But ok, let me try to explain. The 256 MB value that you define in the
DT is the global PCIe memory aperture: it is the maximum amount of
memory that we allow the PCIe driver to allocate for PCIe windows. But
depending on which PCIe devices you have plugged in, and how large
their BARs are, not necessarily all of these 256 MB will be used.

So, you can very well have a 256 MB global PCIe memory aperture, and
still have only one 1 MB PCIe memory window for PCIe 0.0 and a 256 KB
PCIe memory window for PCIe 1.0, and that's it.

Now, the 192 MB comes from the enumeration of your device. Linux
enumerates the BAR of your device:

pci 0000:01:00.0: BAR 1: assigned [mem 0xe0000000-0xe7ffffff]
pci 0000:01:00.0: BAR 3: assigned [mem 0xe8000000-0xe87fffff]
pci 0000:01:00.0: BAR 4: assigned [mem 0xe8800000-0xe8801fff]
pci 0000:01:00.0: BAR 0: assigned [mem 0xe8802000-0xe8802fff]
pci 0000:01:00.0: BAR 2: assigned [mem 0xe8803000-0xe8803fff]
pci 0000:01:00.0: BAR 5: assigned [mem 0xe8804000-0xe8804fff]

and then concludes that at the emulated bridge level, the memory region
to be created is:

pci 0000:00:01.0:   bridge window [mem 0xe0000000-0xebffffff]

which corresponds to the 192 MB window that we see created.

But I believe a 192 MB memory window cannot work with MBus, it should
be rounded up to the next power of 2. Can you try the below patch (not
tested, not even compiled, might need some tweaks to apply to your 3.10
kernel) :

diff --git a/drivers/pci/host/pci-mvebu.c b/drivers/pci/host/pci-mvebu.c
index 13478ec..002229a 100644
--- a/drivers/pci/host/pci-mvebu.c
+++ b/drivers/pci/host/pci-mvebu.c

@@ -372,6 +372,11 @@ static void mvebu_pcie_handle_membase_change(struct mvebu_pcie_port *port)
                 (((port->bridge.memlimit & 0xFFF0) << 16) | 0xFFFFF) -
                 port->memwin_base;

+       pr_info("PCIE %d.%d: creating window at 0x%x, size 0x%x rounded up to 0x%x\n",
+               port->port, port->lane, port->memwin_base,
+               port->memwin_size, roundup_pow_of_two(port->memwin_size));
+       port->memwin_size = roundup_pow_of_two(port->memwin_size);
+
         mvebu_mbus_add_window_by_id(port->mem_target, port->mem_attr,
                                     port->memwin_base, port->memwin_size);
  }

I'm obviously interested in seeing the message that gets shown, as well
as the new mvebu-mbus debugfs output.