mgmt: mcumgr: grp: settings_mgmt: Allow saving single setting

Adds functionality that allows saving a single setting to NVM using
the newly added function in the settings subsystem. This also
replaces calling the subtree save function if it the underlying
Kconfig is enabled to reduce code paths in settings mgmt

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

include/zephyr/mgmt/mcumgr/grp/settings_mgmt/settings_mgmt.h

@@ -58,6 +58,13 @@ enum settings_mgmt_ret_code_t {
 
 	/** The provided key name does not support being saved. */
 	SETTINGS_MGMT_ERR_SAVE_NOT_SUPPORTED,
+
+	/**
+	 * The provided key cannot be saved before the value is longer than the size of the
+	 * largest value that can safely be read
+	 * (CONFIG_SETTINGS_SAVE_SINGLE_SUBTREE_WITHOUT_MODIFICATION_VALUE_SIZE).
+	 */
+	SETTINGS_MGMT_ERR_SAVE_FAILED_VALUE_TOO_LONG_TO_READ,
 };
 
 #ifdef __cplusplus

subsys/mgmt/mcumgr/grp/settings_mgmt/src/settings_mgmt.c

@@ -515,13 +515,28 @@ static int settings_mgmt_save(struct smp_streamer *ctxt)
 	}
 
 	if (save_subtree) {
+#ifdef CONFIG_SETTINGS_SAVE_SINGLE_SUBTREE_WITHOUT_MODIFICATION
+		/*
+		 * This allows saving either a subtree or single setting, if a full setting name
+		 * is provided then the single setting will be saved, otherwise it will save the
+		 * subtree (assuming it is a valid setting subtree).
+		 */
+		rc = settings_save_subtree_or_single_without_modification(key_name, true, true);
+#else
 		rc = settings_save_subtree(key_name);
+#endif
 	} else {
 		rc = settings_save();
 	}
 
 	if (rc != 0) {
 		switch (rc) {
+#ifdef CONFIG_SETTINGS_SAVE_SINGLE_SUBTREE_WITHOUT_MODIFICATION
+		case -EDOM:
+			rc = SETTINGS_MGMT_ERR_SAVE_FAILED_VALUE_TOO_LONG_TO_READ;
+			break;
+		case -ENOSYS:
+#endif
 		case -ENOENT:
 		case -ENOTSUP:
 			rc = SETTINGS_MGMT_ERR_SAVE_NOT_SUPPORTED;