MAX_NUMBER_OF_DESCRIPTORS  581 dev/pci/if_em_hw.h #define MAX_NUMBER_OF_DESCRIPTORS  0xFFF8
MAX_NUMBER_OF_DESCRIPTORS  112 dev/pci/ixgb_hw.h #define MAX_NUMBER_OF_DESCRIPTORS  0xFFF8	/* 13 bits in RDLEN/TDLEN, 128B