net: openthread: rpc: extrapolate radio time

Enhance otLinkRawGetRadioTime() serialization to extrapolate
the radio time from the last radio time reading and the
current local time, if the sync has occurred recently.

This allows to reduce the RPC traffic given that radio time
may need to be read frequently by an application.

Signed-off-by: Damian Krolik <[email protected]>

Author: Damian-Nordic

subsys/net/openthread/rpc/Kconfig

@@ -65,6 +65,16 @@ config OPENTHREAD_RPC_CLIENT_NUM_SENT_COAP_REQUESTS
 	  Defines the number of slots for storing a callback along with its context
 	  for ongoing CoAP requests awaiting a response.
 
+config OPENTHREAD_RPC_CLIENT_RADIO_TIME_REFRESH_PERIOD
+	int "Radio time sync period [s]"
+	default 30
+	help
+	  Defines how frequently the RPC client shall read the RPC server's
+	  radio time instead of extrapolating it from the last reading and the
+	  current local time. The purpose of this setting is to reduce the RPC
+	  traffic given that otLinkRawGetRadioTime() may be used extensively by
+	  the application.
+
 endmenu # "OpenThread over RPC client configuration"
 
 menu "OpenThread over RPC client configuration"

subsys/net/openthread/rpc/client/ot_rpc_link_raw.c

@@ -12,21 +12,46 @@
 
 #include <openthread/link_raw.h>
 
+#include <zephyr/kernel.h>
+
+#define RADIO_TIME_REFRESH_PERIOD (CONFIG_OPENTHREAD_RPC_CLIENT_RADIO_TIME_REFRESH_PERIOD * 1000000)
+
+static bool sync_radio_time_valid;
+static uint64_t sync_local_time;
+static uint64_t sync_radio_time;
+
+static inline uint64_t get_local_time(void)
+{
+	return k_ticks_to_us_floor64(k_uptime_ticks());
+}
+
 uint64_t otLinkRawGetRadioTime(otInstance *aInstance)
 {
 	struct nrf_rpc_cbor_ctx ctx;
-	uint64_t time;
+	uint64_t now;
 
 	ARG_UNUSED(aInstance);
 
-	NRF_RPC_CBOR_ALLOC(&ot_group, ctx, 0);
-	nrf_rpc_cbor_cmd_rsp_no_err(&ot_group, OT_RPC_CMD_LINK_RAW_GET_RADIO_TIME, &ctx);
-
-	time = nrf_rpc_decode_uint64(&ctx);
-
-	if (!nrf_rpc_decoding_done_and_check(&ot_group, &ctx)) {
-		ot_rpc_report_rsp_decoding_error(OT_RPC_CMD_LINK_RAW_GET_RADIO_TIME);
+	now = get_local_time();
+
+	if (!sync_radio_time_valid || now >= sync_local_time + RADIO_TIME_REFRESH_PERIOD) {
+		NRF_RPC_CBOR_ALLOC(&ot_group, ctx, 0);
+		nrf_rpc_cbor_cmd_rsp_no_err(&ot_group, OT_RPC_CMD_LINK_RAW_GET_RADIO_TIME, &ctx);
+
+		/*
+		 * Calculate sync_local_time as if the radio time was read by the peer exactly
+		 * halfway between constructing a request and receiving the response.
+		 */
+		sync_radio_time_valid = true;
+		sync_local_time = now / 2;
+		now = get_local_time();
+		sync_local_time += now / 2;
+		sync_radio_time = nrf_rpc_decode_uint64(&ctx);
+
+		if (!nrf_rpc_decoding_done_and_check(&ot_group, &ctx)) {
+			ot_rpc_report_rsp_decoding_error(OT_RPC_CMD_LINK_RAW_GET_RADIO_TIME);
+		}
 	}
 
-	return time;
+	return sync_radio_time + (now - sync_local_time);
 }

tests/subsys/net/openthread/rpc/client/prj.conf

@@ -10,6 +10,7 @@ CONFIG_TEST_RANDOM_GENERATOR=y
 
 CONFIG_OPENTHREAD_RPC=y
 CONFIG_OPENTHREAD_RPC_CLIENT=y
+CONFIG_OPENTHREAD_RPC_CLIENT_RADIO_TIME_REFRESH_PERIOD=0
 CONFIG_NETWORKING=y
 CONFIG_NRF_RPC_ZCBOR_BACKUPS=1
 CONFIG_NRF_RPC_CBKPROXY_OUT_SLOTS=0