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


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.


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


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.


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