tracing: add RAM backend

If Zephyr is running on a coprocessor we might lack I/O
such as uart or usb to output tracing datas but we might
have gigabytes of RAM available.

This patch allows to output trace datas to a ram buffer, which then
may be retrieved using gdb.

e.g:
(gdb) dump binary memory channel0_0 <ram_tracing_start> \
<ram_tracing_end>

Signed-off-by: Julien Massot <julien.massot@iot.bzh>

subsys/tracing/CMakeLists.txt

@@ -36,6 +36,12 @@ zephyr_sources_ifdef(
   CONFIG_TRACING_BACKEND_POSIX
   tracing_backend_posix.c
   )
+
+zephyr_sources_ifdef(
+  CONFIG_TRACING_BACKEND_RAM
+  tracing_backend_ram.c
+  )
+
 endif()
 
 zephyr_include_directories_ifdef(

subsys/tracing/Kconfig

@@ -165,6 +165,21 @@ config TRACING_BACKEND_POSIX
 	help
 	  Use posix architecture to output tracing data to file system.
 
+config TRACING_BACKEND_RAM
+	bool "Enable RAM backend"
+	help
+	  Use a ram buffer to output tracing data which can
+	  be dumped to a file at runtime with a debugger.
+	  See gdb dump binary memory documentation for example.
+
+config RAM_TRACING_BUFFER_SIZE
+	int "Ram Tracing buffer size"
+	default 4096
+	depends on TRACING_BACKEND_RAM
+	help
+	  Size of the RAM trace buffer. Trace will be discarded if the
+	  length is exceeded.
+
 endchoice
 
 config TRACING_BACKEND_UART_NAME

subsys/tracing/tracing_backend_ram.c

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2021 IoT.bzh
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <ctype.h>
+#include <kernel.h>
+#include <string.h>
+#include <tracing_core.h>
+#include <tracing_buffer.h>
+#include <tracing_backend.h>
+
+uint8_t ram_tracing[CONFIG_RAM_TRACING_BUFFER_SIZE];
+static uint32_t pos;
+static bool buffer_full;
+
+static void tracing_backend_ram_output(
+		const struct tracing_backend *backend,
+		uint8_t *data, uint32_t length)
+{
+	if (buffer_full) {
+		return;
+	}
+
+	if ((pos + length) > CONFIG_RAM_TRACING_BUFFER_SIZE) {
+		buffer_full = true;
+		return;
+	}
+
+	memcpy(ram_tracing + pos, data, length);
+	pos += length;
+}
+
+static void tracing_backend_ram_init(void)
+{
+	memset(ram_tracing, 0, CONFIG_RAM_TRACING_BUFFER_SIZE);
+}
+
+const struct tracing_backend_api tracing_backend_ram_api = {
+	.init = tracing_backend_ram_init,
+	.output  = tracing_backend_ram_output
+};
+
+TRACING_BACKEND_DEFINE(tracing_backend_ram, tracing_backend_ram_api);

subsys/tracing/tracing_core.c

@@ -22,6 +22,8 @@
 #define TRACING_BACKEND_NAME "tracing_backend_usb"
 #elif defined CONFIG_TRACING_BACKEND_POSIX
 #define TRACING_BACKEND_NAME "tracing_backend_posix"
+#elif defined CONFIG_TRACING_BACKEND_RAM
+#define TRACING_BACKEND_NAME "tracing_backend_ram"
 #else
 #define TRACING_BACKEND_NAME ""
 #endif