powerpc/pseries/ddw: Do not try direct mapping with persistent memory and one window

There is a possibility of having just one DMA window available with
a limited capacity which the existing code does not handle that well.
If the window is big enough for the system RAM but less than
MAX_PHYSMEM_BITS (which we want when persistent memory is present),
we create 1:1 window and leave persistent memory without DMA.

This disables 1:1 mapping entirely if there is persistent memory and
either:
- the huge DMA window does not cover the entire address space;
- the default DMA window is removed.

This relies on reverted 54fc3c681d
("powerpc/pseries/ddw: Extend upper limit for huge DMA window for persistent memory")
to return the actual amount RAM in ddw_memory_hotplug_max() (posted
separately).

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211108040320.3857636-4-aik@ozlabs.ru

arch/powerpc/platforms/pseries/iommu.c

@@ -1356,8 +1356,10 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
 		len = order_base_2(query.largest_available_block << page_shift);
 		win_name = DMA64_PROPNAME;
 	} else {
-		direct_mapping = true;
-		win_name = DIRECT64_PROPNAME;
+		direct_mapping = !default_win_removed ||
+			(len == MAX_PHYSMEM_BITS) ||
+			(!pmem_present && (len == max_ram_len));
+		win_name = direct_mapping ? DIRECT64_PROPNAME : DMA64_PROPNAME;
 	}
 
 	ret = create_ddw(dev, ddw_avail, &create, page_shift, len);