142 lines
4.4 KiB
ReStructuredText
142 lines
4.4 KiB
ReStructuredText
.. _nanokernel_fifos:
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Nanokernel FIFOs
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################
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Concepts
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********
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The nanokernel's FIFO object type is an implementation of a traditional
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first in, first out queue. It is mainly intended for use by fibers.
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A nanokernel FIFO allows data items of any size tasks to be sent and received
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asynchronously. The FIFO uses a linked list to hold data items that have been
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sent but not yet received.
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FIFO data items must be aligned on a 4-byte boundary, as the kernel reserves
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the first 32 bits of each item for use as a pointer to the next data item
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in the FIFO's linked list. Consequently, a data item that holds N bytes
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of application data requires N+4 bytes of memory.
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Any number of nanokernel FIFOs can be defined. Each FIFO is a distinct
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variable of type :c:type:`struct nano_fifo`, and is referenced using a
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pointer to that variable. A FIFO must be initialized before it can be used to
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send or receive data items.
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Items can be added to a nanokernel FIFO in a non-blocking manner by any
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context type (i.e. ISR, fiber, or task).
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Items can be removed from a nanokernel FIFO in a non-blocking manner by any
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context type; if the FIFO is empty the :c:macro:`NULL` return code
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indicates that no item was removed. Items can also be removed from a
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nanokernel FIFO in a blocking manner by a fiber or task; if the FIFO is empty
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the thread waits for an item to be added.
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Any number of threads may wait on an empty nanokernel FIFO simultaneously.
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When a data item becomes available it is given to the fiber that has waited
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longest, or to a waiting task if no fiber is waiting.
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.. note::
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A task that waits on an empty nanokernel FIFO does a busy wait. This is
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not an issue for a nanokernel application's background task; however, in
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a microkernel application a task that waits on a nanokernel FIFO remains
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the current task. In contrast, a microkernel task that waits on a
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microkernel data passing object ceases to be the current task, allowing
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other tasks of equal or lower priority to do useful work.
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If multiple tasks in a microkernel application wait on the same nanokernel
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FIFO, higher priority tasks are given data items in preference to lower
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priority tasks. However, the order in which equal priority tasks are given
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data items is unpredictible.
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Purpose
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*******
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Use a nanokernel FIFO to asynchronously transfer data items of arbitrary size
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in a "first in, first out" manner.
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Usage
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*****
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Example: Initializing a Nanokernel FIFO
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=======================================
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This code establishes an empty nanokernel FIFO.
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.. code-block:: c
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struct nano_fifo signal_fifo;
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nano_fifo_init(&signal_fifo);
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Example: Writing to a Nanokernel FIFO from a Fiber
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==================================================
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This code uses a nanokernel FIFO to send data to one or more consumer fibers.
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.. code-block:: c
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struct data_item_t {
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void *fifo_reserved; /* 1st word reserved for use by FIFO */
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...
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};
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struct data_item_t tx_data;
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void producer_fiber(int unused1, int unused2)
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{
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ARG_UNUSED(unused1);
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ARG_UNUSED(unused2);
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while (1) {
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/* create data item to send (e.g. measurement, timestamp, ...) */
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tx_data = ...
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/* send data to consumers */
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nano_fiber_fifo_put(&signal_fifo, &tx_data);
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...
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}
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}
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Example: Reading from a Nanokernel FIFO
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=======================================
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This code uses a nanokernel FIFO to obtain data items from a producer fiber,
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which are then processed in some manner. This design supports the distribution
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of data items to multiple consumer fibers, if desired.
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.. code-block:: c
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void consumer_fiber(int unused1, int unused2)
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{
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struct data_item_t *rx_data;
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ARG_UNUSED(unused1);
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ARG_UNUSED(unused2);
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while (1) {
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rx_data = nano_fiber_fifo_get_wait(&signal_fifo, TICKS_NONE);
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/* process FIFO data */
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...
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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 a nanokernel FIFO are provided by :file:`nanokernel.h`:
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:cpp:func:`nano_fifo_init()`
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Initializes a FIFO.
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:cpp:func:`nano_task_fifo_put()`, :cpp:func:`nano_fiber_fifo_put()`,
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:cpp:func:`nano_isr_fifo_put()`, :cpp:func:`nano_fifo_put()`
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Add an item to a FIFO.
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:cpp:func:`nano_task_fifo_get()`, :cpp:func:`nano_fiber_fifo_get()`,
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:cpp:func:`nano_isr_fifo_get()`, :cpp:func:`nano_fifo_get()`
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Remove an item from a FIFO, or waits for an item for a specified time
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period if it is empty.
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