[PATCH V4 02/15] perf mmap: introduce perf_mmap__read_init()

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

The perf record has specific codes to calculate the ringbuffer position
for both overwrite and non-overwrite mode. Now, only perf record
supports both modes. The perf top will support both modes later.
It is useful to make the specific codes generic.

Introduce a new interface perf_mmap__read_init() to find ringbuffer
position. The perf_mmap__read_init() is actually factored out from
perf_mmap__push().
There are slight differences.
- Add a check for map->refcnt
- Add new return value logic, EAGAIN and EINVAL.

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 tools/perf/util/mmap.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 tools/perf/util/mmap.h |  2 ++
 2 files changed, 45 insertions(+)

diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index 05076e6..414089f 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -267,6 +267,49 @@ static int overwrite_rb_find_range(void *buf, int mask, u64 head, u64 *start, u6
 	return -1;
 }
 
+/*
+ * Report the start and end of the available data in ringbuffer
+ */
+int perf_mmap__read_init(struct perf_mmap *map, bool overwrite,
+			 u64 *startp, u64 *endp)
+{
+	unsigned char *data = map->base + page_size;
+	u64 head = perf_mmap__read_head(map);
+	u64 old = map->prev;
+	unsigned long size;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!refcount_read(&map->refcnt))
+		return -EINVAL;
+
+	*startp = overwrite ? head : old;
+	*endp = overwrite ? old : head;
+
+	if (*startp == *endp)
+		return -EAGAIN;
+
+	size = *endp - *startp;
+	if (size > (unsigned long)(map->mask) + 1) {
+		if (!overwrite) {
+			WARN_ONCE(1, "failed to keep up with mmap data. (warn only once)\n");
+
+			map->prev = head;
+			perf_mmap__consume(map, overwrite);
+			return -EAGAIN;
+		}
+
+		/*
+		 * Backward ring buffer is full. We still have a chance to read
+		 * most of data from it.
+		 */
+		if (overwrite_rb_find_range(data, map->mask, head, startp, endp))
+			return -EINVAL;
+	}
+	return 0;
+}
+
 int perf_mmap__push(struct perf_mmap *md, bool overwrite,
 		    void *to, int push(void *to, void *buf, size_t size))
 {
diff --git a/tools/perf/util/mmap.h b/tools/perf/util/mmap.h
index e43d7b5..0633308 100644
--- a/tools/perf/util/mmap.h
+++ b/tools/perf/util/mmap.h
@@ -94,4 +94,6 @@ int perf_mmap__push(struct perf_mmap *md, bool backward,
 
 size_t perf_mmap__mmap_len(struct perf_mmap *map);
 
+int perf_mmap__read_init(struct perf_mmap *map, bool overwrite,
+			 u64 *startp, u64 *endp);
 #endif /*__PERF_MMAP_H */
-- 
2.5.5