MAXPHYS
#ifndef MAXPHYS
#if MAXPHYS < 512 * 1024
#define AHCI_SG_ENTRIES (roundup(btoc(MAXPHYS) + 1, 8))
len = MIN(br->br_resid, MAXPHYS);
len = MIN(br->br_resid, MAXPHYS);
buf = malloc(MAXPHYS);