tracing/events: Add trace_event boot option

We already have ftrace= boot option, and this adds a similar
boot option for trace events, so allow trace events to be
enabled at boot, for boot debugging purpose.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4A4ACE29.3010407@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -2478,6 +2478,11 @@ and is between 256 and 4096 characters. It is defined in the file
 	trace_buf_size=nn[KMG]
 			[FTRACE] will set tracing buffer size.
 
+	trace_event=[event-list]
+			[FTRACE] Set and start specified trace events in order
+			to facilitate early boot debugging.
+			See also Documentation/trace/events.txt
+
 	trix=		[HW,OSS] MediaTrix AudioTrix Pro
 			Format:
 			<io>,<irq>,<dma>,<dma2>,<sb_io>,<sb_irq>,<sb_dma>,<mpu_io>,<mpu_irq>