Single head:
------------
Breakpoint 2, xf86EnableAccess (pScrn=0x82ac5c0) at xf86Bus.c:694
694         register EntityAccessPtr peAcc = (EntityAccessPtr) pScrn->access;
(gdb) step
707         if (!pScrn->access) {
(gdb)
716         switch (pScrn->resourceType) {
(gdb)
744             pceAcc = pScrn->CurrentAccess->pIoAccess;
(gdb)
745             if (peAcc != pceAcc) { /* current Io != pAccess */
(gdb)
765                 pceAcc = pScrn->CurrentAccess->pMemAccess;
(gdb)
766                 if (pceAcc == peAcc) { /* current Mem == pAccess */
(gdb)
767                     return;
(gdb)
824     }

Dual head:
----------
Breakpoint 1, xf86EnableAccess (pScrn=0x82b0a30) at xf86Bus.c:694
694         register EntityAccessPtr peAcc = (EntityAccessPtr) pScrn->access;
(gdb) step
707         if (!pScrn->access) {
(gdb)
708             if (pScrn->busAccess) {
(gdb)
710                 ((BusAccPtr)pScrn->busAccess)->set_f(pScrn->busAccess);
(gdb)
pciSetBusAccess (ptr=0x82b0170) at xf86pciBus.c:661
661         if (!ptr->primary && !ptr->current)
(gdb)
662             return;
(gdb)
679     }
(gdb)
xf86EnableAccess (pScrn=0x82b0a30) at xf86Bus.c:713
713             return;
(gdb)
824     }