_buf_get(&ring_buf, &type, &value, data, &size);
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if (ret == -EMSGSIZE) {
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printk("Buffer is too small, need %d uint32_t\n", size);
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} else if (ret == -EAGAIN) {
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printk("Ring buffer is empty\n");
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} else {
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printk("got item of type %u value &u of size %u dwords\n",
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type, value, size);
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...
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}
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APIs
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****
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The following APIs for ring buffers are provided by :file:`ring_buffer.h`:
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:c:func:`sys_ring_buf_init()`
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Initializes a ring buffer.
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:c:func:`SYS_RING_BUF_DECLARE_POW2()`,
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:c:func:`SYS_RING_BUF_DECLARE_SIZE()`
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Declare and init a file-scope ring buffer.
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:c:func:`sys_ring_buf_get_space()`
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Returns the amount of free buffer storage space in 32-bit dwords.
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:c:func:`sys_ring_buf_is_empty()`
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Indicates whether a buffer is empty.
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:c:func:`sys_ring_buf_put()`
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Enqueues an item.
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:c:func:`sys_ring_buf_get()`
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De-queues an item. |