external/trace/trace.c

// SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
/*
 * This example code shows how to read from the trace buffer.
 *
 * Copyright 2013-2019 IBM Corp.
 */

#include <external/trace/trace.h>
#include "../ccan/endian/endian.h"
#include "../ccan/short_types/short_types.h"
#include "trace.h"
#include <trace_types.h>
#include <errno.h>

#if defined(__powerpc__) || defined(__powerpc64__)
#define rmb() lwsync()
#else
#define rmb()
#endif

bool trace_empty(const struct trace_reader *tr)
{
	const struct trace_repeat *rep;

	if (tr->rpos == be64_to_cpu(tr->tb->end))
		return true;

	/*
	 * If we have a single element only, and it's a repeat buffer
	 * we've already seen every repeat for (yet which may be
	 * incremented in future), we're also empty.
	 */
	rep = (void *)tr->tb->buf + tr->rpos % be64_to_cpu(tr->tb->buf_size);
	if (be64_to_cpu(tr->tb->end) != tr->rpos + sizeof(*rep))
		return false;

	if (rep->type != TRACE_REPEAT)
		return false;

	if (be16_to_cpu(rep->num) != tr->last_repeat)
		return false;

	return true;
}

/* You can't read in parallel, so some locking required in caller. */
bool trace_get(union trace *t, struct trace_reader *tr)
{
	u64 start, rpos;
	size_t len;

	len = sizeof(*t) < be32_to_cpu(tr->tb->max_size) ? sizeof(*t) :
		be32_to_cpu(tr->tb->max_size);

	if (trace_empty(tr))
		return false;

again:
	/*
	 * The actual buffer is slightly larger than tbsize, so this
	 * memcpy is always valid.
	 */
	memcpy(t, tr->tb->buf + tr->rpos % be64_to_cpu(tr->tb->buf_size), len);

	rmb(); /* read barrier, so we read tr->tb->start after copying record. */

	start = be64_to_cpu(tr->tb->start);
	rpos = tr->rpos;

	/* Now, was that overwritten? */
	if (rpos < start) {
		/* Create overflow record. */
		t->overflow.unused64 = 0;
		t->overflow.type = TRACE_OVERFLOW;
		t->overflow.len_div_8 = sizeof(t->overflow) / 8;
		t->overflow.bytes_missed = cpu_to_be64(start - rpos);
		tr->rpos = start;
		return true;
	}

	/* Repeat entries need special handling */
	if (t->hdr.type == TRACE_REPEAT) {
		u32 num = be16_to_cpu(t->repeat.num);

		/* In case we've read some already... */
		t->repeat.num = cpu_to_be16(num - tr->last_repeat);

		/* Record how many repeats we saw this time. */
		tr->last_repeat = num;

		/* Don't report an empty repeat buffer. */
		if (t->repeat.num == 0) {
			/*
			 * This can't be the last buffer, otherwise
			 * trace_empty would have returned true.
			 */
			assert(be64_to_cpu(tr->tb->end) >
			       rpos + t->hdr.len_div_8 * 8);
			/* Skip to next entry. */
			tr->rpos = rpos + t->hdr.len_div_8 * 8;
			tr->last_repeat = 0;
			goto again;
		}
	} else {
		tr->last_repeat = 0;
		tr->rpos = rpos + t->hdr.len_div_8 * 8;
	}

	return true;
}