$ curl -s https://raw.githubusercontent.com/freebsd/freebsd-src/release/9.0.0/sys/dev/agp/agp_amd64.c | grep -n "ATTBASE\|GARTCTRL"
467:	pci_cfgregwrite(0, 1, 0, AGP3_VIA_ATTBASE, sc->gatt->ag_physical, 4);
468:	pci_cfgregwrite(0, 1, 0, AGP3_VIA_GARTCTRL,
469:	    pci_cfgregread(0, 1, 0, AGP3_VIA_ATTBASE, 4) | 0x180, 4);
$ curl -s https://raw.githubusercontent.com/freebsd/freebsd-src/stable/12/sys/dev/agp/agp_amd64.c | grep -n "ATTBASE\|GARTCTRL"
464:	pci_cfgregwrite(0, 1, 0, AGP3_VIA_ATTBASE, sc->gatt->ag_physical, 4);
465:	pci_cfgregwrite(0, 1, 0, AGP3_VIA_GARTCTRL,
466:	    pci_cfgregread(0, 1, 0, AGP3_VIA_ATTBASE, 4) | 0x180, 4);
=> identical bug upstream; DF inherited verbatim from FreeBSD (header $FreeBSD ... v 1.17 2009/01/12)