ATA_AHCI_DMA_ENTRIES
struct ata_ahci_dma_prd prd_tab[ATA_AHCI_DMA_ENTRIES];
KASSERT(nsegs <= ATA_AHCI_DMA_ENTRIES, ("too many DMA segment entries\n"));
ch->dma->max_iosize = (ATA_AHCI_DMA_ENTRIES - 1) * PAGE_SIZE;