I noticed that pcibios_add_pci_devices() test the platform type to
decide wether to do a device-tree based probe or a direct PCI probe. Why
can't it use ppc_md.probe_mode() like the rest of the PCI code does ?

I can't see a good reason either! :)  I'll have a patch in two shakes of
a lamb's tail.  

On a related note, I don't understand why devtree-based probe is only
desirable for the LPAR case (on pSeries).  

Also, do we anticipate future probe modes for new platforms or
something?  Adding such logic to ppc_md seems like mucho infrastructure
to answer a simple question (lpar or not).  For exmaple, _machine gets
used all over the pmac code.

John