Add documentation for the log system.
Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Bin Meng bmeng.cn@gmail.com ---
Changes in v2: - Drop the special log() functions from the README
doc/README.log | 214 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 214 insertions(+) create mode 100644 doc/README.log
diff --git a/doc/README.log b/doc/README.log new file mode 100644 index 0000000000..f653fe7d79 --- /dev/null +++ b/doc/README.log @@ -0,0 +1,214 @@ +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 + + +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. + + +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 + + +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. + + +Code size +--------- + +Code size impact depends largely on what is enabled. The following numbers are +for snow, which is a Thumb-2 board: + +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 sjg@chromium.org +15-Sep-17