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