tracing: Add synthetic event command generation functions

Add functions used to generate synthetic event commands, built on top
of the dynevent_cmd interface.

synth_event_gen_cmd_start() is used to create a synthetic event
command using a variable arg list and
synth_event_gen_cmd_array_start() does the same thing but using an
array of field descriptors.  synth_event_add_field(),
synth_event_add_field_str() and synth_event_add_fields() can be used
to add single fields one by one or as a group.  Once all desired
fields are added, synth_event_gen_cmd_end() is used to actually
execute the command and create the event.

synth_event_create() does everything, including creating the event, in
a single call.

Link: http://lkml.kernel.org/r/38fef702fad5ef208009f459552f34a94befd860.1580323897.git.zanussi@kernel.org

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/trace_events.h

@@ -357,6 +357,7 @@ extern void trace_put_event_file(struct trace_event_file *file);
 #define MAX_DYNEVENT_CMD_LEN	(2048)
 
 enum dynevent_type {
+	DYNEVENT_TYPE_SYNTH = 1,
 	DYNEVENT_TYPE_NONE,
 };
 
@@ -379,6 +380,42 @@ extern int dynevent_create(struct dynevent_cmd *cmd);
 
 extern int synth_event_delete(const char *name);
 
+extern void synth_event_cmd_init(struct dynevent_cmd *cmd,
+				 char *buf, int maxlen);
+
+extern int __synth_event_gen_cmd_start(struct dynevent_cmd *cmd,
+				       const char *name,
+				       struct module *mod, ...);
+
+#define synth_event_gen_cmd_start(cmd, name, mod, ...)	\
+	__synth_event_gen_cmd_start(cmd, name, mod, ## __VA_ARGS__, NULL)
+
+struct synth_field_desc {
+	const char *type;
+	const char *name;
+};
+
+extern int synth_event_gen_cmd_array_start(struct dynevent_cmd *cmd,
+					   const char *name,
+					   struct module *mod,
+					   struct synth_field_desc *fields,
+					   unsigned int n_fields);
+extern int synth_event_create(const char *name,
+			      struct synth_field_desc *fields,
+			      unsigned int n_fields, struct module *mod);
+
+extern int synth_event_add_field(struct dynevent_cmd *cmd,
+				 const char *type,
+				 const char *name);
+extern int synth_event_add_field_str(struct dynevent_cmd *cmd,
+				     const char *type_name);
+extern int synth_event_add_fields(struct dynevent_cmd *cmd,
+				  struct synth_field_desc *fields,
+				  unsigned int n_fields);
+
+#define synth_event_gen_cmd_end(cmd)	\
+	dynevent_create(cmd)
+
 /*
  * Event file flags:
  *  ENABLED	  - The event is enabled