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>

1 files changed, 100 insertions, 12 deletions

diff --git a/include/linux/printk.h b/include/linux/printk.h
index e061635e0409..ccea5d4a509e 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -279,39 +279,116 @@ static inline void printk_safe_flush_on_panic(void)
 
 extern int kptr_restrict;
 
+/**
+ * pr_fmt - used by the pr_*() macros to generate the printk format string
+ * @fmt: format string passed from a pr_*() macro
+ *
+ * This macro can be used to generate a unified format string for pr_*()
+ * macros. A common use is to prefix all pr_*() messages in a file with a common
+ * string. For example, defining this at the top of a source file:
+ *
+ *        #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+ *
+ * would prefix all pr_info, pr_emerg... messages in the file with the module
+ * name.
+ */
 #ifndef pr_fmt
 #define pr_fmt(fmt) fmt
 #endif
 
-/*
- * These can be used to print at the various log levels.
- * All of these will print unconditionally, although note that pr_debug()
- * and other debug macros are compiled out unless either DEBUG is defined
- * or CONFIG_DYNAMIC_DEBUG is set.
+/**
+ * pr_emerg - Print an emergency-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_EMERG loglevel. It uses pr_fmt() to
+ * generate the format string.
  */
 #define pr_emerg(fmt, ...) \
 	printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_alert - Print an alert-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_ALERT loglevel. It uses pr_fmt() to
+ * generate the format string.
+ */
 #define pr_alert(fmt, ...) \
 	printk(KERN_ALERT pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_crit - Print a critical-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_CRIT loglevel. It uses pr_fmt() to
+ * generate the format string.
+ */
 #define pr_crit(fmt, ...) \
 	printk(KERN_CRIT pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_err - Print an error-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_ERR loglevel. It uses pr_fmt() to
+ * generate the format string.
+ */
 #define pr_err(fmt, ...) \
 	printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_warn - Print a warning-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_WARNING loglevel. It uses pr_fmt()
+ * to generate the format string.
+ */
 #define pr_warn(fmt, ...) \
 	printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_notice - Print a notice-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_NOTICE loglevel. It uses pr_fmt() to
+ * generate the format string.
+ */
 #define pr_notice(fmt, ...) \
 	printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
+/**
+ * pr_info - Print an info-level message
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_INFO loglevel. It uses pr_fmt() to
+ * generate the format string.
+ */
 #define pr_info(fmt, ...) \
 	printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
-/*
- * Like KERN_CONT, pr_cont() should only be used when continuing
- * a line with no newline ('\n') enclosed. Otherwise it defaults
- * back to KERN_DEFAULT.
+
+/**
+ * pr_cont - Continues a previous log message in the same line.
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_CONT loglevel. It should only be
+ * used when continuing a log message with no newline ('\n') enclosed. Otherwise
+ * it defaults back to KERN_DEFAULT loglevel.
  */
 #define pr_cont(fmt, ...) \
 	printk(KERN_CONT fmt, ##__VA_ARGS__)
 
-/* pr_devel() should produce zero code unless DEBUG is defined */
+/**
+ * pr_devel - Print a debug-level message conditionally
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to a printk with KERN_DEBUG loglevel if DEBUG is
+ * defined. Otherwise it does nothing.
+ *
+ * It uses pr_fmt() to generate the format string.
+ */
 #ifdef DEBUG
 #define pr_devel(fmt, ...) \
 	printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
@@ -325,8 +402,19 @@ extern int kptr_restrict;
 #if defined(CONFIG_DYNAMIC_DEBUG)
 #include <linux/dynamic_debug.h>
 
-/* dynamic_pr_debug() uses pr_fmt() internally so we don't need it here */
-#define pr_debug(fmt, ...) \
+/**
+ * pr_debug - Print a debug-level message conditionally
+ * @fmt: format string
+ * @...: arguments for the format string
+ *
+ * This macro expands to dynamic_pr_debug() if CONFIG_DYNAMIC_DEBUG is
+ * set. Otherwise, if DEBUG is defined, it's equivalent to a printk with
+ * KERN_DEBUG loglevel. If DEBUG is not defined it does nothing.
+ *
+ * It uses pr_fmt() to generate the format string (dynamic_pr_debug() uses
+ * pr_fmt() internally).
+ */
+#define pr_debug(fmt, ...)			\
 	dynamic_pr_debug(fmt, ##__VA_ARGS__)
 #elif defined(DEBUG)
 #define pr_debug(fmt, ...) \