[media] pwc: poll(): Check that the device has not beem claimed for streaming already

Some apps which use read() start the streaming through a call to poll(),
this means that if another app has already claimed the device for streaming
(through for example a s_fmt, or a reqbufs), that the poll should fail instead
of getting passed through to vb2_poll.

We only check for this when the app is polling for reads, so that ctrl events
still work.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/pwc/pwc-if.c

@@ -625,10 +625,19 @@ static ssize_t pwc_video_read(struct file *file, char __user *buf,
 static unsigned int pwc_video_poll(struct file *file, poll_table *wait)
 {
 	struct pwc_device *pdev = video_drvdata(file);
+	unsigned long req_events = poll_requested_events(wait);
 
 	if (!pdev->udev)
 		return POLL_ERR;
 
+	if ((req_events & (POLLIN | POLLRDNORM)) &&
+	    pdev->vb_queue.num_buffers == 0 &&
+	    !pdev->iso_init) {
+		/* This poll will start a read stream, check capt_file */
+		if (pwc_test_n_set_capt_file(pdev, file))
+			return POLL_ERR;
+	}
+
 	return vb2_poll(&pdev->vb_queue, file, wait);
 }