[PATCH 5/5] printk: provide a filtering macro for printk

[Marc Andre Tanner <mat@brain-dump.org>]

The macro filters out printk messages based on a configurable verbosity
level (CONFIG_PRINTK_VERBOSITY).

Signed-off-by: Marc Andre Tanner <mat@brain-dump.org>
---
 include/linux/kernel.h |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+), 0 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index c2b3047..1ecc338 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -242,6 +242,29 @@ asmlinkage int printk(const char * fmt, ...)
 asmlinkage int printk_unfiltered(const char *fmt, ...)
 	__attribute__ ((format (printf, 1, 2))) __cold;
 
+#if defined(CONFIG_PRINTK_VERBOSITY) && CONFIG_PRINTK_VERBOSITY > 0
+/*
+ * The idea here is to wrap the actual printk function with a macro which
+ * will filter out all messages above a certain verbosity level. Because
+ * the if condition evaluates to a constant expression the compiler will be
+ * able to eliminate it and the resulting kernel image will be smaller.
+ */
+
+#include <linux/stringify.h>
+
+#define PRINTK_FILTER(fmt) (							\
+	(((const char *)(fmt))[0] != '<' && CONFIG_PRINTK_VERBOSITY >= 4) ||	\
+	(((const char *)(fmt))[0] == '<' && 					\
+	 ((const char *)(fmt))[1] <= *__stringify(CONFIG_PRINTK_VERBOSITY))	\
+)
+
+#define printk(fmt, ...) ( 							\
+	(!__builtin_constant_p(PRINTK_FILTER((fmt))) || PRINTK_FILTER((fmt))) ?	\
+		printk((fmt), ##__VA_ARGS__) : 0				\
+)
+
+#endif /* CONFIG_PRINTK_VERBOSITY */
+
 extern struct ratelimit_state printk_ratelimit_state;
 extern int printk_ratelimit(void);
 extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
-- 
1.6.3.3