docs: pr_*() kerneldocs and basic printk docs

Add kerneldocs comments to the pr_*() macros in printk.h.

Add a new rst node in the core-api manual describing the basic usage of
printk and the related macro aliases.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Link: https://lore.kernel.org/r/20200403093617.18003-1-ricardo.canuelo@collabora.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -18,6 +18,7 @@ it.
 
    kernel-api
    workqueue
+   printk-basics
    printk-formats
    symbol-namespaces
 

Documentation/core-api/printk-basics.rst

@@ -0,0 +1,115 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+Message logging with printk
+===========================
+
+printk() is one of the most widely known functions in the Linux kernel. It's the
+standard tool we have for printing messages and usually the most basic way of
+tracing and debugging. If you're familiar with printf(3) you can tell printk()
+is based on it, although it has some functional differences:
+
+  - printk() messages can specify a log level.
+
+  - the format string, while largely compatible with C99, doesn't follow the
+    exact same specification. It has some extensions and a few limitations
+    (no ``%n`` or floating point conversion specifiers). See :ref:`How to get
+    printk format specifiers right <printk-specifiers>`.
+
+All printk() messages are printed to the kernel log buffer, which is a ring
+buffer exported to userspace through /dev/kmsg. The usual way to read it is
+using ``dmesg``.
+
+printk() is typically used like this::
+
+  printk(KERN_INFO "Message: %s\n", arg);
+
+where ``KERN_INFO`` is the log level (note that it's concatenated to the format
+string, the log level is not a separate argument). The available log levels are:
+
++----------------+--------+-----------------------------------------------+
+| Name           | String |  Alias function                               |
++================+========+===============================================+
+| KERN_EMERG     | "0"    | pr_emerg()                                    |
++----------------+--------+-----------------------------------------------+
+| KERN_ALERT     | "1"    | pr_alert()                                    |
++----------------+--------+-----------------------------------------------+
+| KERN_CRIT      | "2"    | pr_crit()                                     |
++----------------+--------+-----------------------------------------------+
+| KERN_ERR       | "3"    | pr_err()                                      |
++----------------+--------+-----------------------------------------------+
+| KERN_WARNING   | "4"    | pr_warn()                                     |
++----------------+--------+-----------------------------------------------+
+| KERN_NOTICE    | "5"    | pr_notice()                                   |
++----------------+--------+-----------------------------------------------+
+| KERN_INFO      | "6"    | pr_info()                                     |
++----------------+--------+-----------------------------------------------+
+| KERN_DEBUG     | "7"    | pr_debug() and pr_devel() if DEBUG is defined |
++----------------+--------+-----------------------------------------------+
+| KERN_DEFAULT   | ""     |                                               |
++----------------+--------+-----------------------------------------------+
+| KERN_CONT      | "c"    | pr_cont()                                     |
++----------------+--------+-----------------------------------------------+
+
+
+The log level specifies the importance of a message. The kernel decides whether
+to show the message immediately (printing it to the current console) depending
+on its log level and the current *console_loglevel* (a kernel variable). If the
+message priority is higher (lower log level value) than the *console_loglevel*
+the message will be printed to the console.
+
+If the log level is omitted, the message is printed with ``KERN_DEFAULT``
+level.
+
+You can check the current *console_loglevel* with::
+
+  $ cat /proc/sys/kernel/printk
+  4        4        1        7
+
+The result shows the *current*, *default*, *minimum* and *boot-time-default* log
+levels.
+
+To change the current console_loglevel simply write the the desired level to
+``/proc/sys/kernel/printk``. For example, to print all messages to the console::
+
+  # echo 8 > /proc/sys/kernel/printk
+
+Another way, using ``dmesg``::
+
+  # dmesg -n 5
+
+sets the console_loglevel to print KERN_WARNING (4) or more severe messages to
+console. See ``dmesg(1)`` for more information.
+
+As an alternative to printk() you can use the ``pr_*()`` aliases for
+logging. This family of macros embed the log level in the macro names. For
+example::
+
+  pr_info("Info message no. %d\n", msg_num);
+
+prints a ``KERN_INFO`` message.
+
+Besides being more concise than the equivalent printk() calls, they can use a
+common definition for the format string through the pr_fmt() macro. For
+instance, defining this at the top of a source file (before any ``#include``
+directive)::
+
+  #define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__
+
+would prefix every pr_*() message in that file with the module and function name
+that originated the message.
+
+For debugging purposes there are also two conditionally-compiled macros:
+pr_debug() and pr_devel(), which are compiled-out unless ``DEBUG`` (or
+also ``CONFIG_DYNAMIC_DEBUG`` in the case of pr_debug()) is defined.
+
+
+Function reference
+==================
+
+.. kernel-doc:: kernel/printk/printk.c
+   :functions: printk
+
+.. kernel-doc:: include/linux/printk.h
+   :functions: pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info
+      pr_fmt pr_debug pr_devel pr_cont

Documentation/core-api/printk-formats.rst

@@ -2,6 +2,8 @@
 How to get printk format specifiers right
 =========================================
 
+.. _printk-specifiers:
+
 :Author: Randy Dunlap <rdunlap@infradead.org>
 :Author: Andrew Murray <amurray@mpc-data.co.uk>