PCI: Support populating MSI domains of root buses via bridges

Currently, at probing time, the MSI domains of root buses are populated
if either the information of MSI domain is available from firmware (DT
or ACPI), or arch-specific sysdata is used to pass the fwnode of the MSI
domain. These two conditions don't cover all, e.g. Hyper-V virtual PCI
on ARM64, which doesn't have the MSI information in the firmware and
couldn't use arch-specific sysdata because running on an architecture
with PCI_DOMAINS_GENERIC=y.

To support populating MSI domains of the root buses at the probing when
neither of the above condition is true, the ->msi_domain of the
corresponding bridge device is used: in pci_host_bridge_msi_domain(),
which should return the MSI domain of the root bus, the ->msi_domain of
the corresponding bridge is fetched first as a potential value of the
MSI domain of the root bus.

In order to use the approach to populate MSI domains, the driver needs
to dev_set_msi_domain() on the bridge before calling
pci_register_host_bridge(), and makes sure GENERIC_MSI_IRQ_DOMAIN=y.

Another advantage of this new approach is providing an arch-independent
way to populate MSI domains, which allows sharing the driver code as
much as possible between architectures.

Originally-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210726180657.142727-3-boqun.feng@gmail.com
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>

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