tracing: add __print_flags for events

Developers have been asking for the ability in the ftrace event tracer
to display names of bits in a flags variable.

Instead of printing out c2, it would be easier to read FOO|BAR|GOO,
assuming that FOO is bit 1, BAR is bit 6 and GOO is bit 7.

Some examples where this would be useful are the state flags in a context
switch, kmalloc flags, and even permision flags in accessing files.

[
  v2 changes include:

  Frederic Weisbecker's idea of using a mask instead of bits,
  thus we can output GFP_KERNEL instead of GPF_WAIT|GFP_IO|GFP_FS.

  Li Zefan's idea of allowing the caller of __print_flags to add their
  own delimiter (or no delimiter) where we can get for file permissions
  rwx instead of r|w|x.
]

[
  v3 changes:

   Christoph Hellwig's idea of using an array instead of va_args.
]

[ Impact: better displaying of flags in trace output ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index bae51ddfabd..4b58cf1a11c 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -3,12 +3,23 @@
 
 #include <linux/trace_seq.h>
 #include <linux/ring_buffer.h>
-
+#include <linux/percpu.h>
 
 struct trace_array;
 struct tracer;
 struct dentry;
 
+DECLARE_PER_CPU(struct trace_seq, ftrace_event_seq);
+
+struct trace_print_flags {
+	unsigned long		mask;
+	const char		*name;
+};
+
+const char *ftrace_print_flags_seq(struct trace_seq *p, const char *delim,
+				   unsigned long flags,
+				   const struct trace_print_flags *flag_array);
+
 /*
  * The trace entry - the most basic unit of tracing. This is what
  * is printed in the end as a single line in the trace output, such as: