perf_counter: Start documenting HAVE_PERF_COUNTERS requirements

Help out arch porters who want to support perf counters by listing some
basic requirements.

Signed-off-by: Mike Frysinger <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

init/Kconfig

···
 
 config HAVE_PERF_COUNTERS
 	bool
+	help
+	  See tools/perf/design.txt for details.
 
 menu "Performance Counters"
 

tools/perf/design.txt

···
 this process has created on other processes.  It only enables or
 disables the group leaders, not any other members in the groups.
 
+
+Arch requirements
+-----------------
+
+If your architecture does not have hardware performance metrics, you can
+still use the generic software counters based on hrtimers for sampling.
+
+So to start with, in order to add HAVE_PERF_COUNTERS to your Kconfig, you
+will need at least this:
+	- asm/perf_counter.h - a basic stub will suffice at first
+	- support for atomic64 types (and associated helper functions)
+	- set_perf_counter_pending() implemented
+
+If your architecture does have hardware capabilities, you can override the
+weak stub hw_perf_counter_init() to register hardware counters.