virtio: console: make get_inbuf() return port->inbuf if present

Instead of pulling in a buffer from the vq each time it's called,
get_inbuf() now checks if the current active buffer, in port->inbuf is
valid.  If it is, just returns a pointer to it.  This ends up
simplifying a lot of code calling get_inbuf() since the check for
port->inbuf being valid was done by all the callers.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -351,11 +351,12 @@ static struct port_buffer *alloc_buf(size_t buf_size)
 static struct port_buffer *get_inbuf(struct port *port)
 {
 	struct port_buffer *buf;
-	struct virtqueue *vq;
 	unsigned int len;
 
-	vq = port->in_vq;
-	buf = virtqueue_get_buf(vq, &len);
+	if (port->inbuf)
+		return port->inbuf;
+
+	buf = virtqueue_get_buf(port->in_vq, &len);
 	if (buf) {
 		buf->len = len;
 		buf->offset = 0;
@@ -418,18 +419,12 @@ static bool port_has_data(struct port *port)
 	unsigned long flags;
 	bool ret;
 
+	ret = false;
 	spin_lock_irqsave(&port->inbuf_lock, flags);
-	if (port->inbuf) {
-		ret = true;
-		goto out;
-	}
 	port->inbuf = get_inbuf(port);
-	if (port->inbuf) {
+	if (port->inbuf)
 		ret = true;
-		goto out;
-	}
-	ret = false;
-out:
+
 	spin_unlock_irqrestore(&port->inbuf_lock, flags);
 	return ret;
 }
@@ -1489,8 +1484,7 @@ static void in_intr(struct virtqueue *vq)
 		return;
 
 	spin_lock_irqsave(&port->inbuf_lock, flags);
-	if (!port->inbuf)
-		port->inbuf = get_inbuf(port);
+	port->inbuf = get_inbuf(port);
 
 	/*
 	 * Don't queue up data when port is closed.  This condition