perf/abi: Document some more aspects of the perf ABI

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/perf_event.h

@@ -140,27 +140,60 @@ struct hw_perf_event {
 		};
 #endif
 	};
+	/*
+	 * If the event is a per task event, this will point to the task in
+	 * question. See the comment in perf_event_alloc().
+	 */
 	struct task_struct		*target;
+
+/*
+ * hw_perf_event::state flags; used to track the PERF_EF_* state.
+ */
+#define PERF_HES_STOPPED	0x01 /* the counter is stopped */
+#define PERF_HES_UPTODATE	0x02 /* event->count up-to-date */
+#define PERF_HES_ARCH		0x04
+
 	int				state;
+
+	/*
+	 * The last observed hardware counter value, updated with a
+	 * local64_cmpxchg() such that pmu::read() can be called nested.
+	 */
 	local64_t			prev_count;
+
+	/*
+	 * The period to start the next sample with.
+	 */
 	u64				sample_period;
+
+	/*
+	 * The period we started this sample with.
+	 */
 	u64				last_period;
+
+	/*
+	 * However much is left of the current period; note that this is
+	 * a full 64bit value and allows for generation of periods longer
+	 * than hardware might allow.
+	 */
 	local64_t			period_left;
+
+	/*
+	 * State for throttling the event, see __perf_event_overflow() and
+	 * perf_adjust_freq_unthr_context().
+	 */
 	u64                             interrupts_seq;
 	u64				interrupts;
 
+	/*
+	 * State for freq target events, see __perf_event_overflow() and
+	 * perf_adjust_freq_unthr_context().
+	 */
 	u64				freq_time_stamp;
 	u64				freq_count_stamp;
 #endif
 };
 
-/*
- * hw_perf_event::state flags
- */
-#define PERF_HES_STOPPED	0x01 /* the counter is stopped */
-#define PERF_HES_UPTODATE	0x02 /* event->count up-to-date */
-#define PERF_HES_ARCH		0x04
-
 struct perf_event;
 
 /*
@@ -210,7 +243,19 @@ struct pmu {
 
 	/*
 	 * Try and initialize the event for this PMU.
-	 * Should return -ENOENT when the @event doesn't match this PMU.
+	 *
+	 * Returns:
+	 *  -ENOENT	-- @event is not for this PMU
+	 *
+	 *  -ENODEV	-- @event is for this PMU but PMU not present
+	 *  -EBUSY	-- @event is for this PMU but PMU temporarily unavailable
+	 *  -EINVAL	-- @event is for this PMU but @event is not valid
+	 *  -EOPNOTSUPP -- @event is for this PMU, @event is valid, but not supported
+	 *  -EACCESS	-- @event is for this PMU, @event is valid, but no privilidges
+	 *
+	 *  0		-- @event is for this PMU and valid
+	 *
+	 * Other error return values are allowed.
 	 */
 	int (*event_init)		(struct perf_event *event);
 
@@ -221,27 +266,61 @@ struct pmu {
 	void (*event_mapped)		(struct perf_event *event); /*optional*/
 	void (*event_unmapped)		(struct perf_event *event); /*optional*/
 
+	/*
+	 * Flags for ->add()/->del()/ ->start()/->stop(). There are
+	 * matching hw_perf_event::state flags.
+	 */
 #define PERF_EF_START	0x01		/* start the counter when adding    */
 #define PERF_EF_RELOAD	0x02		/* reload the counter when starting */
 #define PERF_EF_UPDATE	0x04		/* update the counter when stopping */
 
 	/*
-	 * Adds/Removes a counter to/from the PMU, can be done inside
-	 * a transaction, see the ->*_txn() methods.
+	 * Adds/Removes a counter to/from the PMU, can be done inside a
+	 * transaction, see the ->*_txn() methods.
+	 *
+	 * The add/del callbacks will reserve all hardware resources required
+	 * to service the event, this includes any counter constraint
+	 * scheduling etc.
+	 *
+	 * Called with IRQs disabled and the PMU disabled on the CPU the event
+	 * is on.
+	 *
+	 * ->add() called without PERF_EF_START should result in the same state
+	 *  as ->add() followed by ->stop().
+	 *
+	 * ->del() must always PERF_EF_UPDATE stop an event. If it calls
+	 *  ->stop() that must deal with already being stopped without
+	 *  PERF_EF_UPDATE.
 	 */
 	int  (*add)			(struct perf_event *event, int flags);
 	void (*del)			(struct perf_event *event, int flags);
 
 	/*
-	 * Starts/Stops a counter present on the PMU. The PMI handler
-	 * should stop the counter when perf_event_overflow() returns
-	 * !0. ->start() will be used to continue.
+	 * Starts/Stops a counter present on the PMU.
+	 *
+	 * The PMI handler should stop the counter when perf_event_overflow()
+	 * returns !0. ->start() will be used to continue.
+	 *
+	 * Also used to change the sample period.
+	 *
+	 * Called with IRQs disabled and the PMU disabled on the CPU the event
+	 * is on -- will be called from NMI context with the PMU generates
+	 * NMIs.
+	 *
+	 * ->stop() with PERF_EF_UPDATE will read the counter and update
+	 *  period/count values like ->read() would.
+	 *
+	 * ->start() with PERF_EF_RELOAD will reprogram the the counter
+	 *  value, must be preceded by a ->stop() with PERF_EF_UPDATE.
 	 */
 	void (*start)			(struct perf_event *event, int flags);
 	void (*stop)			(struct perf_event *event, int flags);
 
 	/*
 	 * Updates the counter value of the event.
+	 *
+	 * For sampling capable PMUs this will also update the software period
+	 * hw_perf_event::period_left field.
 	 */
 	void (*read)			(struct perf_event *event);