If we subscribe to an event with V4L2_EVENT_SUB_FL_SEND_INITIAL, the
driver needs to report back some values that require the camera to be
powered on. But VIDIOC_SUBSCRIBE_EVENT is not part of the ioctls that
turn on the camera.

We could unconditionally turn on the camera during
VIDIOC_SUBSCRIBE_EVENT, but it is more efficient to turn it on only
during V4L2_EVENT_SUB_FL_SEND_INITIAL, which we believe is not a common
usecase.

To avoid a list_del if uvc_pm_get() fails, we move list_add_tail to the
end of the function.

Reviewed-by: Hans de Goede <hansg@kernel.org>
Fixes: d1b618e79548 ("media: uvcvideo: Do not turn on the camera for some ioctls")
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 drivers/media/usb/uvc/uvc_ctrl.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
index 44b6513c526421943bb9841fb53dc5f8e9f93f02..ceff17d65c60761703f473f13d87c8f2b5f25129 100644
--- a/drivers/media/usb/uvc/uvc_ctrl.c
+++ b/drivers/media/usb/uvc/uvc_ctrl.c
@@ -2033,18 +2033,23 @@ static int uvc_ctrl_add_event(struct v4l2_subscribed_event *sev, unsigned elems)
 		goto done;
 	}
 
-	list_add_tail(&sev->node, &mapping->ev_subs);
 	if (sev->flags & V4L2_EVENT_SUB_FL_SEND_INITIAL) {
 		struct v4l2_event ev;
 		u32 changes = V4L2_EVENT_CTRL_CH_FLAGS;
 		s32 val = 0;
 
+		ret = uvc_pm_get(handle->chain->dev);
+		if (ret)
+			goto done;
+
 		if (uvc_ctrl_mapping_is_compound(mapping) ||
 		    __uvc_ctrl_get(handle->chain, ctrl, mapping, &val) == 0)
 			changes |= V4L2_EVENT_CTRL_CH_VALUE;
 
 		uvc_ctrl_fill_event(handle->chain, &ev, ctrl, mapping, val,
 				    changes);
+
+		uvc_pm_put(handle->chain->dev);
 		/*
 		 * Mark the queue as active, allowing this initial event to be
 		 * accepted.
@@ -2053,6 +2058,7 @@ static int uvc_ctrl_add_event(struct v4l2_subscribed_event *sev, unsigned elems)
 		v4l2_event_queue_fh(sev->fh, &ev);
 	}
 
+	list_add_tail(&sev->node, &mapping->ev_subs);
 done:
 	mutex_unlock(&handle->chain->ctrl_mutex);
 	return ret;

-- 
2.49.0.1266.g31b7d2e469-goog