app: Improve pipe handling of CMUX tracing and allow configurable drop

Improve handling of pipe open-close events of CMUX tracing module.

Allow TX timeout and drop policy to be defined by Kconfig.

Remove unnecessary loop from trace backend as nRF library already
loops on -EAGAIN.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

app/Kconfig

@@ -340,7 +340,18 @@ config SM_MODEM_TRACE_BACKEND_CMUX
 
 endchoice # NRF_MODEM_LIB_TRACE_BACKEND
 
-endif
+if SM_MODEM_TRACE_BACKEND_CMUX
+
+config SM_MODEM_TRACE_DROP_ON_TIMEOUT
+	bool "Drop modem trace data if the CMUX trace channel is slow"
+	default y
+
+config SM_MODEM_TRACE_CMUX_TX_TIMEOUT_MS
+	int "Timeout for sending modem trace data over CMUX in milliseconds"
+	default 100
+
+endif # SM_MODEM_TRACE_BACKEND_CMUX
+endif # NRF_MODEM_LIB_TRACE
 
 #
 # Hardware RX byte counting for UART

app/src/sm_cmux.c

@@ -10,6 +10,7 @@
 #endif
 #include "sm_util.h"
 #include "sm_uart_handler.h"
+#include "sm_trace_backend_cmux.h"
 #include <zephyr/logging/log.h>
 #include <zephyr/modem/cmux.h>
 #include <zephyr/modem/pipe.h>
@@ -339,6 +340,9 @@ static void stop_work_fn(struct k_work *work)
 
 	/* Will stop the UART when calling the close_pipe() function. */
 	if (sm_cmux_is_started()) {
+#if defined(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX)
+		sm_trace_backend_detach();
+#endif
 		modem_cmux_release(&cmux.instance);
 
 		close_pipe(&cmux.uart_pipe);
@@ -437,6 +441,10 @@ static int cmux_start(void)
 		return ret;
 	}
 
+#if defined(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX)
+	sm_trace_backend_attach(sm_cmux_reserve(CMUX_MODEM_TRACE_CHANNEL));
+#endif
+
 	ret = modem_pipe_open(cmux.uart_pipe, K_SECONDS(CONFIG_SM_MODEM_PIPE_TIMEOUT));
 	if (!ret) {
 		cmux.uart_pipe_open = true;

app/src/sm_trace_backend_cmux.c

@@ -9,13 +9,14 @@
 #include <zephyr/modem/pipe.h>
 #include <modem/nrf_modem_lib_trace.h>
 #include <modem/trace_backend.h>
-#include "sm_cmux.h"
+#include "sm_util.h"
 
 LOG_MODULE_REGISTER(modem_trace_backend, CONFIG_MODEM_TRACE_BACKEND_LOG_LEVEL);
 
 static K_SEM_DEFINE(tx_idle_sem, 0, 1);
 
 static trace_backend_processed_cb trace_processed_callback;
+static struct modem_pipe *trace_pipe;
 
 static void modem_pipe_event_handler(struct modem_pipe *pipe,
 				     enum modem_pipe_event event, void *user_data)
@@ -25,7 +26,14 @@ static void modem_pipe_event_handler(struct modem_pipe *pipe,
 	case MODEM_PIPE_EVENT_TRANSMIT_IDLE:
 		k_sem_give(&tx_idle_sem);
 		break;
-
+	case MODEM_PIPE_EVENT_CLOSED:
+		LOG_INF("Trace pipe closed");
+		trace_pipe = NULL;
+		break;
+	case MODEM_PIPE_EVENT_OPENED:
+		LOG_INF("Trace pipe opened");
+		trace_pipe = pipe;
+		break;
 	default:
 		break;
 	}
@@ -38,58 +46,73 @@ int trace_backend_init(trace_backend_processed_cb trace_processed_cb)
 	}
 
 	trace_processed_callback = trace_processed_cb;
+	trace_pipe = NULL;
 
 	return 0;
 }
 
 int trace_backend_deinit(void)
 {
-	sm_cmux_release(CMUX_MODEM_TRACE_CHANNEL);
-
+	if (trace_pipe) {
+		modem_pipe_release(trace_pipe);
+		trace_pipe = NULL;
+	}
 	return 0;
 }
 
+void sm_trace_backend_attach(struct modem_pipe *pipe)
+{
+	modem_pipe_attach(pipe, modem_pipe_event_handler, NULL);
+}
+
+void sm_trace_backend_detach(void)
+{
+	if (trace_pipe) {
+		modem_pipe_release(trace_pipe);
+		trace_pipe = NULL;
+	}
+}
+
 int trace_backend_write(const void *data, size_t len)
 {
-	struct modem_pipe *pipe = sm_cmux_reserve(CMUX_MODEM_TRACE_CHANNEL);
-	const uint8_t *buf = (const uint8_t *)data;
-	size_t sent_len = 0;
 	int ret = 0;
 
-	if (!sm_cmux_dlci_is_open(CMUX_MODEM_TRACE_CHANNEL)) {
-		LOG_DBG("Dropped %u bytes.", len);
+	if (!trace_pipe || !sm_pipe_is_open(trace_pipe)) {
+		LOG_DBG_RATELIMIT("Pipe closed, dropped %u bytes.", len);
 		trace_processed_callback(len);
 		return len;
 	}
 
-	modem_pipe_attach(pipe, modem_pipe_event_handler, NULL);
-
-	while (sent_len < len) {
-		ret = modem_pipe_transmit(pipe, buf, len - sent_len);
-		if (ret < 0) {
-			LOG_WRN("TX error (%d). Dropped %u bytes.", ret, len - sent_len);
+	/* No need to retry here.
+	 * The nrf_modem_lib_trace.c:trace_fragment_write() handles
+	 * retrying if the backend returns -EAGAIN.
+	 *
+	 * It is important to call trace_processed_callback(len) for
+	 * every fragment written to ensure the trace fifos are not
+	 * overflowing. Even if we drop the fragment.
+	 */
+
+	if (k_sem_take(&tx_idle_sem, K_MSEC(CONFIG_SM_MODEM_TRACE_CMUX_TX_TIMEOUT_MS)) != 0) {
+		if (IS_ENABLED(CONFIG_SM_MODEM_TRACE_DROP_ON_TIMEOUT)) {
+			LOG_WRN_RATELIMIT("Timeout, dropping %u bytes.", len);
 			trace_processed_callback(len);
-			return ret;
-		} else if (ret == 0) {
-			if (k_sem_take(&tx_idle_sem, K_SECONDS(1)) != 0) {
-				LOG_WRN("TX timeout.");
-				break;
-			}
-		} else {
-			sent_len += ret;
-			buf += ret;
+			return len;
 		}
+		LOG_WRN_RATELIMIT("TX timeout.");
+		return -EAGAIN;
 	}
 
-	if (sent_len) {
-		trace_processed_callback(sent_len);
-	}
-
-	if (sent_len < len) {
-		LOG_DBG("Sent %u out of %u bytes.", sent_len, len);
+	ret = modem_pipe_transmit(trace_pipe, data, len);
+	if (ret < 0) {
+		LOG_WRN("TX error (%d). Dropped %u bytes.", ret, len);
+		trace_processed_callback(len);
+		return ret;
+	} else if (ret == 0) {
+		return -EAGAIN;
 	}
+	trace_processed_callback(ret);
 
-	return sent_len;
+	return ret;
 }
 
 struct nrf_modem_lib_trace_backend trace_backend = {

app/src/sm_trace_backend_cmux.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2026 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+void sm_trace_backend_attach(struct modem_pipe *pipe);
+
+void sm_trace_backend_detach(void);