iwlwifi: pcie: Refactor iwl_rxq_space

Simplify iwl_rxq_space to improve readability and reduce
the ambiguity spares to a single element.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/iwlwifi/pcie/rx.c

@@ -112,15 +112,16 @@
  */
 static int iwl_rxq_space(const struct iwl_rxq *rxq)
 {
-	int s = rxq->read - rxq->write;
-
-	if (s <= 0)
-		s += RX_QUEUE_SIZE;
-	/* keep some buffer to not confuse full and empty queue */
-	s -= 2;
-	if (s < 0)
-		s = 0;
-	return s;
+	/* Make sure RX_QUEUE_SIZE is a power of 2 */
+	BUILD_BUG_ON(RX_QUEUE_SIZE & (RX_QUEUE_SIZE - 1));
+
+	/*
+	 * There can be up to (RX_QUEUE_SIZE - 1) free slots, to avoid ambiguity
+	 * between empty and completely full queues.
+	 * The following is equivalent to modulo by RX_QUEUE_SIZE and is well
+	 * defined for negative dividends.
+	 */
+	return (rxq->read - rxq->write - 1) & (RX_QUEUE_SIZE - 1);
 }
 
 /*