The credit interface is relevant when a queue’s fcrd_en context bit is set. It allows the user logic to prioritize and
meter descriptors fetched for each queue. You can specify the DMA direction, qid, and
credit value. For a typical use case, the descriptor engine uses credit inputs to fetch
descriptors. Internally, credits received and consumed are tracked for each queue. If
credits are added when the queue is not enabled, the credits will be returned through
the Traffic Manager Output Interface with tm_dsc_sts_qinv asserted, and the credits in tm_dsc_sts_avl are not valid. Monitor tm_dsc_sts interface to keep an account for each queue on how many credits
are consumed.