fbarray: add reverse finding

Add function to look up used/free indexes starting from specified
index, but going backwards instead of forward. Semantics are kept
similar to the existing function, except for the fact that, given
the same input, the results returned will be in reverse order.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>

lib/librte_eal/common/eal_common_fbarray.c

@@ -352,6 +352,60 @@ find_contig(const struct rte_fbarray *arr, unsigned int start, bool used)
 	return result;
 }
 
+static int
+find_prev(const struct rte_fbarray *arr, unsigned int start, bool used)
+{
+	const struct used_mask *msk = get_used_mask(arr->data, arr->elt_sz,
+			arr->len);
+	unsigned int idx, first, first_mod;
+	uint64_t ignore_msk;
+
+	/*
+	 * mask only has granularity of MASK_ALIGN, but start may not be aligned
+	 * on that boundary, so construct a special mask to exclude anything we
+	 * don't want to see to avoid confusing clz.
+	 */
+	first = MASK_LEN_TO_IDX(start);
+	first_mod = MASK_LEN_TO_MOD(start);
+	/* we're going backwards, so mask must start from the top */
+	ignore_msk = first_mod == MASK_ALIGN - 1 ?
+				-1ULL : /* prevent overflow */
+				~(-1ULL << (first_mod + 1));
+
+	/* go backwards, include zero */
+	idx = first;
+	do {
+		uint64_t cur = msk->data[idx];
+		int found;
+
+		/* if we're looking for free entries, invert mask */
+		if (!used)
+			cur = ~cur;
+
+		/* ignore everything before start on first iteration */
+		if (idx == first)
+			cur &= ignore_msk;
+
+		/* check if we have any entries */
+		if (cur == 0)
+			continue;
+
+		/*
+		 * find last set bit - that will correspond to whatever it is
+		 * that we're looking for. we're counting trailing zeroes, thus
+		 * the value we get is counted from end of mask, so calculate
+		 * position from start of mask.
+		 */
+		found = MASK_ALIGN - __builtin_clzll(cur) - 1;
+
+		return MASK_GET_IDX(idx, found);
+	} while (idx-- != 0); /* decrement after check  to include zero*/
+
+	/* we didn't find anything */
+	rte_errno = used ? ENOENT : ENOSPC;
+	return -1;
+}
+
 static int
 set_used(struct rte_fbarray *arr, unsigned int idx, bool used)
 {
@@ -676,7 +730,7 @@ rte_fbarray_is_used(struct rte_fbarray *arr, unsigned int idx)
 }
 
 static int
-fbarray_find(struct rte_fbarray *arr, unsigned int start, bool used)
+fbarray_find(struct rte_fbarray *arr, unsigned int start, bool next, bool used)
 {
 	int ret = -1;
 
@@ -708,8 +762,10 @@ fbarray_find(struct rte_fbarray *arr, unsigned int start, bool used)
 			goto out;
 		}
 	}
-
-	ret = find_next(arr, start, used);
+	if (next)
+		ret = find_next(arr, start, used);
+	else
+		ret = find_prev(arr, start, used);
 out:
 	rte_rwlock_read_unlock(&arr->rwlock);
 	return ret;
@@ -718,13 +774,25 @@ out:
 int __rte_experimental
 rte_fbarray_find_next_free(struct rte_fbarray *arr, unsigned int start)
 {
-	return fbarray_find(arr, start, false);
+	return fbarray_find(arr, start, true, false);
 }
 
 int __rte_experimental
 rte_fbarray_find_next_used(struct rte_fbarray *arr, unsigned int start)
 {
-	return fbarray_find(arr, start, true);
+	return fbarray_find(arr, start, true, true);
+}
+
+int __rte_experimental
+rte_fbarray_find_prev_free(struct rte_fbarray *arr, unsigned int start)
+{
+	return fbarray_find(arr, start, false, false);
+}
+
+int __rte_experimental
+rte_fbarray_find_prev_used(struct rte_fbarray *arr, unsigned int start)
+{
+	return fbarray_find(arr, start, false, true);
 }
 
 static int

lib/librte_eal/common/include/rte_fbarray.h

@@ -336,6 +336,40 @@ rte_fbarray_find_contig_free(struct rte_fbarray *arr,
 int __rte_experimental
 rte_fbarray_find_contig_used(struct rte_fbarray *arr, unsigned int start);
 
+/**
+ * Find index of previous free element, starting at specified index.
+ *
+ * @param arr
+ *   Valid pointer to allocated and correctly set up ``rte_fbarray`` structure.
+ *
+ * @param start
+ *   Element index to start search from.
+ *
+ * @return
+ *  - non-negative integer on success.
+ *  - -1 on failure, with ``rte_errno`` indicating reason for failure.
+ */
+int __rte_experimental
+rte_fbarray_find_prev_free(struct rte_fbarray *arr, unsigned int start);
+
+
+/**
+ * Find index of previous used element, starting at specified index.
+ *
+ * @param arr
+ *   Valid pointer to allocated and correctly set up ``rte_fbarray`` structure.
+ *
+ * @param start
+ *   Element index to start search from.
+ *
+ * @return
+ *  - non-negative integer on success.
+ *  - -1 on failure, with ``rte_errno`` indicating reason for failure.
+ */
+int __rte_experimental
+rte_fbarray_find_prev_used(struct rte_fbarray *arr, unsigned int start);
+
+
 
 /**
  * Dump ``rte_fbarray`` metadata.

lib/librte_eal/rte_eal_version.map

@@ -269,6 +269,8 @@ EXPERIMENTAL {
 	rte_fbarray_find_next_used;
 	rte_fbarray_find_next_n_free;
 	rte_fbarray_find_next_n_used;
+	rte_fbarray_find_prev_free;
+	rte_fbarray_find_prev_used;
 	rte_fbarray_find_contig_free;
 	rte_fbarray_find_contig_used;
 	rte_fbarray_get;