The purpose of calling init_waitqueue_head()

Hi, I am trying to understanding a linux driver code here: https://android.googlesource.com/kernel/exynos/+/a6a1de6334417e0800440cdf971... In here https://android.googlesource.com/kernel/exynos/+/a6a1de6334417e0800440cdf971..., it calls 'init_waitqueue_head() where wq is a "wait_queue_head_t wq;" mutex_init(&ctrl->lock); init_waitqueue_head(&ctrl->wq); But I search the whole directory for wq usage, I don't find anywhere wq is used. Can you please tell me what is the purpose of calling "init_waitqueue_head"? Thank you.