README.log 8.7 KB

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