[PATCH 03/10] perf tool: new iterfaces to read event from ring buffer

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

The perf_evlist__mmap_read only support forward mode. It needs a common
function to support both forward and backward mode.
The perf_evlist__mmap_read_backward is buggy.

Introduce a new mmap read interface to support both forward and backward
mode. It can read event one by one from the specific range of ring
buffer.

The standard process to mmap__read event would be as below.
 perf_mmap__read_init
 while (event = perf_mmap__read_event) {
   //process the event
   perf_mmap__consume
 }
 perf_mmap__read_done

The following patches will use it to replace the old interfaces.

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 tools/perf/util/evlist.c | 41 +++++++++++++++++++++++++++++++++++++++++
 tools/perf/util/evlist.h |  2 ++
 2 files changed, 43 insertions(+)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 7d23cf5..b36211e 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -897,6 +897,47 @@ perf_mmap__read(struct perf_mmap *md, bool check_messup, u64 start,
 	return event;
 }
 
+/*
+ * Read the first event in the specified range of the ring buffer.
+ * Used by most of the perf tools and tests
+ */
+union perf_event *
+perf_mmap__read_event(struct perf_mmap_read *read)
+{
+	struct perf_mmap *md = read->md;
+	union perf_event *event;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!refcount_read(&md->refcnt))
+		return NULL;
+
+	/* In backward, the ringbuffer is already paused. */
+	if (!read->backward) {
+		read->end = perf_mmap__read_head(md);
+		if (!read->end)
+			return NULL;
+	}
+
+	event = perf_mmap__read(md, !read->backward && read->overwrite,
+				read->start, read->end, &md->prev);
+	read->start = md->prev;
+	return event;
+}
+
+/*
+ * Mandatory for backward
+ * The direction of backward mmap__read_event is from head to tail.
+ * The last mmap__read_event will set tail to md->prev.
+ * Need to correct the md->prev.
+ */
+void
+perf_mmap__read_done(struct perf_mmap_read *read)
+{
+	read->md->prev = read->head;
+}
+
 union perf_event *perf_mmap__read_forward(struct perf_mmap *md, bool check_messup)
 {
 	u64 head;
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 1ce4857..53baf26 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -207,6 +207,8 @@ int perf_mmap__read_init(struct perf_mmap *md, struct perf_mmap_read *read,
 			 bool overwrite, bool backward);
 int perf_mmap__read_to_file(struct perf_mmap_read *read,
 			    struct perf_data_file *file);
+union perf_event *perf_mmap__read_event(struct perf_mmap_read *read);
+void perf_mmap__read_done(struct perf_mmap_read *read);
 
 int perf_evlist__open(struct perf_evlist *evlist);
 void perf_evlist__close(struct perf_evlist *evlist);
-- 
2.5.5