perf mmap: Discard 'prev' in perf_mmap__read()

/* When check_messup is true, 'end' must points to a good entry */

static union perf_event *perf_mmap__read(struct perf_mmap *map,

					 u64 start, u64 end, u64 *prev)

					 u64 *startp, u64 end)

{

	unsigned char *data = map->base + page_size;

	union perf_event *event = NULL;

	int diff = end - start;

	int diff = end - *startp;

	if (diff >= (int)sizeof(event->header)) {

		size_t size;

		event = (union perf_event *)&data[start & map->mask];

		event = (union perf_event *)&data[*startp & map->mask];

		size = event->header.size;

		if (size < sizeof(event->header) || diff < (int)size) {

			event = NULL;

			goto broken_event;

		}

		if (size < sizeof(event->header) || diff < (int)size)

			return NULL;

		/*

		 * Event straddles the mmap boundary -- header should always

		 * be inside due to u64 alignment of output.

		 */

		if ((start & map->mask) + size != ((start + size) & map->mask)) {

			unsigned int offset = start;

		if ((*startp & map->mask) + size != ((*startp + size) & map->mask)) {

			unsigned int offset = *startp;

			unsigned int len = min(sizeof(*event), size), cpy;

			void *dst = map->event_copy;

			event = (union perf_event *)map->event_copy;

		}

		start += size;

		*startp += size;

	}

broken_event:

	if (prev)

		*prev = start;

	return event;

}

union perf_event *perf_mmap__read_forward(struct perf_mmap *map)

{

	u64 head;

	u64 old = map->prev;

	/*

	 * Check if event was unmapped due to a POLLHUP/POLLERR.

	head = perf_mmap__read_head(map);

	return perf_mmap__read(map, old, head, &map->prev);

	return perf_mmap__read(map, &map->prev, head);

}

union perf_event *perf_mmap__read_backward(struct perf_mmap *map)

{

	u64 head, end;

	u64 start = map->prev;

	/*

	 * Check if event was unmapped due to a POLLHUP/POLLERR.

	else

		end = head + map->mask + 1;

	return perf_mmap__read(map, start, end, &map->prev);

	return perf_mmap__read(map, &map->prev, end);

}

void perf_mmap__read_catchup(struct perf_mmap *map)