sys_heap: introduce min_chunk_size()

With this we can remove magic constants, especially those used with
big_heap().

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

lib/os/heap-validate.c

@@ -19,7 +19,7 @@
 
 static size_t max_chunkid(struct z_heap *h)
 {
-	return h->len - bytes_to_chunksz(h, 1);
+	return h->len - min_chunk_size(h);
 }
 
 static bool in_bounds(struct z_heap *h, chunkid_t c)

lib/os/heap.c

@@ -92,7 +92,7 @@ static void *split_alloc(struct z_heap *h, int bidx, size_t sz)
 
 	CHECK(rem < h->len);
 
-	if (rem >= (big_heap(h) ? 2 : 1)) {
+	if (rem >= min_chunk_size(h)) {
 		chunkid_t c2 = c + sz;
 		chunkid_t c3 = right_chunk(h, c);
 
@@ -224,7 +224,7 @@ void sys_heap_init(struct sys_heap *heap, void *mem, size_t bytes)
 	size_t chunk0_size = chunksz(sizeof(struct z_heap) +
 				     nb_buckets * sizeof(struct z_heap_bucket));
 
-	CHECK(chunk0_size < buf_sz);
+	CHECK(chunk0_size + min_chunk_size(h) < buf_sz);
 
 	for (int i = 0; i < nb_buckets; i++) {
 		h->buckets[i].list_size = 0;

lib/os/heap.h

@@ -199,10 +199,16 @@ static inline size_t bytes_to_chunksz(struct z_heap *h, size_t bytes)
 	return chunksz(chunk_header_bytes(h) + bytes);
 }
 
-static int bucket_idx(struct z_heap *h, size_t sz)
+static inline int min_chunk_size(struct z_heap *h)
 {
-	/* A chunk of size 2 is the minimum size on big heaps */
-	return 31 - __builtin_clz(sz) - (big_heap(h) ? 1 : 0);
+	return bytes_to_chunksz(h, 1);
 }
 
+static inline int bucket_idx(struct z_heap *h, size_t sz)
+{
+	size_t usable_sz = sz - min_chunk_size(h) + 1;
+	return 31 - __builtin_clz(usable_sz);
+}
+
+
 #endif /* ZEPHYR_INCLUDE_LIB_OS_HEAP_H_ */