logging.rst 10 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.
  20. See enum :c:type:`log_level_t`
  21. Logging category
  22. ----------------
  23. Logging can come from a wide variety of places within U-Boot. Each log message
  24. has a category which is intended to allow messages to be filtered according to
  25. their source.
  26. See enum :c:type:`log_category_t`
  27. Enabling logging
  28. ----------------
  29. The following options are used to enable logging at compile time:
  30. * CONFIG_LOG - Enables the logging system
  31. * CONFIG_LOG_MAX_LEVEL - Max log level to build (anything higher is compiled
  32. out)
  33. * CONFIG_LOG_CONSOLE - Enable writing log records to the console
  34. If CONFIG_LOG is not set, then no logging will be available.
  35. The above have SPL and TPL versions also, e.g. CONFIG_SPL_LOG_MAX_LEVEL and
  36. CONFIG_TPL_LOG_MAX_LEVEL.
  37. Temporary logging within a single file
  38. --------------------------------------
  39. Sometimes it is useful to turn on logging just in one file. You can use this
  40. .. code-block:: c
  41. #define LOG_DEBUG
  42. to enable building in of all logging statements in a single file. Put it at
  43. the top of the file, before any #includes.
  44. To actually get U-Boot to output this you need to also set the default logging
  45. level - e.g. set CONFIG_LOG_DEFAULT_LEVEL to 7 (:c:data:`LOGL_DEBUG`) or more.
  46. Otherwise debug output is suppressed and will not be generated.
  47. Using DEBUG
  48. -----------
  49. U-Boot has traditionally used a #define called DEBUG to enable debugging on a
  50. file-by-file basis. The debug() macro compiles to a printf() statement if
  51. DEBUG is enabled, and an empty statement if not.
  52. With logging enabled, debug() statements are interpreted as logging output
  53. with a level of LOGL_DEBUG and a category of LOGC_NONE.
  54. The logging facilities are intended to replace DEBUG, but if DEBUG is defined
  55. at the top of a file, then it takes precedence. This means that debug()
  56. statements will result in output to the console and this output will not be
  57. logged.
  58. Logging statements
  59. ------------------
  60. The main logging function is:
  61. .. code-block:: c
  62. log(category, level, format_string, ...)
  63. Also debug() and error() will generate log records - these use LOG_CATEGORY
  64. as the category, so you should #define this right at the top of the source
  65. file to ensure the category is correct.
  66. Generally each log format_string ends with a newline. If it does not, then the
  67. next log statement will have the LOGRECF_CONT flag set. This can be used to
  68. continue the statement on the same line as the previous one without emitting
  69. new header information (such as category/level). This behaviour is implemented
  70. with log_console. Here is an example that prints a list all on one line with
  71. the tags at the start:
  72. .. code-block:: c
  73. log_debug("Here is a list:");
  74. for (i = 0; i < count; i++)
  75. log_debug(" item %d", i);
  76. log_debug("\n");
  77. Also see the special category LOGL_CONT and level LOGC_CONT.
  78. You can also define CONFIG_LOG_ERROR_RETURN to enable the log_ret() macro. This
  79. can be used whenever your function returns an error value:
  80. .. code-block:: c
  81. return log_ret(uclass_first_device_err(UCLASS_MMC, &dev));
  82. This will write a log record when an error code is detected (a value < 0). This
  83. can make it easier to trace errors that are generated deep in the call stack.
  84. The log_msg_ret() variant will print a short string if CONFIG_LOG_ERROR_RETURN
  85. is enabled. So long as the string is unique within the function you can normally
  86. determine exactly which call failed:
  87. .. code-block:: c
  88. ret = gpio_request_by_name(dev, "cd-gpios", 0, &desc, GPIOD_IS_IN);
  89. if (ret)
  90. return log_msg_ret("gpio", ret);
  91. Some functions return 0 for success and any other value is an error. For these,
  92. log_retz() and log_msg_retz() are available.
  93. Convenience functions
  94. ~~~~~~~~~~~~~~~~~~~~~
  95. A number of convenience functions are available to shorten the code needed
  96. for logging:
  97. * log_err(_fmt...)
  98. * log_warning(_fmt...)
  99. * log_notice(_fmt...)
  100. * log_info(_fmt...)
  101. * log_debug(_fmt...)
  102. * log_content(_fmt...)
  103. * log_io(_fmt...)
  104. With these the log level is implicit in the name. The category is set by
  105. LOG_CATEGORY, which you can only define once per file, above all #includes, e.g.
  106. .. code-block:: c
  107. #define LOG_CATEGORY LOGC_ALLOC
  108. or
  109. .. code-block:: c
  110. #define LOG_CATEGORY UCLASS_SPI
  111. Remember that all uclasses IDs are log categories too.
  112. Logging destinations
  113. --------------------
  114. If logging information goes nowhere then it serves no purpose. U-Boot provides
  115. several possible determinations for logging information, all of which can be
  116. enabled or disabled independently:
  117. * console - goes to stdout
  118. * syslog - broadcast RFC 3164 messages to syslog servers on UDP port 514
  119. The syslog driver sends the value of environmental variable 'log_hostname' as
  120. HOSTNAME if available.
  121. Filters
  122. -------
  123. Filters are attached to log drivers to control what those drivers emit. FIlters
  124. can either allow or deny a log message when they match it. Only records which
  125. are allowed by a filter make it to the driver.
  126. Filters can be based on several criteria:
  127. * minimum or maximum log level
  128. * in a set of categories
  129. * in a set of files
  130. If no filters are attached to a driver then a default filter is used, which
  131. limits output to records with a level less than CONFIG_MAX_LOG_LEVEL.
  132. Log command
  133. -----------
  134. The 'log' command provides access to several features:
  135. * level - list log levels or set the default log level
  136. * categories - list log categories
  137. * drivers - list log drivers
  138. * filter-list - list filters
  139. * filter-add - add a new filter
  140. * filter-remove - remove filters
  141. * format - access the console log format
  142. * rec - output a log record
  143. Type 'help log' for details.
  144. Log format
  145. ~~~~~~~~~~
  146. You can control the log format using the 'log format' command. The basic
  147. format is::
  148. LEVEL.category,file.c:123-func() message
  149. In the above, file.c:123 is the filename where the log record was generated and
  150. func() is the function name. By default ('log format default') only the message
  151. is displayed on the console. You can control which fields are present, but not
  152. the field order.
  153. Adding Filters
  154. ~~~~~~~~~~~~~~
  155. To add new filters at runtime, use the 'log filter-add' command. For example, to
  156. suppress messages from the SPI and MMC subsystems, run::
  157. log filter-add -D -c spi -c mmc
  158. You will also need to add another filter to allow other messages (because the
  159. default filter no longer applies)::
  160. log filter-add -A -l info
  161. Log levels may be either symbolic names (like above) or numbers. For example, to
  162. disable all debug and above (log level 7) messages from ``drivers/core/lists.c``
  163. and ``drivers/core/ofnode.c``, run::
  164. log filter-add -D -f drivers/core/lists.c,drivers/core/ofnode.c -L 7
  165. To view active filters, use the 'log filter-list' command. Some example output
  166. is::
  167. => log filter-list
  168. num policy level categories files
  169. 2 deny >= DEBUG drivers/core/lists.c,drivers/core/ofnode.c
  170. 0 deny <= IO spi
  171. mmc
  172. 1 allow <= INFO
  173. Note that filters are processed in-order from top to bottom, not in the order of
  174. their filter number. Filters are added to the top of the list if they deny when
  175. they match, and to the bottom if they allow when they match. For more
  176. information, consult the usage of the 'log' command, by running 'help log'.
  177. Code size
  178. ---------
  179. Code size impact depends largely on what is enabled. The following numbers are
  180. generated by 'buildman -S' for snow, which is a Thumb-2 board (all units in
  181. bytes)::
  182. This series: adds bss +20.0 data +4.0 rodata +4.0 text +44.0
  183. CONFIG_LOG: bss -52.0 data +92.0 rodata -635.0 text +1048.0
  184. CONFIG_LOG_MAX_LEVEL=7: bss +188.0 data +4.0 rodata +49183.0 text +98124.0
  185. The last option turns every debug() statement into a logging call, which
  186. bloats the code hugely. The advantage is that it is then possible to enable
  187. all logging within U-Boot.
  188. To Do
  189. -----
  190. There are lots of useful additions that could be made. None of the below is
  191. implemented! If you do one, please add a test in test/log/log_test.c
  192. log filter-add -D -f drivers/core/lists.c,drivers/core/ofnode.c -l 6
  193. Convenience functions to support setting the category:
  194. * log_arch(level, format_string, ...) - category LOGC_ARCH
  195. * log_board(level, format_string, ...) - category LOGC_BOARD
  196. * log_core(level, format_string, ...) - category LOGC_CORE
  197. * log_dt(level, format_string, ...) - category LOGC_DT
  198. More logging destinations:
  199. * device - goes to a device (e.g. serial)
  200. * buffer - recorded in a memory buffer
  201. Convert debug() statements in the code to log() statements
  202. Support making printf() emit log statements at L_INFO level
  203. Convert error() statements in the code to log() statements
  204. Figure out what to do with BUG(), BUG_ON() and warn_non_spl()
  205. Add a way to browse log records
  206. Add a way to record log records for browsing using an external tool
  207. Add commands to add and remove log devices
  208. Allow sharing of printf format strings in log records to reduce storage size
  209. for large numbers of log records
  210. Consider making log() calls emit an automatic newline, perhaps with a logn()
  211. function to avoid that
  212. Passing log records through to linux (e.g. via device tree /chosen)
  213. Provide a command to access the number of log records generated, and the
  214. number dropped due to them being generated before the log system was ready.
  215. Add a printf() format string pragma so that log statements are checked properly
  216. Add a command to delete existing log records.