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>

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 092a0e8a479aa19569fa0704e050a853bc38883c..d61ee4459eee6aafaf4954c04cd8b3eab4f1f5c7 100644 (file)
--- a/include/linux/perf_event.h
+++ b/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);