logging: support custom timestamp formatting

This feature adds support for using a custom function to format
the timestamp used for all logging backends. When the kconfig
option LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP is set the custom
formatting function will always be used.

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>

include/zephyr/logging/log_output_custom.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021 Converge
+ * Copyright (c) 2023 Nobleo Technology
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -36,6 +37,63 @@ void log_custom_output_msg_process(const struct log_output *log_output,
  */
 void log_custom_output_msg_set(log_format_func_t format);
 
+
+/**
+ * @brief Prototype of a printer function that can print the given timestamp
+ * into a specific logger instance.
+ *
+ * Example usage:
+ * @code{.c}
+ * log_timestamp_printer_t *printer = ...;
+ * printer(log_instance, "%02u:%02u", hours, minutes);
+ * @endcode
+ *
+ * @param output The logger instance to write to
+ * @param fmt The format string
+ * @param ... optional arguments for the format string
+ */
+typedef int (*log_timestamp_printer_t)(const struct log_output *output, const char *fmt, ...);
+
+/**
+ * @brief Prototype of the function that will apply custom formatting
+ * to a timestamp when LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP
+ *
+ * Example function:
+ * @code{.c}
+ * int custom_timestamp_formatter(const struct log_output* output,
+ *                                const log_timestamp_t timestamp,
+ *                                const log_timestamp_printer_t printer) {
+ *     return printer(output, "%d ", timestamp);
+ * }
+ * @endcode
+ *
+ * @param output The logger instance to write to
+ * @param timestamp
+ * @param printer The printing function to use when formatting the timestamp.
+ */
+typedef int (*log_timestamp_format_func_t)(const struct log_output *output,
+					   const log_timestamp_t timestamp,
+					   const log_timestamp_printer_t printer);
+
+/** @brief Format the timestamp with a external function.
+ *
+ * Function is using provided context with the buffer and output function to
+ * process formatted string and output the data.
+ *
+ * @param output Pointer to the log output instance.
+ * @param timestamp
+ * @param printer The printing function to use when formatting the timestamp.
+ */
+int log_custom_timestamp_print(const struct log_output *output, const log_timestamp_t timestamp,
+			      const log_timestamp_printer_t printer);
+
+/** @brief Set the timestamp formatting function that will be applied
+ * when LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP
+ *
+ * @param format Pointer to the external formatter function
+ */
+void log_custom_timestamp_set(log_timestamp_format_func_t format);
+
 /**
  * @}
  */

subsys/logging/CMakeLists.txt

@@ -61,7 +61,7 @@ if(NOT CONFIG_LOG_MODE_MINIMAL)
     log_output_syst.c
   )
 
-  if(CONFIG_LOG_CUSTOM_FORMAT_SUPPORT)
+  if(CONFIG_LOG_CUSTOM_FORMAT_SUPPORT OR CONFIG_LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP)
     zephyr_sources(log_output_custom.c)
   endif()
 

subsys/logging/Kconfig.formatting

@@ -168,4 +168,10 @@ config LOG_OUTPUT_FORMAT_LINUX_TIMESTAMP
 	  which timestamps are printed as fixed point values with seconds on the
 	  left side of the point and microseconds on the right side.
 
+config LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP
+	bool "Custom timestamp format support"
+	help
+	  Enable support for custom formatter for the timestamp.
+	  It will be applied to all backends.
+
 endmenu

subsys/logging/log_output.c

@@ -6,6 +6,7 @@
 
 #include <zephyr/logging/log_output.h>
 #include <zephyr/logging/log_ctrl.h>
+#include <zephyr/logging/log_output_custom.h>
 #include <zephyr/logging/log.h>
 #include <zephyr/sys/__assert.h>
 #include <zephyr/sys/cbprintf.h>
@@ -215,7 +216,8 @@ static int timestamp_print(const struct log_output *output,
 	bool format =
 		(flags & LOG_OUTPUT_FLAG_FORMAT_TIMESTAMP) |
 		(flags & LOG_OUTPUT_FLAG_FORMAT_SYSLOG) |
-		IS_ENABLED(CONFIG_LOG_OUTPUT_FORMAT_LINUX_TIMESTAMP);
+		IS_ENABLED(CONFIG_LOG_OUTPUT_FORMAT_LINUX_TIMESTAMP) |
+		IS_ENABLED(CONFIG_LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP);
 
 
 	if (!format) {
@@ -249,8 +251,10 @@ static int timestamp_print(const struct log_output *output,
 		ms = (remainder * 1000U) / freq;
 		us = (1000 * (remainder * 1000U - (ms * freq))) / freq;
 
-		if (IS_ENABLED(CONFIG_LOG_BACKEND_NET) &&
+		if (IS_ENABLED(CONFIG_LOG_OUTPUT_FORMAT_CUSTOM_TIMESTAMP)) {
-		    flags & LOG_OUTPUT_FLAG_FORMAT_SYSLOG) {
+			length = log_custom_timestamp_print(output, timestamp, print_formatted);
+		} else if (IS_ENABLED(CONFIG_LOG_BACKEND_NET) &&
+			   flags & LOG_OUTPUT_FLAG_FORMAT_SYSLOG) {
 #if defined(CONFIG_NEWLIB_LIBC)
 			char time_str[sizeof("1970-01-01T00:00:00")];
 			struct tm *tm;

subsys/logging/log_output_custom.c

@@ -1,10 +1,12 @@
 /*
  * Copyright (c) 2022 Converge
+ * Copyright (c) 2023 Nobleo Technology
  *
  * SPDX-License-Identifier: Apache-2.0
  */
 #include <zephyr/logging/log.h>
 #include <zephyr/logging/log_output.h>
+#include <zephyr/logging/log_output_custom.h>
 
 static log_format_func_t log_custom_format_func;
 
@@ -20,3 +22,22 @@ void log_custom_output_msg_set(log_format_func_t format)
 {
 	log_custom_format_func = format;
 }
+
+static log_timestamp_format_func_t log_timestamp_format_func;
+
+int log_custom_timestamp_print(const struct log_output *output, const log_timestamp_t timestamp,
+			      const log_timestamp_printer_t printer)
+{
+	__ASSERT(log_timestamp_format_func != NULL, "custom timestamp format function not set");
+
+	if (log_timestamp_format_func) {
+		return log_timestamp_format_func(output, timestamp, printer);
+	}
+
+	return 0;
+}
+
+void log_custom_timestamp_set(log_timestamp_format_func_t format)
+{
+	log_timestamp_format_func = format;
+}