quoted

+void dcr_unmap_generic(dcr_host_t host, unsigned int dcr_c)
+{
+	if (host.type == NATIVE)
+		dcr_unmap_native(host.host.native, dcr_c);
+	else
+		dcr_unmap_mmio(host.host.mmio, dcr_c);

What happens if host.type == INVALID?  Same question for the other
accessors in dcr_*_generic.

I guess looking back on it, I assumed that MAP_OK would return 0, meaning that behavior was undefined,
but I agree it's probably safer to have some error reporting there...  There starts to become a speed tradeoff
at some point, which would make function pointers more attractive.  If the ioremap does fail, or the
dcr-access-method can't be determined, then dcr_unmap_mmio would probably SEGV anyway, although that's
not something I'd really want to rely on.  I'll put an error case in there.

quoted

+enum host_type_t {MMIO, NATIVE, INVALID};

Should these be DCR_HOST_MMIO, DCR_HOST_NATIVE, DCR_HOST_INVALID?

I worry about the generic nature of the names.

Also seems reasonable, 

Steve