Here it is:
None if MWI is not enabled.
May be speed, but practicaly hardly observable. It is described in PCI specs.
If the hardware has this property why not using it? In any case that does not hurt.
pci_set_cacheline_size() is NOT exported, while pci_try_set_mwi() is and
sets (as a side effect) the cache line size.
It fails only if cache line size cannot be set. Then MWI is not enabled.
The device continues to work (nicely) as it did up to now.