These days most OpenBSD users should have the machdep.allowaperture sysctl set to 0 (the default). Having it set to seomething else poses security risks and can actually cause problems, in particular on systems that have multiple GPUs where one of tha GPUs is supported by inteldrm(4) or radeondrm(4) and the other isn't.
You'll only need to set machdep.allowaperture to a non-zero value if inteldrm(4) or radeordrm(4) doesn't attach on your machine and you can't use efifb(4) either.
