tracing: update sample event documentation

# builds the trace events example kernel modules;

# then to use one (as root):  insmod <module_name.ko>

# If you include a trace header outside of include/trace/events

# then the file that does the #define CREATE_TRACE_POINTS must

# have that tracer file in its main search path. This is because

# define_trace.h will include it, and must be able to find it from

# the include/trace directory.

#

# Here trace-events-sample.c does the CREATE_TRACE_POINTS.

#

CFLAGS_trace-events-sample.o := -I$(src)

obj-$(CONFIG_SAMPLE_TRACE_EVENTS) += trace-events-sample.o

 * If TRACE_SYSTEM is defined, that will be the directory created

 * in the ftrace directory under /debugfs/tracing/events/<system>

 *

 * The define_trace.h belowe will also look for a file name of

 * The define_trace.h below will also look for a file name of

 * TRACE_SYSTEM.h where TRACE_SYSTEM is what is defined here.

 * In this case, it would look for sample.h

 *

 * If you want a different system than file name, you can override

 * the header name by defining TRACE_INCLUDE_FILE

 * If the header name will be different than the system name

 * (as in this case), then you can override the header name that

 * define_trace.h will look up by defining TRACE_INCLUDE_FILE

 *

 * If this file was called, goofy.h, then we would define:

 * This file is called trace-events-sample.h but we want the system

 * to be called "sample". Therefore we must define the name of this

 * file:

 *

 * #define TRACE_INCLUDE_FILE goofy

 * #define TRACE_INCLUDE_FILE trace-events-sample

 *

 * As we do an the bottom of this file.

 */

#undef TRACE_SYSTEM

#define TRACE_SYSTEM sample

 *

 * #define TRACE_INCLUDE_PATH ../../samples/trace_events

 *

 * But I chose to simply make it use the current directory and then in

 * the Makefile I added:

 * But the safest and easiest way to simply make it use the directory

 * that the file is in is to add in the Makefile:

 *

 * CFLAGS_trace-events-sample.o := -I$(PWD)/samples/trace_events/

 * CFLAGS_trace-events-sample.o := -I$(src)

 *

 * This will make sure the current path is part of the include

 * structure for our file so that we can find it.

 * structure for our file so that define_trace.h can find it.

 *

 * I could have made only the top level directory the include:

 *

 *

 * #define TRACE_INCLUDE_PATH samples/trace_events

 *

 * But then if something defines "samples" or "trace_events" then we

 * could risk that being converted too, and give us an unexpected

 * But then if something defines "samples" or "trace_events" as a macro

 * then we could risk that being converted too, and give us an unexpected

 * result.

 */

#undef TRACE_INCLUDE_PATH