perf tools: Support reading from backward ring buffer

	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);

}

union perf_event *

perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)

{

	struct perf_mmap *md = &evlist->mmap[idx];

	u64 head, end;

	u64 start = md->prev;

	/*

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

	 */

	if (!atomic_read(&md->refcnt))

		return NULL;

	head = perf_mmap__read_head(md);

	if (!head)

		return NULL;

	/*

	 * 'head' pointer starts from 0. Kernel minus sizeof(record) form

	 * it each time when kernel writes to it, so in fact 'head' is

	 * negative. 'end' pointer is made manually by adding the size of

	 * the ring buffer to 'head' pointer, means the validate data can

	 * read is the whole ring buffer. If 'end' is positive, the ring

	 * buffer has not fully filled, so we must adjust 'end' to 0.

	 *

	 * However, since both 'head' and 'end' is unsigned, we can't

	 * simply compare 'end' against 0. Here we compare '-head' and

	 * the size of the ring buffer, where -head is the number of bytes

	 * kernel write to the ring buffer.

	 */

	if (-head < (u64)(md->mask + 1))

		end = 0;

	else

		end = head + md->mask + 1;

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

}

void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx)

{

	struct perf_mmap *md = &evlist->mmap[idx];

	u64 head;

	if (!atomic_read(&md->refcnt))

		return;

	head = perf_mmap__read_head(md);

	md->prev = head;

}

static bool perf_mmap__empty(struct perf_mmap *md)

{

	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;

union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);

union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist,

						  int idx);

void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx);

void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);

int perf_evlist__open(struct perf_evlist *evlist);