app: Fix CMUX tracing and allow configurable drop

Fix CMUX tracing after the latest pipe changes.

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

@@ -349,7 +349,18 @@ 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

@@ -5,9 +5,8 @@
  */
 #include "sm_cmux.h"
 #include "sm_at_host.h"
-#if defined(CONFIG_SM_PPP)
 #include "sm_ppp.h"
-#endif
+#include "sm_trace_backend_cmux.h"
 #include "sm_util.h"
 #include "sm_uart_handler.h"
 #include <zephyr/logging/log.h>
@@ -123,13 +122,17 @@ static void stop_work_fn(struct k_work *work)
 	ARG_UNUSED(work);
 
 	if (sm_cmux_is_started()) {
-		modem_cmux_release(&cmux.instance);
-
-		cmux.at_channel = 0;
 		if (IS_ENABLED(CONFIG_SM_PPP)) {
 			sm_ppp_detach();
 		}
 
+		if (IS_ENABLED(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX)) {
+			sm_trace_backend_detach();
+		}
+
+		modem_cmux_release(&cmux.instance);
+		cmux.at_channel = 0;
+
 		/* Return AT host to UART pipe */
 		sm_at_host_set_pipe(sm_at_host_get_urc_ctx(), cmux.uart_pipe);
 		cmux.uart_pipe = NULL;
@@ -189,6 +192,15 @@ static int cmux_start(void)
 		sm_at_host_release(sm_at_host_get_ctx_from(ppp_pipe));
 		sm_ppp_attach(ppp_pipe);
 	}
+	if (IS_ENABLED(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX)) {
+		/* Reserve trace channel pipe for trace backend */
+		struct modem_pipe *trace_pipe = sm_cmux_get_dlci(CMUX_MODEM_TRACE_CHANNEL);
+
+		LOG_DBG("Reserving CMUX trace channel pipe %p for trace backend",
+			(void *)trace_pipe);
+		sm_at_host_release(sm_at_host_get_ctx_from(trace_pipe));
+		sm_trace_backend_attach(trace_pipe);
+	}
 
 	/* Attach CMUX to UART pipe (AT host will be detached by transition) */
 	ret = modem_cmux_attach(&cmux.instance, cmux.uart_pipe);

app/src/sm_cmux.h

@@ -16,9 +16,7 @@ enum cmux_channel {
 #if defined(CONFIG_SM_PPP)
 	CMUX_PPP_CHANNEL = 2,
 #endif
-#if defined(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX)
 	CMUX_MODEM_TRACE_CHANNEL = 3,
-#endif
 	CMUX_USER_CHANNEL_0,
 	CMUX_USER_CHANNEL_1,
 	CMUX_EXT_CHANNEL_COUNT

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);