PCI: Allow root and child buses to have different pci_ops

PCI host bridges often have different ways to access the root and child
bus config spaces. The host bridge drivers have invented their own
abstractions to handle this. Let's support having different root and
child bus pci_ops so these per driver abstractions can be removed.

Link: https://lore.kernel.org/r/20200821035420.380495-2-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/probe.c

@@ -1036,6 +1036,7 @@ static struct pci_bus *pci_alloc_child_bus(struct pci_bus *parent,
 					   struct pci_dev *bridge, int busnr)
 {
 	struct pci_bus *child;
+	struct pci_host_bridge *host;
 	int i;
 	int ret;
 
@@ -1045,11 +1046,16 @@ static struct pci_bus *pci_alloc_child_bus(struct pci_bus *parent,
 		return NULL;
 
 	child->parent = parent;
-	child->ops = parent->ops;
 	child->msi = parent->msi;
 	child->sysdata = parent->sysdata;
 	child->bus_flags = parent->bus_flags;
 
+	host = pci_find_host_bridge(parent);
+	if (host->child_ops)
+		child->ops = host->child_ops;
+	else
+		child->ops = parent->ops;
+
 	/*
 	 * Initialize some portions of the bus device, but don't register
 	 * it now as the parent is not properly set up yet.

include/linux/pci.h

@@ -523,6 +523,7 @@ struct pci_host_bridge {
 	struct device	dev;
 	struct pci_bus	*bus;		/* Root bus */
 	struct pci_ops	*ops;
+	struct pci_ops	*child_ops;
 	void		*sysdata;
 	int		busnr;
 	struct list_head windows;	/* resource_entry */