printk: introduce printk_once()

This pattern shows up frequently in the kernel:

  static int once = 1;
  ...

		if (once) {
			once = 0;
			printk(KERN_ERR "message\n");
		}
  ...

So add a printk_once() helper macro that reduces this to a single line
of:

		printk_once(KERN_ERR "message\n");

It works analogously to WARN_ONCE() & friends. (We use a macro not
an inline because vararg expansion in inlines looks awkward and the
macro is simple enough.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 343df9ef2412..3c183d9864ae 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -242,6 +242,19 @@ extern struct ratelimit_state printk_ratelimit_state;
 extern int printk_ratelimit(void);
 extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
                                    unsigned int interval_msec);
+
+/*
+ * Print a one-time message (analogous to WARN_ONCE() et al):
+ */
+#define printk_once(x...) ({                    \
+        static int __print_once = 1;            \
+                                                \
+        if (__print_once) {                     \
+                __print_once = 0;               \
+                printk(x);                      \
+        }                                       \
+})
+
 #else
 static inline int vprintk(const char *s, va_list args)
         __attribute__ ((format (printf, 1, 0)));
@@ -253,6 +266,10 @@ static inline int printk_ratelimit(void) { return 0; }
 static inline bool printk_timed_ratelimit(unsigned long *caller_jiffies, \
                                           unsigned int interval_msec)   \
                 { return false; }
+
+/* No effect, but we still get type checking even in the !PRINTK case: */
+#define printk_once(x...) printk(x)
+
 #endif
 
 extern int printk_needs_cpu(int cpu);