net: ipa: rework last transaction determination

When quiescing a channel, we find the "last" transaction, which is
the latest one to have been allocated.  (New transaction allocation
will have been prevented by the time this is called.)

Currently we do this by looking for the first non-empty transaction
list in each state, then return the last entry from that last.
Instead, determine the last entry in each list (if any) and return
that entry if found.

Temporarily (locally) introduce list_last_entry_or_null() as a
helper for this, mirroring list_first_entry_or_null().  This macro
definition will be removed by an upcoming patch.

Remove the temporary warnings added by the previous commit.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ipa/gsi.c

@@ -710,7 +710,6 @@ static void gsi_evt_ring_program(struct gsi *gsi, u32 evt_ring_id)
 static struct gsi_trans *gsi_channel_trans_last(struct gsi_channel *channel)
 {
 	struct gsi_trans_info *trans_info = &channel->trans_info;
-	const struct list_head *list;
 	struct gsi_trans *trans;
 
 	spin_lock_bh(&trans_info->spinlock);
@@ -719,29 +718,30 @@ static struct gsi_trans *gsi_channel_trans_last(struct gsi_channel *channel)
 	 * before we disabled transmits, so check for that.
 	 */
 	if (channel->toward_ipa) {
-		list = &trans_info->alloc;
-		if (!list_empty(list))
+		trans = list_last_entry_or_null(&trans_info->alloc,
+						struct gsi_trans, links);
+		if (trans)
 			goto done;
-		list = &trans_info->committed;
-		if (!list_empty(list))
+		trans = list_last_entry_or_null(&trans_info->committed,
+						struct gsi_trans, links);
+		if (trans)
 			goto done;
-		list = &trans_info->pending;
-		if (!list_empty(list))
+		trans = list_last_entry_or_null(&trans_info->pending,
+						struct gsi_trans, links);
+		if (trans)
 			goto done;
 	}
 
 	/* Otherwise (TX or RX) we want to wait for anything that
 	 * has completed, or has been polled but not released yet.
 	 */
-	list = &trans_info->complete;
-	if (!list_empty(list))
+	trans = list_last_entry_or_null(&trans_info->complete,
+					struct gsi_trans, links);
+	if (trans)
 		goto done;
-	list = &trans_info->polled;
-	if (list_empty(list))
-		list = NULL;
+	trans = list_last_entry_or_null(&trans_info->polled,
+					struct gsi_trans, links);
 done:
-	trans = list ? list_last_entry(list, struct gsi_trans, links) : NULL;
-
 	/* Caller will wait for this, so take a reference */
 	if (trans)
 		refcount_inc(&trans->refcount);

drivers/net/ipa/gsi_private.h

@@ -16,6 +16,20 @@ struct gsi_channel;
 
 #define GSI_RING_ELEMENT_SIZE	16	/* bytes; must be a power of 2 */
 
+/**
+ * list_last_entry_or_null - get the last element from a list
+ * @ptr:	the list head to take the element from.
+ * @type:	the type of the struct this is embedded in.
+ * @member:	the name of the list_head within the struct.
+ *
+ * Note that if the list is empty, it returns NULL.
+ */
+#define list_last_entry_or_null(ptr, type, member) ({ \
+	struct list_head *head__ = (ptr); \
+	struct list_head *pos__ = READ_ONCE(head__->prev); \
+	pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
+})
+
 /**
  * gsi_trans_move_complete() - Mark a GSI transaction completed
  * @trans:	Transaction to commit

drivers/net/ipa/gsi_trans.c

@@ -309,23 +309,15 @@ void gsi_trans_move_polled(struct gsi_trans *trans)
 {
 	struct gsi_channel *channel = &trans->gsi->channel[trans->channel_id];
 	struct gsi_trans_info *trans_info = &channel->trans_info;
-	u16 trans_index;
 
 	spin_lock_bh(&trans_info->spinlock);
 
 	list_move_tail(&trans->links, &trans_info->polled);
 
-	trans = list_first_entry(&trans_info->polled,
-				 struct gsi_trans, links);
-
 	spin_unlock_bh(&trans_info->spinlock);
 
 	/* This completed transaction is now polled */
 	trans_info->completed_id++;
-
-	WARN_ON(trans_info->polled_id == trans_info->completed_id);
-	trans_index = trans_info->polled_id % channel->tre_count;
-	WARN_ON(trans != &trans_info->trans[trans_index]);
 }
 
 /* Reserve some number of TREs on a channel.  Returns true if successful */
@@ -413,11 +405,8 @@ struct gsi_trans *gsi_channel_trans_alloc(struct gsi *gsi, u32 channel_id,
 /* Free a previously-allocated transaction */
 void gsi_trans_free(struct gsi_trans *trans)
 {
-	struct gsi_channel *channel = &trans->gsi->channel[trans->channel_id];
 	refcount_t *refcount = &trans->refcount;
 	struct gsi_trans_info *trans_info;
-	struct gsi_trans *polled;
-	u16 trans_index;
 	bool last;
 
 	/* We must hold the lock to release the last reference */
@@ -433,9 +422,6 @@ void gsi_trans_free(struct gsi_trans *trans)
 	if (last)
 		list_del(&trans->links);
 
-	polled = list_first_entry_or_null(&trans_info->polled,
-					  struct gsi_trans, links);
-
 	spin_unlock_bh(&trans_info->spinlock);
 
 	if (!last)
@@ -456,14 +442,6 @@ void gsi_trans_free(struct gsi_trans *trans)
 	/* This transaction is now free */
 	trans_info->polled_id++;
 
-	if (polled) {
-		trans_index = trans_info->polled_id % channel->tre_count;
-		WARN_ON(polled != &trans_info->trans[trans_index]);
-	} else {
-		WARN_ON(trans_info->polled_id !=
-			trans_info->completed_id);
-	}
-
 	/* Releasing the reserved TREs implicitly frees the sgl[] and
 	 * (if present) info[] arrays, plus the transaction itself.
 	 */