doc/README.log

   1 Logging in U-Boot
   2 =================
   3
   4 Introduction
   5 ------------
   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
  11 Most of the time this internal detail is not useful. Displaying it on the
  12 console would delay booting (U-Boot's primary purpose) and confuse users.
  13
  14 But for digging into what is happening in a particular area, or for debugging
  15 a problem it is often useful to see what U-Boot is doing in more detail than
  16 is visible from the basic console output.
  17
  18 U-Boot's logging feature aims to satisfy this goal for both users and
  19 developers.
  20
  21
  22 Logging levels
  23 --------------
  24
  25 There are a number logging levels available, in increasing order of verbosity:
  26
  27    LOGL_EMERG   - Printed before U-Boot halts
  28    LOGL_ALERT   - Indicates action must be taken immediate or U-Boot will crash
  29    LOGL_CRIT    - Indicates a critical error that will cause boot failure
  30    LOGL_ERR     - Indicates an error that may cause boot failure
  31    LOGL_WARNING - Warning about an unexpected condition
  32    LOGL_NOTE    - Important information about progress
  33    LOGL_INFO    - Information about normal boot progress
  34    LOGL_DEBUG   - Debug information (useful for debugging a driver or subsystem)
  35    LOGL_DEBUG_CONTENT   - Debug message showing full message content
  36    LOGL_DEBUG_IO        - Debug message showing hardware I/O access
  37
  38
  39 Logging category
  40 ----------------
  41
  42 Logging can come from a wide variety of places within U-Boot. Each log message
  43 has a category which is intended to allow messages to be filtered according to
  44 their source.
  45
  46 The following main categories are defined:
  47
  48    LOGC_NONE    - Unknown category (e.g. a debug() statement)
  49    UCLASS_...   - Related to a particular uclass (e.g. UCLASS_USB)
  50    LOGC_ARCH    - Related to architecture-specific code
  51    LOGC_BOARD   - Related to board-specific code
  52    LOGC_CORE    - Related to core driver-model support
  53    LOGC_DT      - Related to device tree control
  54    LOGC_EFI     - Related to EFI implementation
  55
  56
  57 Enabling logging
  58 ----------------
  59
  60 The following options are used to enable logging at compile time:
  61
  62    CONFIG_LOG           - Enables the logging system
  63    CONFIG_MAX_LOG_LEVEL - Max log level to build (anything higher is compiled
  64                                 out)
  65    CONFIG_LOG_CONSOLE   - Enable writing log records to the console
  66
  67 If CONFIG_LOG is not set, then no logging will be available.
  68
  69 The above have SPL versions also, e.g. CONFIG_SPL_MAX_LOG_LEVEL.
  70
  71
  72 Log commands
  73 ------------
  74
  75 The 'log' command provides access to several features:
  76
  77    level - access the default log level
  78    format - access the console log format
  79    rec - output a log record
  80    test - run tests
  81
  82 Type 'help log' for details.
  83
  84
  85 Using DEBUG
  86 -----------
  87
  88 U-Boot has traditionally used a #define called DEBUG to enable debugging on a
  89 file-by-file basis. The debug() macro compiles to a printf() statement if
  90 DEBUG is enabled, and an empty statement if not.
  91
  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
  95 The logging facilities are intended to replace DEBUG, but if DEBUG is defined
  96 at the top of a file, then it takes precedence. This means that debug()
  97 statements will result in output to the console and this output will not be
  98 logged.
  99
 100
 101 Logging destinations
 102 --------------------
 103
 104 If logging information goes nowhere then it serves no purpose. U-Boot provides
 105 several possible determinations for logging information, all of which can be
 106 enabled or disabled independently:
 107
 108    console - goes to stdout
 109
 110
 111 Log format
 112 ----------
 113
 114 You can control the log format using the 'log format' command. The basic
 115 format is:
 116
 117    LEVEL.category,file.c:123-func() message
 118
 119 In the above, file.c:123 is the filename where the log record was generated and
 120 func() is the function name. By default ('log format default') only the
 121 function name and message are displayed on the console. You can control which
 122 fields are present, but not the field order.
 123
 124
 125 Filters
 126 -------
 127
 128 Filters are attached to log drivers to control what those drivers emit. Only
 129 records that pass through the filter make it to the driver.
 130
 131 Filters can be based on several criteria:
 132
 133    - maximum log level
 134    - in a set of categories
 135    - in a set of files
 136
 137 If no filters are attached to a driver then a default filter is used, which
 138 limits output to records with a level less than CONFIG_LOG_MAX_LEVEL.
 139
 140
 141 Logging statements
 142 ------------------
 143
 144 The main logging function is:
 145
 146    log(category, level, format_string, ...)
 147
 148 Also debug() and error() will generate log records  - these use LOG_CATEGORY
 149 as the category, so you should #define this right at the top of the source
 150 file to ensure the category is correct.
 151
 152 You can also define CONFIG_LOG_ERROR_RETURN to enable the log_ret() macro. This
 153 can be used whenever your function returns an error value:
 154
 155    return log_ret(uclass_first_device(UCLASS_MMC, &dev));
 156
 157 This will write a log record when an error code is detected (a value < 0). This
 158 can make it easier to trace errors that are generated deep in the call stack.
 159
 160
 161 Code size
 162 ---------
 163
 164 Code size impact depends largely on what is enabled. The following numbers are
 165 for snow, which is a Thumb-2 board:
 166
 167 This series: adds bss +20.0 data +4.0 rodata +4.0 text +44.0
 168 CONFIG_LOG: bss -52.0 data +92.0 rodata -635.0 text +1048.0
 169 CONFIG_LOG_MAX_LEVEL=7: bss +188.0 data +4.0 rodata +49183.0 text +98124.0
 170
 171 The last option turns every debug() statement into a logging call, which
 172 bloats the code hugely. The advantage is that it is then possible to enable
 173 all logging within U-Boot.
 174
 175
 176 To Do
 177 -----
 178
 179 There are lots of useful additions that could be made. None of the below is
 180 implemented! If you do one, please add a test in test/py/tests/test_log.py
 181
 182 Convenience functions to support setting the category:
 183
 184    log_arch(level, format_string, ...) - category LOGC_ARCH
 185    log_board(level, format_string, ...) - category LOGC_BOARD
 186    log_core(level, format_string, ...) - category LOGC_CORE
 187    log_dt(level, format_string, ...) - category LOGC_DT
 188
 189 Convenience functions to support a category defined for a single file, for
 190 example:
 191
 192    #define LOG_CATEGORY   UCLASS_USB
 193
 194 all of these can use LOG_CATEGORY as the category, and a log level
 195 corresponding to the function name:
 196
 197    logc(level, format_string, ...)
 198
 199 More logging destinations:
 200
 201    device - goes to a device (e.g. serial)
 202    buffer - recorded in a memory buffer
 203
 204 Convert debug() statements in the code to log() statements
 205
 206 Support making printf() emit log statements a L_INFO level
 207
 208 Convert error() statements in the code to log() statements
 209
 210 Figure out what to do with BUG(), BUG_ON() and warn_non_spl()
 211
 212 Figure out what to do with assert()
 213
 214 Add a way to browse log records
 215
 216 Add a way to record log records for browsing using an external tool
 217
 218 Add commands to add and remove filters
 219
 220 Add commands to add and remove log devices
 221
 222 Allow sharing of printf format strings in log records to reduce storage size
 223 for large numbers of log records
 224
 225 Add a command-line option to sandbox to set the default logging level
 226
 227 Convert core driver model code to use logging
 228
 229 Convert uclasses to use logging with the correct category
 230
 231 Consider making log() calls emit an automatic newline, perhaps with a logn()
 232    function to avoid that
 233
 234 Passing log records through to linux (e.g. via device tree /chosen)
 235
 236 Provide a command to access the number of log records generated, and the
 237 number dropped due to them being generated before the log system was ready.
 238
 239 Add a printf() format string pragma so that log statements are checked properly
 240
 241 Enhance the log console driver to show level / category / file / line
 242 information
 243
 244 Add a command to add new log records and delete existing records.
 245
 246 Provide additional log() functions - e.g. logc() to specify the category
 247
 248 --
 249 Simon Glass <sjg@chromium.org>
 250 15-Sep-17