nrf_modem_lib: trace_backends: rtt: early alloc buf

Allocate the modem trace buffer for the RTT backend on boot, instead of
when `nrf_modem_lib_init` is called. This allows the user to delay the
call to `nrf_modem_lib_init`, giving them a chance to attach to the RTT
channel before the modem starts.

Without this change the start of trace logs from the RTT backend are
invariably lost, as the user cannot attach until after data generation
has started.

Update tests to work with the move of `SEGGER_RTT_AllocUpBuffer` into
a `SYS_INIT` function.

Signed-off-by: Jordan Yates <[email protected]>

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -626,6 +626,7 @@ Modem libraries
 
 * :ref:`nrf_modem_lib_readme` library:
 
+  * Updated the RTT trace backend to allocate the RTT channel at boot, instead of when the modem is activated.
   * Removed support for deprecated RAI socket options ``SO_RAI_LAST``, ``SO_RAI_NO_DATA``, ``SO_RAI_ONE_RESP``, ``SO_RAI_ONGOING``, and ``SO_RAI_WAIT_MORE``.
 
 Multiprotocol Service Layer libraries

lib/nrf_modem_lib/trace_backends/rtt/rtt.c

@@ -5,6 +5,7 @@
  */
 
 #include <stdio.h>
+#include <zephyr/init.h>
 #include <zephyr/logging/log.h>
 #include <modem/trace_backend.h>
 #include <SEGGER_RTT.h>
@@ -18,21 +19,15 @@ static trace_backend_processed_cb trace_processed_callback;
 
 int trace_backend_init(trace_backend_processed_cb trace_processed_cb)
 {
-	const int segger_rtt_mode = SEGGER_RTT_MODE_BLOCK_IF_FIFO_FULL;
-
 	if (trace_processed_cb == NULL) {
 		return -EFAULT;
 	}
-
-	trace_processed_callback = trace_processed_cb;
-
-	trace_rtt_channel = SEGGER_RTT_AllocUpBuffer("modem_trace", rtt_buffer, sizeof(rtt_buffer),
-						     segger_rtt_mode);
-
 	if (trace_rtt_channel <= 0) {
 		return -EBUSY;
 	}
 
+	trace_processed_callback = trace_processed_cb;
+
 	return 0;
 }
 
@@ -72,3 +67,18 @@ struct nrf_modem_lib_trace_backend trace_backend = {
 	.deinit = trace_backend_deinit,
 	.write = trace_backend_write,
 };
+
+/* Allocate RTT buffer ASAP so external tooling can attach before `nrf_modem_init` */
+IF_DISABLED(CONFIG_UNITY, (static))
+int nrf_modem_lib_trace_rtt_channel_alloc(void)
+{
+	const int segger_rtt_mode = SEGGER_RTT_MODE_BLOCK_IF_FIFO_FULL;
+
+	trace_rtt_channel = SEGGER_RTT_AllocUpBuffer("modem_trace", rtt_buffer, sizeof(rtt_buffer),
+						     segger_rtt_mode);
+
+	return trace_rtt_channel < 0 ? -EBUSY : 0;
+}
+
+/* The RTT core initialises in PRE_KERNEL_1 */
+SYS_INIT(nrf_modem_lib_trace_rtt_channel_alloc, POST_KERNEL, 0);

tests/lib/nrf_modem_lib/trace_backends/rtt/src/main.c

@@ -10,6 +10,9 @@
 
 #include "trace_backend.h"
 
+/* SYS_INIT function */
+int nrf_modem_lib_trace_rtt_channel_alloc(void);
+
 extern struct nrf_modem_lib_trace_backend trace_backend;
 
 #include "cmock_SEGGER_RTT.h"
@@ -51,8 +54,10 @@ void test_trace_backend_init_rtt(void)
 	__cmock_SEGGER_RTT_AllocUpBuffer_ExpectAnyArgsAndReturn(trace_rtt_channel);
 	__cmock_SEGGER_RTT_AllocUpBuffer_AddCallback(&rtt_allocupbuffer_callback);
 
-	ret = trace_backend.init(callback);
+	ret = nrf_modem_lib_trace_rtt_channel_alloc();
+	TEST_ASSERT_EQUAL(0, ret);
 
+	ret = trace_backend.init(callback);
 	TEST_ASSERT_EQUAL(0, ret);
 }
 
@@ -63,6 +68,9 @@ void test_trace_backend_init_rtt_ebusy(void)
 	/* Simulate failure by returning negative RTT channel. */
 	__cmock_SEGGER_RTT_AllocUpBuffer_ExpectAnyArgsAndReturn(-1);
 
+	ret = nrf_modem_lib_trace_rtt_channel_alloc();
+	TEST_ASSERT_EQUAL(-EBUSY, ret);
+
 	ret = trace_backend.init(callback);
 	TEST_ASSERT_EQUAL(-EBUSY, ret);
 }