rapidio/tsi721: fix queue wrapping bug in inbound doorbell handler

Fix a bug that causes a kernel panic when the number of received doorbells
is larger than number of entries in the inbound doorbell queue (current
default value = 512).

Another possible indication for this bug is large number of spurious
doorbells reported by tsi721 driver after reaching the queue size maximum.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Chul Kim <chul.kim@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: <stable@vger.kernel.org>		[3.2.x+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rapidio/devices/tsi721.c

@@ -410,13 +410,14 @@ static void tsi721_db_dpc(struct work_struct *work)
 	 */
 	mport = priv->mport;
 
-	wr_ptr = ioread32(priv->regs + TSI721_IDQ_WP(IDB_QUEUE));
-	rd_ptr = ioread32(priv->regs + TSI721_IDQ_RP(IDB_QUEUE));
+	wr_ptr = ioread32(priv->regs + TSI721_IDQ_WP(IDB_QUEUE)) % IDB_QSIZE;
+	rd_ptr = ioread32(priv->regs + TSI721_IDQ_RP(IDB_QUEUE)) % IDB_QSIZE;
 
 	while (wr_ptr != rd_ptr) {
 		idb_entry = (u64 *)(priv->idb_base +
 					(TSI721_IDB_ENTRY_SIZE * rd_ptr));
 		rd_ptr++;
+		rd_ptr %= IDB_QSIZE;
 		idb.msg = *idb_entry;
 		*idb_entry = 0;