ALSA: line6: hwdep: add support for O_NONBLOCK opening mode

Currently line6 hwdep interface ignores O_NONBLOCK flag when
opening device and it renders it somewhat useless when using poll.

Check for O_NONBLOCK flag when opening device and don't block read()
if it is set.
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Link: https://lore.kernel.org/r/20200502193120.79115-2-anarsoul@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/driver.c

@@ -550,6 +550,7 @@ static int line6_hwdep_open(struct snd_hwdep *hw, struct file *file)
 	/* NOTE: hwdep layer provides atomicity here */
 
 	line6->messages.active = 1;
+	line6->messages.nonblock = file->f_flags & O_NONBLOCK ? 1 : 0;
 
 	return 0;
 }
@@ -579,6 +580,9 @@ line6_hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 	while (kfifo_len(&line6->messages.fifo) == 0) {
 		mutex_unlock(&line6->messages.read_lock);
 
+		if (line6->messages.nonblock)
+			return -EAGAIN;
+
 		rv = wait_event_interruptible(
 			line6->messages.wait_queue,
 			kfifo_len(&line6->messages.fifo) != 0);

sound/usb/line6/driver.h

@@ -163,6 +163,7 @@ struct usb_line6 {
 		struct mutex read_lock;
 		wait_queue_head_t wait_queue;
 		unsigned int active:1;
+		unsigned int nonblock:1;
 		STRUCT_KFIFO_REC_2(LINE6_BUFSIZE_LISTEN * LINE6_RAW_MESSAGES_MAXCOUNT)
 			fifo;
 	} messages;