iwlwifi: pcie: Refactor iwl_queue_space

Reduce the ambiguity spares to a single element if the window size is not
smaller than the queue size. If smaller, no spares are required at all.

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

1 files changed, 24 insertions, 12 deletions

diff --git a/drivers/net/wireless/iwlwifi/pcie/tx.c b/drivers/net/wireless/iwlwifi/pcie/tx.c
index 011167c22da8..12c9c0030da6 100644
--- a/drivers/net/wireless/iwlwifi/pcie/tx.c
+++ b/drivers/net/wireless/iwlwifi/pcie/tx.c
@@ -65,18 +65,30 @@
  ***************************************************/
 static int iwl_queue_space(const struct iwl_queue *q)
 {
-        int s = q->read_ptr - q->write_ptr;
-
-        if (q->read_ptr > q->write_ptr)
-                s -= q->n_bd;
-
-        if (s <= 0)
-                s += q->n_window;
-        /* keep some reserve to not confuse empty and full situations */
-        s -= 2;
-        if (s < 0)
-                s = 0;
-        return s;
+        unsigned int max;
+        unsigned int used;
+
+        /*
+         * To avoid ambiguity between empty and completely full queues, there
+         * should always be less than q->n_bd elements in the queue.
+         * If q->n_window is smaller than q->n_bd, there is no need to reserve
+         * any queue entries for this purpose.
+         */
+        if (q->n_window < q->n_bd)
+                max = q->n_window;
+        else
+                max = q->n_bd - 1;
+
+        /*
+         * q->n_bd is a power of 2, so the following is equivalent to modulo by
+         * q->n_bd and is well defined for negative dividends.
+         */
+        used = (q->write_ptr - q->read_ptr) & (q->n_bd - 1);
+
+        if (WARN_ON(used > max))
+                return 0;
+
+        return max - used;
 }
 
 /*