Flush DAHDI->trunkdev FIFO at open time

If we don't do this, we always have a full FIFO at the time userspace
opens the trunkdev, adding latency for no good reason.

Change-Id: Ifbda3766309cc7d6be485073e36607bdd6742f3a

drivers/dahdi/trunkdev/frame_fifo.c

@@ -53,6 +53,15 @@ void frame_fifo_init(struct frame_fifo *fifo, unsigned int threshold,
 	spin_lock_init(&fifo->lock);
 }
 
+/*! Flush (drop) the entire content of the FIFO */
+void frame_fifo_flush(struct frame_fifo *fifo)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&fifo->lock, flags);
+	fifo->next_out = fifo->next_in;
+	spin_unlock_irqrestore(&fifo->lock, flags);
+}
+
 #define FIFO_BUF_END(f)	((f)->buf + sizeof((f)->buf))
 
 /*! put one received frames into the FIFO.

drivers/dahdi/trunkdev/frame_fifo.h

@@ -20,6 +20,8 @@ void frame_fifo_init(struct frame_fifo *fifo, unsigned int threshold,
 		     void (*threshold_cb)(struct frame_fifo *fifo, unsigned int frames, void *priv),
 		     void *priv);
 
+void frame_fifo_flush(struct frame_fifo *fifo);
+
 /* number of frames currently available in FIFO */
 static inline unsigned int __frame_fifo_frames(struct frame_fifo *fifo)
 {

drivers/dahdi/trunkdev/trunkdev.c

@@ -456,6 +456,9 @@ static long dahdi_unlocked_ioctl(struct file *file, unsigned int cmd, unsigned l
 		span->alarms &= ~(ALL_RY_ALARMS|DAHDI_ALARM_LOS);
 		dahdi_alarm_notify(span);
 		spin_unlock_irqrestore(&span->lock, flags);
+		/* FIFO should be full at this point, let's flush it as now we actually
+		 * have a receiver */
+		frame_fifo_flush(&td->to_trunk);
 
 		open.spanno = span->spanno;
 		if (copy_to_user((__user void *) data, &open, sizeof(open)))