firewire: core: use long bus reset on gap count error

When resetting the bus after a gap count error, use a long rather than
short bus reset.

IEEE 1394-1995 uses only long bus resets. IEEE 1394a adds the option of
short bus resets. When video or audio transmission is in progress and a
device is hot-plugged elsewhere on the bus, the resulting bus reset can
cause video frame drops or audio dropouts. Short bus resets reduce or
eliminate this problem. Accordingly, short bus resets are almost always
preferred.

However, on a mixed 1394/1394a bus, a short bus reset can trigger an
immediate additional bus reset. This double bus reset can be interpreted
differently by different nodes on the bus, resulting in an inconsistent gap
count after the bus reset. An inconsistent gap count will cause another bus
reset, leading to a neverending bus reset loop. This only happens for some
bus topologies, not for all mixed 1394/1394a buses.

By instead sending a long bus reset after a gap count inconsistency, we
avoid the doubled bus reset, restoring the bus to normal operation.
Signed-off-by: Adam Goldman <adamg@pobox.com>
Link: https://sourceforge.net/p/linux1394/mailman/message/58741624/Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

drivers/firewire/core-card.c

@@ -500,7 +500,19 @@ static void bm_work(struct work_struct *work)
 		fw_notice(card, "phy config: new root=%x, gap_count=%d\n",
 			  new_root_id, gap_count);
 		fw_send_phy_config(card, new_root_id, generation, gap_count);
-		reset_bus(card, true);
+		/*
+		 * Where possible, use a short bus reset to minimize
+		 * disruption to isochronous transfers. But in the event
+		 * of a gap count inconsistency, use a long bus reset.
+		 *
+		 * As noted in 1394a 8.4.6.2, nodes on a mixed 1394/1394a bus
+		 * may set different gap counts after a bus reset. On a mixed
+		 * 1394/1394a bus, a short bus reset can get doubled. Some
+		 * nodes may treat the double reset as one bus reset and others
+		 * may treat it as two, causing a gap count inconsistency
+		 * again. Using a long bus reset prevents this.
+		 */
+		reset_bus(card, card->gap_count != 0);
 		/* Will allocate broadcast channel after the reset. */
 		goto out;
 	}