firewire: unnecessary BM delay after generation rollover

Noticed by Jarod Wilson:  The bus manager work was unnecessarily delayed
each time the bus generation counter rolled over.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jarod Wilson <jwilson@redhat.com>

drivers/firewire/fw-card.c

@@ -232,7 +232,7 @@ fw_card_bm_work(struct work_struct *work)
 	root_id = root_node->node_id;
 	grace = time_after(jiffies, card->reset_jiffies + DIV_ROUND_UP(HZ, 10));
 
-	if (card->bm_generation + 1 == generation ||
+	if (is_next_generation(generation, card->bm_generation) ||
 	    (card->bm_generation != generation && grace)) {
 		/*
 		 * This first step is to figure out who is IRM and

drivers/firewire/fw-topology.c

@@ -523,7 +523,7 @@ fw_core_handle_bus_reset(struct fw_card *card,
 	 * previously processed one, we cannot reliably compare the
 	 * old and new topologies.
 	 */
-	if ((generation & 0xff) != ((card->generation + 1) & 0xff) &&
+	if (!is_next_generation(generation, card->generation) &&
 	    card->local_node != NULL) {
 		fw_notify("skipped bus generations, destroying all nodes\n");
 		fw_destroy_nodes(card);

drivers/firewire/fw-transaction.h

@@ -275,6 +275,15 @@ static inline void fw_card_put(struct fw_card *card)
 
 extern void fw_schedule_bm_work(struct fw_card *card, unsigned long delay);
 
+/*
+ * Check whether new_generation is the immediate successor of old_generation.
+ * Take counter roll-over at 255 (as per to OHCI) into account.
+ */
+static inline bool is_next_generation(int new_generation, int old_generation)
+{
+	return (new_generation & 0xff) == ((old_generation + 1) & 0xff);
+}
+
 /*
  * The iso packet format allows for an immediate header/payload part
  * stored in 'header' immediately after the packet info plus an