printk: add Kconfig option to set default console loglevel

Add a configuration option to set the default console loglevel.  This
is, as before, still possible to override at runtime through bootargs
(loglevel=<x>), sysrq and /proc/printk.

There are cases where adding additional arguments on the commandline is
impractical, and changing the default for the kernel when being built
makes more sense.  Provide such a method here, for those who choose to
do so.

Also, while touching this code, clarify the difference between
MESSAGE_LOGLEVEL_DEFAULT and CONSOLE_LOGLEVEL_DEFAULT.

Link: http://lkml.kernel.org/r/1479676829-30031-1-git-send-email-olof@lixom.net
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/printk.h

@@ -50,10 +50,15 @@ static inline const char *printk_skip_headers(const char *buffer)
 #define CONSOLE_LOGLEVEL_SILENT  0 /* Mum's the word */
 #define CONSOLE_LOGLEVEL_MIN	 1 /* Minimum loglevel we let people use */
 #define CONSOLE_LOGLEVEL_QUIET	 4 /* Shhh ..., when booted with "quiet" */
-#define CONSOLE_LOGLEVEL_DEFAULT 7 /* anything MORE serious than KERN_DEBUG */
 #define CONSOLE_LOGLEVEL_DEBUG	10 /* issue debug messages */
 #define CONSOLE_LOGLEVEL_MOTORMOUTH 15	/* You can't shut this one up */
 
+/*
+ * Default used to be hard-coded at 7, we're now allowing it to be set from
+ * kernel config.
+ */
+#define CONSOLE_LOGLEVEL_DEFAULT CONFIG_CONSOLE_LOGLEVEL_DEFAULT
+
 extern int console_printk[];
 
 #define console_loglevel (console_printk[0])

lib/Kconfig.debug

@@ -15,6 +15,21 @@ config PRINTK_TIME
 	  The behavior is also controlled by the kernel command line
 	  parameter printk.time=1. See Documentation/kernel-parameters.txt
 
+config CONSOLE_LOGLEVEL_DEFAULT
+	int "Default console loglevel (1-15)"
+	range 1 15
+	default "7"
+	help
+	  Default loglevel to determine what will be printed on the console.
+
+	  Setting a default here is equivalent to passing in loglevel=<x> in
+	  the kernel bootargs. loglevel=<x> continues to override whatever
+	  value is specified here as well.
+
+	  Note: This does not affect the log level of un-prefixed prink()
+	  usage in the kernel. That is controlled by the MESSAGE_LOGLEVEL_DEFAULT
+	  option.
+
 config MESSAGE_LOGLEVEL_DEFAULT
 	int "Default message log level (1-7)"
 	range 1 7
@@ -26,6 +41,10 @@ config MESSAGE_LOGLEVEL_DEFAULT
 	  that are auditing their logs closely may want to set it to a lower
 	  priority.
 
+	  Note: This does not affect what message level gets printed on the console
+	  by default. To change that, use loglevel=<x> in the kernel bootargs,
+	  or pick a different CONSOLE_LOGLEVEL_DEFAULT configuration value.
+
 config BOOT_PRINTK_DELAY
 	bool "Delay each boot printk message by N milliseconds"
 	depends on DEBUG_KERNEL && PRINTK && GENERIC_CALIBRATE_DELAY