Logging in U-Boot ================= Introduction ------------ U-Boot's internal operation involves many different steps and actions. From setting up the board to displaying a start-up screen to loading an Operating System, there are many component parts each with many actions. Most of the time this internal detail is not useful. Displaying it on the console would delay booting (U-Boot's primary purpose) and confuse users. But for digging into what is happening in a particular area, or for debugging a problem it is often useful to see what U-Boot is doing in more detail than is visible from the basic console output. U-Boot's logging feature aims to satisfy this goal for both users and developers. Logging levels -------------- There are a number logging levels available, in increasing order of verbosity: LOGL_EMERG - Printed before U-Boot halts LOGL_ALERT - Indicates action must be taken immediate or U-Boot will crash LOGL_CRIT - Indicates a critical error that will cause boot failure LOGL_ERR - Indicates an error that may cause boot failure LOGL_WARNING - Warning about an unexpected condition LOGL_NOTE - Important information about progress LOGL_INFO - Information about normal boot progress LOGL_DEBUG - Debug information (useful for debugging a driver or subsystem) LOGL_DEBUG_CONTENT - Debug message showing full message content LOGL_DEBUG_IO - Debug message showing hardware I/O access Logging category ---------------- Logging can come from a wide variety of places within U-Boot. Each log message has a category which is intended to allow messages to be filtered according to their source. The following main categories are defined: LOGC_NONE - Unknown category (e.g. a debug() statement) UCLASS_... - Related to a particular uclass (e.g. UCLASS_USB) LOGC_ARCH - Related to architecture-specific code LOGC_BOARD - Related to board-specific code LOGC_CORE - Related to core driver-model support LOGC_DT - Related to device tree control LOGC_EFI - Related to EFI implementation Enabling logging ---------------- The following options are used to enable logging at compile time: CONFIG_LOG - Enables the logging system CONFIG_MAX_LOG_LEVEL - Max log level to build (anything higher is compiled out) CONFIG_LOG_CONSOLE - Enable writing log records to the console If CONFIG_LOG is not set, then no logging will be available. The above have SPL versions also, e.g. CONFIG_SPL_MAX_LOG_LEVEL. Log commands ------------ The 'log' command provides access to several features: level - access the default log level format - access the console log format rec - output a log record test - run tests Type 'help log' for details. Using DEBUG ----------- U-Boot has traditionally used a #define called DEBUG to enable debugging on a file-by-file basis. The debug() macro compiles to a printf() statement if DEBUG is enabled, and an empty statement if not. With logging enabled, debug() statements are interpreted as logging output with a level of LOGL_DEBUG and a category of LOGC_NONE. The logging facilities are intended to replace DEBUG, but if DEBUG is defined at the top of a file, then it takes precedence. This means that debug() statements will result in output to the console and this output will not be logged. Logging destinations -------------------- If logging information goes nowhere then it serves no purpose. U-Boot provides several possible determinations for logging information, all of which can be enabled or disabled independently: console - goes to stdout Log format ---------- You can control the log format using the 'log format' command. The basic format is: LEVEL.category,file.c:123-func() message In the above, file.c:123 is the filename where the log record was generated and func() is the function name. By default ('log format default') only the function name and message are displayed on the console. You can control which fields are present, but not the field order. Filters ------- Filters are attached to log drivers to control what those drivers emit. Only records that pass through the filter make it to the driver. Filters can be based on several criteria: - maximum log level - in a set of categories - in a set of files If no filters are attached to a driver then a default filter is used, which limits output to records with a level less than CONFIG_LOG_MAX_LEVEL. Logging statements ------------------ The main logging function is: log(category, level, format_string, ...) Also debug() and error() will generate log records - these use LOG_CATEGORY as the category, so you should #define this right at the top of the source file to ensure the category is correct. You can also define CONFIG_LOG_ERROR_RETURN to enable the log_ret() macro. This can be used whenever your function returns an error value: return log_ret(uclass_first_device(UCLASS_MMC, &dev)); This will write a log record when an error code is detected (a value < 0). This can make it easier to trace errors that are generated deep in the call stack. Code size --------- Code size impact depends largely on what is enabled. The following numbers are generated by 'buildman -S' for snow, which is a Thumb-2 board (all units in bytes): This series: adds bss +20.0 data +4.0 rodata +4.0 text +44.0 CONFIG_LOG: bss -52.0 data +92.0 rodata -635.0 text +1048.0 CONFIG_LOG_MAX_LEVEL=7: bss +188.0 data +4.0 rodata +49183.0 text +98124.0 The last option turns every debug() statement into a logging call, which bloats the code hugely. The advantage is that it is then possible to enable all logging within U-Boot. To Do ----- There are lots of useful additions that could be made. None of the below is implemented! If you do one, please add a test in test/py/tests/test_log.py Convenience functions to support setting the category: log_arch(level, format_string, ...) - category LOGC_ARCH log_board(level, format_string, ...) - category LOGC_BOARD log_core(level, format_string, ...) - category LOGC_CORE log_dt(level, format_string, ...) - category LOGC_DT Convenience functions to support a category defined for a single file, for example: #define LOG_CATEGORY UCLASS_USB all of these can use LOG_CATEGORY as the category, and a log level corresponding to the function name: logc(level, format_string, ...) More logging destinations: device - goes to a device (e.g. serial) buffer - recorded in a memory buffer Convert debug() statements in the code to log() statements Support making printf() emit log statements a L_INFO level Convert error() statements in the code to log() statements Figure out what to do with BUG(), BUG_ON() and warn_non_spl() Figure out what to do with assert() Add a way to browse log records Add a way to record log records for browsing using an external tool Add commands to add and remove filters Add commands to add and remove log devices Allow sharing of printf format strings in log records to reduce storage size for large numbers of log records Add a command-line option to sandbox to set the default logging level Convert core driver model code to use logging Convert uclasses to use logging with the correct category Consider making log() calls emit an automatic newline, perhaps with a logn() function to avoid that Passing log records through to linux (e.g. via device tree /chosen) Provide a command to access the number of log records generated, and the number dropped due to them being generated before the log system was ready. Add a printf() format string pragma so that log statements are checked properly Enhance the log console driver to show level / category / file / line information Add a command to add new log records and delete existing records. Provide additional log() functions - e.g. logc() to specify the category -- Simon Glass 15-Sep-17