drm/crc: Add support for polling on the data fd.

This will make it possible for userspace to know whether reading
will block, without blocking on the fd. This makes it possible to
drain all queued CRC's in blocking mode, without having to reopen
the fd.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180202142743.68527-1-maarten.lankhorst@linux.intel.comReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[mlankhorst: Use POLLIN|POLLRDNORM, based on Ville's suggestion]

drivers/gpu/drm/drm_debugfs_crc.c

@@ -307,10 +307,29 @@ static ssize_t crtc_crc_read(struct file *filep, char __user *user_buf,
 	return LINE_LEN(crc->values_cnt);
 }
 
+static unsigned int crtc_crc_poll(struct file *file, poll_table *wait)
+{
+	struct drm_crtc *crtc = file->f_inode->i_private;
+	struct drm_crtc_crc *crc = &crtc->crc;
+	unsigned ret;
+
+	poll_wait(file, &crc->wq, wait);
+
+	spin_lock_irq(&crc->lock);
+	if (crc->source && crtc_crc_data_count(crc))
+		ret = POLLIN | POLLRDNORM;
+	else
+		ret = 0;
+	spin_unlock_irq(&crc->lock);
+
+	return ret;
+}
+
 static const struct file_operations drm_crtc_crc_data_fops = {
 	.owner = THIS_MODULE,
 	.open = crtc_crc_open,
 	.read = crtc_crc_read,
+	.poll = crtc_crc_poll,
 	.release = crtc_crc_release,
 };