ALSA: line6: Add poll callback for hwdep

At least POD HD500 uses message-based communication, both sides can
send messages. Add poll callback so application can wait for device
messages without using busy loop.
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Link: https://lore.kernel.org/r/20200502193120.79115-3-anarsoul@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/driver.c

@@ -630,11 +630,27 @@ line6_hwdep_write(struct snd_hwdep *hwdep, const char __user *data, long count,
 	return rv;
 }
 
+static __poll_t
+line6_hwdep_poll(struct snd_hwdep *hwdep, struct file *file, poll_table *wait)
+{
+	__poll_t rv;
+	struct usb_line6 *line6 = hwdep->private_data;
+
+	poll_wait(file, &line6->messages.wait_queue, wait);
+
+	mutex_lock(&line6->messages.read_lock);
+	rv = kfifo_len(&line6->messages.fifo) == 0 ? 0 : EPOLLIN | EPOLLRDNORM;
+	mutex_unlock(&line6->messages.read_lock);
+
+	return rv;
+}
+
 static const struct snd_hwdep_ops hwdep_ops = {
 	.open    = line6_hwdep_open,
 	.release = line6_hwdep_release,
 	.read    = line6_hwdep_read,
 	.write   = line6_hwdep_write,
+	.poll    = line6_hwdep_poll,
 };
 
 /* Insert into circular buffer */