logging.rst 8.8 KB

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  1. .. SPDX-License-Identifier: GPL-2.0+
  2. .. Copyright (c) 2017 Simon Glass <sjg@chromium.org>
  3. Logging in U-Boot
  4. =================
  5. Introduction
  6. ------------
  7. U-Boot's internal operation involves many different steps and actions. From
  8. setting up the board to displaying a start-up screen to loading an Operating
  9. System, there are many component parts each with many actions.
  10. Most of the time this internal detail is not useful. Displaying it on the
  11. console would delay booting (U-Boot's primary purpose) and confuse users.
  12. But for digging into what is happening in a particular area, or for debugging
  13. a problem it is often useful to see what U-Boot is doing in more detail than
  14. is visible from the basic console output.
  15. U-Boot's logging feature aims to satisfy this goal for both users and
  16. developers.
  17. Logging levels
  18. --------------
  19. There are a number logging levels available, in increasing order of verbosity:
  20. * LOGL_EMERG - Printed before U-Boot halts
  21. * LOGL_ALERT - Indicates action must be taken immediate or U-Boot will crash
  22. * LOGL_CRIT - Indicates a critical error that will cause boot failure
  23. * LOGL_ERR - Indicates an error that may cause boot failure
  24. * LOGL_WARNING - Warning about an unexpected condition
  25. * LOGL_NOTE - Important information about progress
  26. * LOGL_INFO - Information about normal boot progress
  27. * LOGL_DEBUG - Debug information (useful for debugging a driver or subsystem)
  28. * LOGL_DEBUG_CONTENT - Debug message showing full message content
  29. * LOGL_DEBUG_IO - Debug message showing hardware I/O access
  30. Logging category
  31. ----------------
  32. Logging can come from a wide variety of places within U-Boot. Each log message
  33. has a category which is intended to allow messages to be filtered according to
  34. their source.
  35. The following main categories are defined:
  36. * LOGC_NONE - Unknown category (e.g. a debug() statement)
  37. * UCLASS\_... - Related to a particular uclass (e.g. UCLASS_USB)
  38. * LOGC_ARCH - Related to architecture-specific code
  39. * LOGC_BOARD - Related to board-specific code
  40. * LOGC_CORE - Related to core driver-model support
  41. * LOGC_DT - Related to device tree control
  42. * LOGC_EFI - Related to EFI implementation
  43. Enabling logging
  44. ----------------
  45. The following options are used to enable logging at compile time:
  46. * CONFIG_LOG - Enables the logging system
  47. * CONFIG_LOG_MAX_LEVEL - Max log level to build (anything higher is compiled
  48. out)
  49. * CONFIG_LOG_CONSOLE - Enable writing log records to the console
  50. If CONFIG_LOG is not set, then no logging will be available.
  51. The above have SPL and TPL versions also, e.g. CONFIG_SPL_LOG_MAX_LEVEL and
  52. CONFIG_TPL_LOG_MAX_LEVEL.
  53. Temporary logging within a single file
  54. --------------------------------------
  55. Sometimes it is useful to turn on logging just in one file. You can use this
  56. .. code-block:: c
  57. #define LOG_DEBUG
  58. to enable building in of all logging statements in a single file. Put it at
  59. the top of the file, before any #includes.
  60. To actually get U-Boot to output this you need to also set the default logging
  61. level - e.g. set CONFIG_LOG_DEFAULT_LEVEL to 7 (LOGL_DEBUG) or more. Otherwise
  62. debug output is suppressed and will not be generated.
  63. Convenience functions
  64. ---------------------
  65. A number of convenience functions are available to shorten the code needed
  66. for logging:
  67. * log_err(_fmt...)
  68. * log_warning(_fmt...)
  69. * log_notice(_fmt...)
  70. * log_info(_fmt...)
  71. * log_debug(_fmt...)
  72. * log_content(_fmt...)
  73. * log_io(_fmt...)
  74. With these the log level is implicit in the name. The category is set by
  75. LOG_CATEGORY, which you can only define once per file, above all #includes, e.g.
  76. .. code-block:: c
  77. #define LOG_CATEGORY LOGC_ALLOC
  78. Remember that all uclasses IDs are log categories too.
  79. Log command
  80. -----------
  81. The 'log' command provides access to several features:
  82. * level - access the default log level
  83. * format - access the console log format
  84. * rec - output a log record
  85. * test - run tests
  86. Type 'help log' for details.
  87. Using DEBUG
  88. -----------
  89. U-Boot has traditionally used a #define called DEBUG to enable debugging on a
  90. file-by-file basis. The debug() macro compiles to a printf() statement if
  91. DEBUG is enabled, and an empty statement if not.
  92. With logging enabled, debug() statements are interpreted as logging output
  93. with a level of LOGL_DEBUG and a category of LOGC_NONE.
  94. The logging facilities are intended to replace DEBUG, but if DEBUG is defined
  95. at the top of a file, then it takes precedence. This means that debug()
  96. statements will result in output to the console and this output will not be
  97. logged.
  98. Logging destinations
  99. --------------------
  100. If logging information goes nowhere then it serves no purpose. U-Boot provides
  101. several possible determinations for logging information, all of which can be
  102. enabled or disabled independently:
  103. * console - goes to stdout
  104. * syslog - broadcast RFC 3164 messages to syslog servers on UDP port 514
  105. The syslog driver sends the value of environmental variable 'log_hostname' as
  106. HOSTNAME if available.
  107. Log format
  108. ----------
  109. You can control the log format using the 'log format' command. The basic
  110. format is::
  111. LEVEL.category,file.c:123-func() message
  112. In the above, file.c:123 is the filename where the log record was generated and
  113. func() is the function name. By default ('log format default') only the
  114. function name and message are displayed on the console. You can control which
  115. fields are present, but not the field order.
  116. Filters
  117. -------
  118. Filters are attached to log drivers to control what those drivers emit. Only
  119. records that pass through the filter make it to the driver.
  120. Filters can be based on several criteria:
  121. * maximum log level
  122. * in a set of categories
  123. * in a set of files
  124. If no filters are attached to a driver then a default filter is used, which
  125. limits output to records with a level less than CONFIG_MAX_LOG_LEVEL.
  126. Logging statements
  127. ------------------
  128. The main logging function is:
  129. .. code-block:: c
  130. log(category, level, format_string, ...)
  131. Also debug() and error() will generate log records - these use LOG_CATEGORY
  132. as the category, so you should #define this right at the top of the source
  133. file to ensure the category is correct.
  134. You can also define CONFIG_LOG_ERROR_RETURN to enable the log_ret() macro. This
  135. can be used whenever your function returns an error value:
  136. .. code-block:: c
  137. return log_ret(uclass_first_device(UCLASS_MMC, &dev));
  138. This will write a log record when an error code is detected (a value < 0). This
  139. can make it easier to trace errors that are generated deep in the call stack.
  140. Code size
  141. ---------
  142. Code size impact depends largely on what is enabled. The following numbers are
  143. generated by 'buildman -S' for snow, which is a Thumb-2 board (all units in
  144. bytes)::
  145. This series: adds bss +20.0 data +4.0 rodata +4.0 text +44.0
  146. CONFIG_LOG: bss -52.0 data +92.0 rodata -635.0 text +1048.0
  147. CONFIG_LOG_MAX_LEVEL=7: bss +188.0 data +4.0 rodata +49183.0 text +98124.0
  148. The last option turns every debug() statement into a logging call, which
  149. bloats the code hugely. The advantage is that it is then possible to enable
  150. all logging within U-Boot.
  151. To Do
  152. -----
  153. There are lots of useful additions that could be made. None of the below is
  154. implemented! If you do one, please add a test in test/py/tests/test_log.py
  155. Convenience functions to support setting the category:
  156. * log_arch(level, format_string, ...) - category LOGC_ARCH
  157. * log_board(level, format_string, ...) - category LOGC_BOARD
  158. * log_core(level, format_string, ...) - category LOGC_CORE
  159. * log_dt(level, format_string, ...) - category LOGC_DT
  160. More logging destinations:
  161. * device - goes to a device (e.g. serial)
  162. * buffer - recorded in a memory buffer
  163. Convert debug() statements in the code to log() statements
  164. Support making printf() emit log statements at L_INFO level
  165. Convert error() statements in the code to log() statements
  166. Figure out what to do with BUG(), BUG_ON() and warn_non_spl()
  167. Figure out what to do with assert()
  168. Add a way to browse log records
  169. Add a way to record log records for browsing using an external tool
  170. Add commands to add and remove filters
  171. Add commands to add and remove log devices
  172. Allow sharing of printf format strings in log records to reduce storage size
  173. for large numbers of log records
  174. Add a command-line option to sandbox to set the default logging level
  175. Convert core driver model code to use logging
  176. Convert uclasses to use logging with the correct category
  177. Consider making log() calls emit an automatic newline, perhaps with a logn()
  178. function to avoid that
  179. Passing log records through to linux (e.g. via device tree /chosen)
  180. Provide a command to access the number of log records generated, and the
  181. number dropped due to them being generated before the log system was ready.
  182. Add a printf() format string pragma so that log statements are checked properly
  183. Enhance the log console driver to show level / category / file / line
  184. information
  185. Add a command to add new log records and delete existing records.
  186. Provide additional log() functions - e.g. logc() to specify the category