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