Therefore I can't change it to my (modified) sbus_mmio_map() function
because it would break other non-SPARC platforms, and AIUI there is nothing
in the memory API that allows me to move a subregion to a different
MemoryRegion parent, even if I can get a reference to it with
sysbus_mmio_get_region() after the sysbus_mmio_map() call - or have I
misunderstood something?
Init functions like esp_init should be purely convenience functions
which create, set properties on, and map the sysbus/qdev device. If
they make use of private knowledge about the internals of the device
then this is wrong and should be fixed. For esp_init() it looks like
the handling of dma_memory_read, dma_memory_write, dma_opaque, it_shift
and dma_enabled are wrong.
If you fix that then you can just ignore the convenience function,
and create, configure and map the device as appropriate for you.