doc: misc: Add documentation for MPSC Packet Buffer

Add documentation for Multi Producer Single Consumer Packet Buffer.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

doc/reference/data_structures/index.rst

@@ -30,5 +30,6 @@ needed will be provided by the user.
 
   slist.rst
   dlist.rst
+  mpsc_pbuf.rst
   rbtree.rst
   ring_buffers.rst

doc/reference/data_structures/mpsc_pbuf.rst

@@ -0,0 +1,147 @@
+.. _mpsc_pbuf:
+
+Multi Producer Single Consumer Packet Buffer
+============================================
+
+A :dfn:`Multi Producer Single Consumer Packet Buffer (MPSC_PBUF)` is a circular
+buffer, whose contents are stored in first-in-first-out order. Variable size
+packets are stored in the buffer. Packet buffer works under assumption that there
+is a single context that consumes the data. However, it is possible that another
+context may interfere to flush the data and never come back (panic case).
+Packet is produced in two steps: first requested amount of data is allocated,
+producer fills the data and commits it. Consuming a packet is also performed in
+two steps: consumer claims the packet, gets pointer to it and length and later
+on packet is freed. This approach reduces memory copying.
+
+A :dfn:`MPSC Packet Buffer` has the following key properties:
+
+* Allocate, commit scheme used for packet producing.
+* Claim, free scheme used for packet consuming.
+* Allocator ensures that continue memory of requested length is allocated.
+* Following policies can be applied when requested space cannot be allocated:
+
+  * **Overwrite** - oldest entries are dropped until requested amount of memory can
+    be allocated. For each dropped packet user callback is called.
+  * **No overwrite** - When requested amount of space cannot be allocated,
+    allocation fails.
+* Dedicated, optimized API for storing short packets.
+* Allocation with timeout.
+
+Internals
+---------
+
+Each packet in the buffer contains ``MPSC_PBUF`` specific header which is used
+for internal management. Header consists of 2 bit flags. In order to optimize
+memory usage, header can be added on top of the user header using
+:c:macro:`MPSC_PBUF_HDR` and remaining bits in the first word can be application
+specific. Header consists of following flags:
+
+* valid - bit set to one when packet contains valid user packet
+* busy - bit set when packet is being consumed (claimed but not free)
+
+Header state:
+
++-------+------+----------------------+
+| valid | busy | description          |
++-------+------+----------------------+
+| 0     | 0    | space is free        |
++-------+------+----------------------+
+| 1     | 0    | valid packet         |
++-------+------+----------------------+
+| 1     | 1    | claimed valid packet |
++-------+------+----------------------+
+| 0     | 1    | internal skip packet |
++-------+------+----------------------+
+
+Packet buffer space contains free space, valid user packets and internal skip
+packets. Internal skip packets indicates padding, e.g. at the of the buffer.
+
+Allocation
+^^^^^^^^^^
+
+Using pairs for read and write indexes, available space is determined. If
+space can be allocated, temporary write index is moved and pointer to a space
+witing buffer is returned. Packet header is reset. If allocation required
+wrapping of the write index, a skip packet is added to the end of buffer. If
+space cannot be allocated and overwrite is disabled then ``NULL`` pointer is
+returned or context blocks if allocation was with timeout.
+
+Allocation with overwrite
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If overwrite is enabled, oldest packets are dropped until requested amount of
+space can be allocated. When packets are dropped ``busy`` flag is checked in the
+header to ensure that currently consumed packet is not overwritten. In that case,
+skip packet is added before busy packet and packets following the busy packet
+are dropped. When busy packet is being freed, such situation is detected and
+packet is converted to skip packet to avoid double processing.
+
+Usage
+-----
+
+Packet header definition
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Packet header details can be found in :zephyr_file:`include/sys/mpsc_packet.h`.
+API functions can be found in :zephyr_file:`include/sys/mpsc_pbuf.h`. Headers
+are split to avoid include spam when declaring the packet.
+
+User header structure must start with internal header:
+
+.. code-block:: c
+
+   #include <sys/mpsc_packet.h>
+
+   struct foo_header {
+           MPSC_PBUF_HDR;
+           uint32_t length: 32 - MPSC_PBUF_HDR_BITS;
+   };
+
+Packet buffer configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Configuration structure contains buffer details, configuration flags and
+callbacks. Following callbacks are used by the packet buffer:
+
+* Drop notification - callback called whenever a packet is dropped due to
+  overwrite.
+* Get packet length - callback to determine packet length
+
+Packet producing
+^^^^^^^^^^^^^^^^
+
+Standard, two step method:
+
+.. code-block:: c
+
+   foo_packet *packet = mpsc_pbuf_alloc(buffer, len, K_NO_WAIT);
+
+   fill_data(packet);
+
+   mpsc_pbuf_commit(buffer, packet);
+
+Performance optimized storing of small packets:
+
+* 32 bit word packet
+* 32 bit word with pointer packet
+
+Note that since packets are written by value, they should already contain
+``valid`` bit set in the header.
+
+.. code-block:: c
+
+   mpsc_pbuf_put_word(buffer, data);
+   mpsc_pbuf_put_word_ext(buffer, data, ptr);
+
+Packet consuming
+^^^^^^^^^^^^^^^^
+
+Two step method:
+
+.. code-block:: c
+
+   foo_packet *packet = mpsc_pbuf_claim(buffer);
+
+   process(packet);
+
+   mpsc_pbuf_free(buffer, packet);