modem: cmux: Simplify resync mechanism

Some modems don't start sending resync flags as described in 3G TS 27.010 V2.0.0 5.2.5, which results in the CMUX being stuck in resync mode for said modems. This patch simplifies the resync mechanism to simply drop invalid frames, and wait for atleast two consequtive frame flags (stop+start). Signed-off-by: Bjarki Arge Andreasen <[email protected]>
noahp · Jan 11, 2024 · 3debfc8 · 3debfc8
1 parent 06b5792
commit 3debfc8
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Showing 2 changed files with 9 additions and 42 deletions.
diff --git a/include/zephyr/modem/cmux.h b/include/zephyr/modem/cmux.h
@@ -63,10 +63,7 @@ enum modem_cmux_state {
 
 enum modem_cmux_receive_state {
 	MODEM_CMUX_RECEIVE_STATE_SOF = 0,
-	MODEM_CMUX_RECEIVE_STATE_RESYNC_0,
-	MODEM_CMUX_RECEIVE_STATE_RESYNC_1,
-	MODEM_CMUX_RECEIVE_STATE_RESYNC_2,
-	MODEM_CMUX_RECEIVE_STATE_RESYNC_3,
+	MODEM_CMUX_RECEIVE_STATE_RESYNC,
 	MODEM_CMUX_RECEIVE_STATE_ADDRESS,
 	MODEM_CMUX_RECEIVE_STATE_ADDRESS_CONT,
 	MODEM_CMUX_RECEIVE_STATE_CONTROL,

diff --git a/subsys/modem/modem_cmux.c b/subsys/modem/modem_cmux.c
@@ -642,60 +642,30 @@ static void modem_cmux_on_frame(struct modem_cmux *cmux)
 	modem_cmux_on_dlci_frame(cmux);
 }
 
-static void modem_cmux_transmit_resync(struct modem_cmux *cmux)
-{
-	static const uint8_t resync[3] = {0xF9, 0xF9, 0xF9};
-
-	modem_pipe_transmit(cmux->pipe, resync, sizeof(resync));
-}
-
 static void modem_cmux_process_received_byte(struct modem_cmux *cmux, uint8_t byte)
 {
 	uint8_t fcs;
 
 	switch (cmux->receive_state) {
 	case MODEM_CMUX_RECEIVE_STATE_SOF:
 		if (byte == 0xF9) {
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_ADDRESS;
+			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC;
 			break;
 		}
 
-		modem_cmux_transmit_resync(cmux);
-		cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_0;
-		break;
-
-	case MODEM_CMUX_RECEIVE_STATE_RESYNC_0:
-		if (byte == 0xF9) {
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_1;
-		}
-
-		break;
-
-	case MODEM_CMUX_RECEIVE_STATE_RESYNC_1:
-		if (byte == 0xF9) {
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_2;
-		} else {
-			modem_cmux_transmit_resync(cmux);
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_0;
-		}
-
 		break;
 
-	case MODEM_CMUX_RECEIVE_STATE_RESYNC_2:
-		if (byte == 0xF9) {
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_3;
-		} else {
-			modem_cmux_transmit_resync(cmux);
-			cmux->receive_state = MODEM_CMUX_RECEIVE_STATE_RESYNC_0;
-		}
-
-		break;
-
-	case MODEM_CMUX_RECEIVE_STATE_RESYNC_3:
+	case MODEM_CMUX_RECEIVE_STATE_RESYNC:
+		/*
+		 * Allow any number of consequtive flags (0xF9).
+		 * 0xF9 could also be a valid address field for DLCI 62.
+		 */
 		if (byte == 0xF9) {
 			break;
 		}
 
+		__fallthrough;
+
 	case MODEM_CMUX_RECEIVE_STATE_ADDRESS:
 		/* Initialize */
 		cmux->receive_buf_len = 0;