+lib-$(CONFIG_HAS_DMA) += dma-virt.o

There probably should be a config option for it for two reasons:

 - do not bloat kernels that don't need it.
 - the feature can only work for 32-bit architectures or for
   64-bit architectures that set ARCH_DMA_ADDR_T_64BIT…
   Altenatiely this option would have to force
   ARCH_DMA_ADDR_T_64BIT when not yet set for 64-bit architectures.

And yes, this is currently broken already for, but we'd better fix it.