PCI: Introduce PCI bridge emulated config space common logic

Some PCI host controllers do not expose a configuration space for the
root port PCI bridge. Due to this, the Marvell Armada 370/38x/XP PCI
controller driver (pci-mvebu) emulates a root port PCI bridge
configuration space, and uses that to (among other things) dynamically
create the memory windows that correspond to the PCI MEM and I/O
regions.

Since we now need to add a very similar logic for the Marvell Armada
37xx PCI controller driver (pci-aardvark), instead of duplicating the
code, we create in this commit a common logic called pci-bridge-emul.

The idea of this logic is to emulate a root port PCI bridge
configuration space by providing configuration space read/write
operations, and faking behind the scenes the configuration space of a
PCI bridge. A PCI host controller driver simply has to call
pci_bridge_emul_conf_read() and pci_bridge_emul_conf_write() to
read/write the configuration space of the bridge.

By default, the PCI bridge configuration space is simply emulated by a
chunk of memory, but the PCI host controller can override the behavior
of the read and write operations on a per-register basis to do
additional actions if needed. We take care of complying with the
behavior of the PCI configuration space registers in terms of bits
that are read-write, read-only, reserved and write-1-to-clear.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>

drivers/pci/Kconfig

@@ -98,6 +98,9 @@ config PCI_ECAM
 config PCI_LOCKLESS_CONFIG
 	bool
 
+config PCI_BRIDGE_EMUL
+	bool
+
 config PCI_IOV
 	bool "PCI IOV support"
 	depends on PCI

drivers/pci/Makefile

@@ -19,6 +19,7 @@ obj-$(CONFIG_HOTPLUG_PCI)	+= hotplug/
 obj-$(CONFIG_PCI_MSI)		+= msi.o
 obj-$(CONFIG_PCI_ATS)		+= ats.o
 obj-$(CONFIG_PCI_IOV)		+= iov.o
+obj-$(CONFIG_PCI_BRIDGE_EMUL)	+= pci-bridge-emul.o
 obj-$(CONFIG_ACPI)		+= pci-acpi.o
 obj-$(CONFIG_PCI_LABEL)		+= pci-label.o
 obj-$(CONFIG_X86_INTEL_MID)	+= pci-mid.o