1.. _retention_api:
2
3Retention System
4################
5
6The retention system provides an API which allows applications to read and
7write data from and to memory areas or devices that retain the data while the
8device is powered. This allows for sharing information between different
9applications or within a single application without losing state information
10when a device reboots. The stored data should not persist in the event of a
11power failure (or during some low-power modes on some devices) nor should it be
12stored to a non-volatile storage like :ref:`flash_api`, :ref:`eeprom_api`, or
13battery-backed RAM.
14
15The retention system builds on top of the retained data driver, and adds
16additional software-level features to it for ensuring the validity of data.
17Optionally, a magic header can be used to check if the front of
18the retained data memory section contains this specific value, and an optional
19checksum (1, 2, or 4-bytes in size) of the stored data can be appended to the
20end of the data. Additionally, the retention system API allows partitioning of
21the retained data sections into multiple distinct areas. For example, a 64-byte
22retained data area could be split up into 4 bytes for a boot mode, 16 bytes for
23a timestamp, 44 bytes for a last log message. All of these sections can be
24accessed or updated independently. The prefix and checksum can be set
25per-instance using devicetree.
26
27Devicetree setup
28****************
29
30To use the retention system, a retained data driver must be setup for the board
31you are using, there is a zephyr driver which can be used which will use some
32RAM as non-init for this purpose. The retention system is then initialised as a
33child node of this device 1 or more times - note that the memory region will
34need to be decremented to account for this reserved portion of RAM. See the
35following example (examples in this guide are based on the
36:ref:`nrf52840dk_nrf52840` board and memory layout):
37
38.. code-block:: devicetree
39
40	/ {
41		sram@2003FC00 {
42			compatible = "zephyr,memory-region", "mmio-sram";
43			reg = <0x2003FC00 DT_SIZE_K(1)>;
44			zephyr,memory-region = "RetainedMem";
45			status = "okay";
46
47			retainedmem {
48				compatible = "zephyr,retained-ram";
49				status = "okay";
50				#address-cells = <1>;
51				#size-cells = <1>;
52
53				/* This creates a 256-byte partition */
54				retention0: retention@0 {
55					compatible = "zephyr,retention";
56					status = "okay";
57
58					/* The total size of this area is 256
59					 * bytes which includes the prefix and
60					 * checksum, this means that the usable
61					 * data storage area is 256 - 3 = 253
62					 * bytes
63					 */
64					reg = <0x0 0x100>;
65
66					/* This is the prefix which must appear
67					 * at the front of the data
68					 */
69					prefix = [08 04];
70
71					/* This uses a 1-byte checksum */
72					checksum = <1>;
73				};
74
75				/* This creates a 768-byte partition */
76				retention1: retention@100 {
77					compatible = "zephyr,retention";
78					status = "okay";
79
80					/* Start position must be after the end
81					 * of the previous partition. The total
82					 * size of this area is 768 bytes which
83					 * includes the prefix and checksum,
84					 * this means that the usable data
85					 * storage area is 768 - 6 = 762 bytes
86					 */
87					reg = <0x100 0x300>;
88
89					/* This is the prefix which must appear
90					 * at the front of the data
91					 */
92					prefix = [00 11 55 88 fa bc];
93
94					/* If omitted, there will be no
95					 * checksum
96					 */
97				};
98			};
99		};
100	};
101
102	/* Reduce SRAM0 usage by 1KB to account for non-init area */
103	&sram0 {
104		reg = <0x20000000 DT_SIZE_K(255)>;
105	};
106
107The retention areas can then be accessed using the data retention API (once
108enabled with :kconfig:option:`CONFIG_RETENTION`, which requires that
109:kconfig:option:`CONFIG_RETAINED_MEM` be enabled) by getting the device by
110using:
111
112.. code-block:: C
113
114	#include <zephyr/device.h>
115	#include <zephyr/retention/retention.h>
116
117	const struct device *retention1 = DEVICE_DT_GET(DT_NODELABEL(retention1));
118	const struct device *retention2 = DEVICE_DT_GET(DT_NODELABEL(retention2));
119
120When the write function is called, the magic header and checksum (if enabled)
121will be set on the area, and it will be marked as valid from that point
122onwards.
123
124.. _boot_mode_api:
125
126Boot mode
127*********
128
129An addition to the retention subsystem is a boot mode interface, this can be
130used to dynamically change the state of an application or run a different
131application with a minimal set of functions when a device is rebooted (an
132example is to have a buttonless way of entering mcuboot's serial recovery
133feature from the main application).
134
135To use the boot mode feature, a data retention entry must exist in the device
136tree, which is dedicated for use as the boot mode selection (the user area data
137size only needs to be a single byte), and this area be assigned to the chosen
138node of ``zephyr,boot-mode``. See the following example:
139
140.. code-block:: devicetree
141
142	/ {
143		sram@2003FFFF {
144			compatible = "zephyr,memory-region", "mmio-sram";
145			reg = <0x2003FFFF 0x1>;
146			zephyr,memory-region = "RetainedMem";
147			status = "okay";
148
149			retainedmem {
150				compatible = "zephyr,retained-ram";
151				status = "okay";
152				#address-cells = <1>;
153				#size-cells = <1>;
154
155				retention0: retention@0 {
156					compatible = "zephyr,retention";
157					status = "okay";
158					reg = <0x0 0x1>;
159				};
160			};
161		};
162
163		chosen {
164			zephyr,boot-mode = &retention0;
165		};
166	};
167
168	/* Reduce SRAM0 usage by 1 byte to account for non-init area */
169	&sram0 {
170		reg = <0x20000000 0x3FFFF>;
171	};
172
173The boot mode interface can be enabled with
174:kconfig:option:`CONFIG_RETENTION_BOOT_MODE` and then accessed by using the
175boot mode functions. If using mcuboot with serial recovery, it can be built
176with ``CONFIG_MCUBOOT_SERIAL`` and ``CONFIG_BOOT_SERIAL_BOOT_MODE`` enabled
177which will allow rebooting directly into the serial recovery mode by using:
178
179.. code-block:: C
180
181	#include <zephyr/retention/bootmode.h>
182	#include <zephyr/sys/reboot.h>
183
184	bootmode_set(BOOT_MODE_TYPE_BOOTLOADER);
185	sys_reboot(0);
186
187API Reference
188*************
189
190Retention system API
191====================
192
193.. doxygengroup:: retention_api
194
195Boot mode interface
196===================
197
198.. doxygengroup:: boot_mode_interface
199