--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (c) 2017 Simon Glass <sjg@chromium.org>
+
+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_LOG_MAX_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 and TPL versions also, e.g. CONFIG_SPL_LOG_MAX_LEVEL and
+CONFIG_TPL_LOG_MAX_LEVEL.
+
+
+Temporary logging within a single file
+--------------------------------------
+
+Sometimes it is useful to turn on logging just in one file. You can use this
+
+.. code-block:: c
+
+ #define LOG_DEBUG
+
+to enable building in of all logging statements in a single file. Put it at
+the top of the file, before any #includes.
+
+To actually get U-Boot to output this you need to also set the default logging
+level - e.g. set CONFIG_LOG_DEFAULT_LEVEL to 7 (LOGL_DEBUG) or more. Otherwise
+debug output is suppressed and will not be generated.
+
+
+Convenience functions
+---------------------
+
+A number of convenience functions are available to shorten the code needed
+for logging:
+
+* log_err(_fmt...)
+* log_warning(_fmt...)
+* log_notice(_fmt...)
+* log_info(_fmt...)
+* log_debug(_fmt...)
+* log_content(_fmt...)
+* log_io(_fmt...)
+
+With these the log level is implicit in the name. The category is set by
+LOG_CATEGORY, which you can only define once per file, above all #includes, e.g.
+
+.. code-block:: c
+
+ #define LOG_CATEGORY LOGC_ALLOC
+
+Remember that all uclasses IDs are log categories too.
+
+
+Log command
+-----------
+
+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
+* syslog - broadcast RFC 3164 messages to syslog servers on UDP port 514
+
+The syslog driver sends the value of environmental variable 'log_hostname' as
+HOSTNAME if available.
+
+
+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_MAX_LOG_LEVEL.
+
+
+Logging statements
+------------------
+
+The main logging function is:
+
+.. code-block:: c
+
+ 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:
+
+.. code-block:: c
+
+ 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
+generated by 'buildman -S' for snow, which is a Thumb-2 board (all units in
+bytes)::
+
+ 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
+
+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 at 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
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