media: uvcvideo: Turn on the camera if V4L2_EVENT_SUB_FL_SEND_INITIAL

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: d1b618e ("media: uvcvideo: Do not turn on the camera for some ioctls")
Cc: stable@vger.kernel.org
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://lore.kernel.org/r/20250701-uvc-grannular-invert-v4-5-8003b9b89f68@chromium.org
Signed-off-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>

Author: ribalda

drivers/media/usb/uvc/uvc_ctrl.c

@@ -2072,18 +2072,24 @@ 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.
@@ -2092,6 +2098,8 @@ 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;