dmaengine: at_hdmac: fix race condition in atc_advance_work()

The BUG_ON() directive is triggered probably due to a latency
modification following inclusion of commit c10d7367 ("softirq:
reduce latencies").  This condition has not been met before 3.9-rc1 and
doesn't trigger without this patch.

We now make sure that DMA channel is idle before calling
atc_complete_all() which makes the BUG_ON() "protection" useless.
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/dma/at_hdmac.c

@@ -310,8 +310,6 @@ static void atc_complete_all(struct at_dma_chan *atchan)
 
 	dev_vdbg(chan2dev(&atchan->chan_common), "complete all\n");
 
-	BUG_ON(atc_chan_is_enabled(atchan));
-
 	/*
 	 * Submit queued descriptors ASAP, i.e. before we go through
 	 * the completed ones.
@@ -368,6 +366,9 @@ static void atc_advance_work(struct at_dma_chan *atchan)
 {
 	dev_vdbg(chan2dev(&atchan->chan_common), "advance_work\n");
 
+	if (atc_chan_is_enabled(atchan))
+		return;
+
 	if (list_empty(&atchan->active_list) ||
 	    list_is_singular(&atchan->active_list)) {
 		atc_complete_all(atchan);
@@ -1078,9 +1079,7 @@ static void atc_issue_pending(struct dma_chan *chan)
 		return;
 
 	spin_lock_irqsave(&atchan->lock, flags);
-	if (!atc_chan_is_enabled(atchan)) {
 	atc_advance_work(atchan);
-	}
 	spin_unlock_irqrestore(&atchan->lock, flags);
 }