Settings

The settings subsystem gives modules a way to store persistent per-device configuration and runtime state. A variety of storage implementations are provided behind a common API using FCB, NVS, or a file system. These different implementations give the application developer flexibility to select an appropriate storage medium, and even change it later as needs change. This subsystem is used by various Zephyr components and can be used simultaneously by user applications.

Settings items are stored as key-value pair strings. By convention, the keys can be organized by the package and subtree defining the key, for example the key id/serial would define the serial configuration element for the package id.

Convenience routines are provided for converting a key value to and from a string type.

For an example of the settings subsystem refer to Settings API sample.

Note

As of Zephyr release 2.1 the recommended backend for non-filesystem storage is NVS.

Handlers

Settings handlers for subtree implement a set of handler functions. These are registered using a call to settings_register().

h_get: This gets called when asking for a settings element value by its name using settings_runtime_get() from the runtime backend.
h_set: This gets called when the value is loaded from persisted storage with settings_load(), or when using settings_runtime_set() from the runtime backend.
h_commit: This gets called after the settings have been loaded in full. Sometimes you don’t want an individual setting value to take effect right away, for example if there are multiple settings which are interdependent.
h_export: This gets called to write all current settings. This happens when settings_save() tries to save the settings or transfer to any user-implemented back-end.

Backends

Backends are meant to load and save data to/from setting handlers, and implement a set of handler functions. These are registered using a call to settings_src_register() for backends that can load data, and/or settings_dst_register() for backends that can save data. The current implementation allows for multiple source backends but only a single destination backend.

csi_load: This gets called when loading values from persistent storage using settings_load().
csi_save: This gets called when saving a single setting to persistent storage using settings_save_one().
csi_save_start: This gets called when starting a save of all current settings using settings_save().
csi_save_end: This gets called after having saved of all current settings using settings_save().

Zephyr Storage Backends

Zephyr has three storage backends: a Flash Circular Buffer (CONFIG_SETTINGS_FCB), a file in the filesystem (CONFIG_SETTINGS_FILE), or non-volatile storage (CONFIG_SETTINGS_NVS).

You can declare multiple sources for settings; settings from all of these are restored when settings_load() is called.

There can be only one target for writing settings; this is where data is stored when you call settings_save(), or settings_save_one().

FCB read target is registered using settings_fcb_src(), and write target using settings_fcb_dst(). As a side-effect, settings_fcb_src() initializes the FCB area, so it must be called before calling settings_fcb_dst(). File read target is registered using settings_file_src(), and write target by using settings_file_dst(). Non-volatile storage read target is registered using settings_nvs_src(), and write target by using settings_nvs_dst().

Storage Location

The FCB and non-volatile storage (NVS) backends both look for a fixed partition with label “storage” by default. A different partition can be selected by setting the zephyr,settings-partition property of the chosen node in the devicetree.

The file path used by the file backend to store settings is selected via the option CONFIG_SETTINGS_FILE_PATH.

Loading data from persisted storage

A call to settings_load() uses an h_set implementation to load settings data from storage to volatile memory. After all data is loaded, the h_commit handler is issued, signalling the application that the settings were successfully retrieved.

Technically FCB and file backends may store some history of the entities. This means that the newest data entity is stored after any older existing data entities. Starting with Zephyr 2.1, the back-end must filter out all old entities and call the callback with only the newest entity.

Storing data to persistent storage

A call to settings_save_one() uses a backend implementation to store settings data to the storage medium. A call to settings_save() uses an h_export implementation to store different data in one operation using settings_save_one(). A key need to be covered by a h_export only if it is supposed to be stored by settings_save() call.

For both FCB and file back-end only storage requests with data which changes most actual key’s value are stored, therefore there is no need to check whether a value changed by the application. Such a storage mechanism implies that storage can contain multiple value assignments for a key , while only the last is the current value for the key.

Garbage collection

When storage becomes full (FCB) or consumes too much space (file), the backend removes non-recent key-value pairs records and unnecessary key-delete records.

Secure domain settings

Currently settings doesn’t provide scheme of being secure, and non-secure configuration storage simultaneously for the same instance. It is recommended that secure domain uses its own settings instance and it might provide data for non-secure domain using dedicated interface if needed (case dependent).

Example: Device Configuration

This is a simple example, where the settings handler only implements h_set and h_export. h_set is called when the value is restored from storage (or when set initially), and h_export is used to write the value to storage thanks to storage_func(). The user can also implement some other export functionality, for example, writing to the shell console).

#define DEFAULT_FOO_VAL_VALUE 1

static int8 foo_val = DEFAULT_FOO_VAL_VALUE;

static int foo_settings_set(const char *name, size_t len,
                            settings_read_cb read_cb, void *cb_arg)
{
    const char *next;
    int rc;

    if (settings_name_steq(name, "bar", &next) && !next) {
        if (len != sizeof(foo_val)) {
            return -EINVAL;
        }

        rc = read_cb(cb_arg, &foo_val, sizeof(foo_val));
        if (rc >= 0) {
            /* key-value pair was properly read.
             * rc contains value length.
             */
            return 0;
        }
        /* read-out error */
        return rc;
    }

    return -ENOENT;
}

static int foo_settings_export(int (*storage_func)(const char *name,
                                                   const void *value,
                                                   size_t val_len))
{
    return storage_func("foo/bar", &foo_val, sizeof(foo_val));
}

struct settings_handler my_conf = {
    .name = "foo",
    .h_set = foo_settings_set,
    .h_export = foo_settings_export
};

Example: Persist Runtime State

This is a simple example showing how to persist runtime state. In this example, only h_set is defined, which is used when restoring value from persisted storage.

In this example, the main function increments foo_val, and then persists the latest number. When the system restarts, the application calls settings_load() while initializing, and foo_val will continue counting up from where it was before restart.

#include <zephyr/kernel.h>
#include <zephyr/sys/reboot.h>
#include <zephyr/settings/settings.h>
#include <zephyr/sys/printk.h>
#include <inttypes.h>

#define DEFAULT_FOO_VAL_VALUE 0

static uint8_t foo_val = DEFAULT_FOO_VAL_VALUE;

static int foo_settings_set(const char *name, size_t len,
                            settings_read_cb read_cb, void *cb_arg)
{
    const char *next;
    int rc;

    if (settings_name_steq(name, "bar", &next) && !next) {
        if (len != sizeof(foo_val)) {
            return -EINVAL;
        }

        rc = read_cb(cb_arg, &foo_val, sizeof(foo_val));
        if (rc >= 0) {
            return 0;
        }

        return rc;
    }


    return -ENOENT;
}

struct settings_handler my_conf = {
    .name = "foo",
    .h_set = foo_settings_set
};

int main(void)
{
    settings_subsys_init();
    settings_register(&my_conf);
    settings_load();

    foo_val++;
    settings_save_one("foo/bar", &foo_val, sizeof(foo_val));

    printk("foo: %d\n", foo_val);

    k_msleep(1000);
    sys_reboot(SYS_REBOOT_COLD);
}

Example: Custom Backend Implementation

This is a simple example showing how to register a simple custom backend handler (CONFIG_SETTINGS_CUSTOM).

static int settings_custom_load(struct settings_store *cs,
                                const struct settings_load_arg *arg)
{
    //...
}

static int settings_custom_save(struct settings_store *cs, const char *name,
                                const char *value, size_t val_len)
{
    //...
}

/* custom backend interface */
static struct settings_store_itf settings_custom_itf = {
    .csi_load = settings_custom_load,
    .csi_save = settings_custom_save,
};

/* custom backend node */
static struct settings_store settings_custom_store = {
    .cs_itf = &settings_custom_itf
};

int settings_backend_init(void)
{
    /* register custom backend */
    settings_dst_register(&settings_custom_store);
    settings_src_register(&settings_custom_store);
    return 0;
}