doc: lwm2m: Time series data document update

Added documentation for Time series data.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>

doc/connectivity/networking/api/lwm2m.rst

@@ -429,6 +429,65 @@ This is especially useful if the server is composite-observing the resources bei
 written to. Locking will then ensure that the client only updates and sends notifications
 to the server after all operations are done, resulting in fewer messages in general.
 
+Support for time series data
+****************************
+
+LwM2M version 1.1 adds support for SenML CBOR and SenML JSON data formats. These data formats add
+support for time series data. Time series formats can be used for READ, NOTIFY and SEND operations.
+When data cache is enabled for a resource, each write will create a timestamped entry in a cache,
+and its content is then returned as a content in in READ, NOTIFY or SEND operation for a given
+resource.
+
+Data cache is only supported for resources with a fixed data size.
+
+Supported resource types:
+
+* Signed and unsigned 8-64-bit integers
+* Float
+* Boolean
+
+Enabling and configuring
+========================
+
+Enable data cache by selecting :kconfig:option:`CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT`.
+Application needs to allocate an array of :c:struct:`lwm2m_time_series_elem` structures and then
+enable the cache by calling :c:func:`lwm2m_engine_enable_cache` for a given resource. Earch resource
+must be enabled separately and each resource needs their own storage.
+
+.. code-block:: c
+
+  /* Allocate data cache storage */
+  static struct lwm2m_time_series_elem temperature_cache[10];
+  /* Enable data cache */
+  lwm2m_engine_enable_cache(LWM2M_PATH(IPSO_OBJECT_TEMP_SENSOR_ID, 0, SENSOR_VALUE_RID),
+          temperature_cache, ARRAY_SIZE(temperature_cache));
+
+LwM2M engine have room for four resources that have cache enabled. Limit can be increased by
+changing :kconfig:option:`CONFIG_LWM2M_MAX_CACHED_RESOURCES`. This affects a static memory usage of
+engine.
+
+Data caches depends on one of the SenML data formats
+:kconfig:option:`CONFIG_LWM2M_RW_SENML_CBOR_SUPPORT` or
+:kconfig:option:`CONFIG_LWM2M_RW_SENML_JSON_SUPPORT` and needs :kconfig:option:`CONFIG_POSIX_CLOCK`
+so it can request a timestamp from the system and :kconfig:option:`CONFIG_RING_BUFFER` for ring
+buffer.
+
+Read and Write operations
+=========================
+
+Full content of data cache is written into a payload when any READ, SEND or NOTIFY operation
+internally reads the content of a given resource. This has a side effect that any read callbacks
+registered for a that resource are ignored when cache is enabled.
+Data is written into a cache when any of the ``lwm2m_engine_set_*`` functions are called. To filter
+the data entering the cache, application may register a validation callback using
+:c:func:`lwm2m_engine_register_validate_callback`.
+
+Limitations
+===========
+
+Cache size should be manually set so small that the content can fit normal packets sizes.
+When cache is full, new values are dropped.
+
 LwM2M engine and application events
 ***********************************