TX_MAX_DESC
#define desc_reclaimable(q) ((int)((q)->processed - (q)->cleaned - TX_MAX_DESC))
#define TX_START_MAX_DESC (TX_MAX_DESC << 2) /* maximum number of descriptors
#define TX_CLEAN_MAX_DESC (TX_MAX_DESC << 4) /* maximum tx descriptors
if (txq->size - txq->in_use <= TX_MAX_DESC)
!TXQ_RING_NEEDS_ENQUEUE(qs) && avail > TX_MAX_DESC) {