Merge tag 'v3.10.101' into update

[GitHub/mt8127/android_kernel_alcatel_ttab.git] / include / linux / tracepoint.h

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 36e5e9998865182f48db41277751426a731f9eb2..fe6f0b3fe49c75cc56e3560bbcee18325b55ae43 100644 (file)
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -247,6 +247,50 @@ static inline void tracepoint_synchronize_unregister(void)
 
 #endif /* CONFIG_TRACEPOINTS */
 
+#ifdef CONFIG_TRACING
+/**
+ * tracepoint_string - register constant persistent string to trace system
+ * @str - a constant persistent string that will be referenced in tracepoints
+ *
+ * If constant strings are being used in tracepoints, it is faster and
+ * more efficient to just save the pointer to the string and reference
+ * that with a printf "%s" instead of saving the string in the ring buffer
+ * and wasting space and time.
+ *
+ * The problem with the above approach is that userspace tools that read
+ * the binary output of the trace buffers do not have access to the string.
+ * Instead they just show the address of the string which is not very
+ * useful to users.
+ *
+ * With tracepoint_string(), the string will be registered to the tracing
+ * system and exported to userspace via the debugfs/tracing/printk_formats
+ * file that maps the string address to the string text. This way userspace
+ * tools that read the binary buffers have a way to map the pointers to
+ * the ASCII strings they represent.
+ *
+ * The @str used must be a constant string and persistent as it would not
+ * make sense to show a string that no longer exists. But it is still fine
+ * to be used with modules, because when modules are unloaded, if they
+ * had tracepoints, the ring buffers are cleared too. As long as the string
+ * does not change during the life of the module, it is fine to use
+ * tracepoint_string() within a module.
+ */
+#define tracepoint_string(str)                                         \
+       ({                                                              \
+               static const char *___tp_str __tracepoint_string = str; \
+               ___tp_str;                                              \
+       })
+#define __tracepoint_string    __attribute__((section("__tracepoint_str")))
+#else
+/*
+ * tracepoint_string() is used to save the string address for userspace
+ * tracing tools. When tracing isn't configured, there's no need to save
+ * anything.
+ */
+# define tracepoint_string(str) str
+# define __tracepoint_string
+#endif
+
 /*
  * The need for the DECLARE_TRACE_NOARGS() is to handle the prototype
  * (void). "void" is a special value in a function prototype and can