PCI: Introduce domain_nr in pci_host_bridge

Currently we retrieve the PCI domain number of the host bridge from the
bus sysdata (or pci_config_window if PCI_DOMAINS_GENERIC=y). Actually
we have the information at PCI host bridge probing time, and it makes
sense that we store it into pci_host_bridge. One benefit of doing so is
the requirement for supporting PCI on Hyper-V for ARM64, because the
host bridge of Hyper-V doesn't have pci_config_window, whereas ARM64 is
a PCI_DOMAINS_GENERIC=y arch, so we cannot retrieve the PCI domain
number from pci_config_window on ARM64 Hyper-V guest.

As the preparation for ARM64 Hyper-V PCI support, we introduce the
domain_nr in pci_host_bridge and a sentinel value to allow drivers to
set domain numbers properly at probing time. Currently
CONFIG_PCI_DOMAINS_GENERIC=y archs are only users of this
newly-introduced field.

Link: https://lore.kernel.org/r/20210726180657.142727-2-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>

drivers/pci/probe.c

@@ -594,6 +594,7 @@ static void pci_init_host_bridge(struct pci_host_bridge *bridge)
 	bridge->native_pme = 1;
 	bridge->native_ltr = 1;
 	bridge->native_dpc = 1;
+	bridge->domain_nr = PCI_DOMAIN_NR_NOT_SET;
 
 	device_initialize(&bridge->dev);
 }
@@ -898,7 +899,10 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
 	bus->ops = bridge->ops;
 	bus->number = bus->busn_res.start = bridge->busnr;
 #ifdef CONFIG_PCI_DOMAINS_GENERIC
-	bus->domain_nr = pci_bus_find_domain_nr(bus, parent);
+	if (bridge->domain_nr == PCI_DOMAIN_NR_NOT_SET)
+		bus->domain_nr = pci_bus_find_domain_nr(bus, parent);
+	else
+		bus->domain_nr = bridge->domain_nr;
 #endif
 
 	b = pci_find_bus(pci_domain_nr(bus), bridge->busnr);

include/linux/pci.h

@@ -526,6 +526,16 @@ static inline int pci_channel_offline(struct pci_dev *pdev)
 	return (pdev->error_state != pci_channel_io_normal);
 }
 
+/*
+ * Currently in ACPI spec, for each PCI host bridge, PCI Segment
+ * Group number is limited to a 16-bit value, therefore (int)-1 is
+ * not a valid PCI domain number, and can be used as a sentinel
+ * value indicating ->domain_nr is not set by the driver (and
+ * CONFIG_PCI_DOMAINS_GENERIC=y archs will set it with
+ * pci_bus_find_domain_nr()).
+ */
+#define PCI_DOMAIN_NR_NOT_SET (-1)
+
 struct pci_host_bridge {
 	struct device	dev;
 	struct pci_bus	*bus;		/* Root bus */
@@ -533,6 +543,7 @@ struct pci_host_bridge {
 	struct pci_ops	*child_ops;
 	void		*sysdata;
 	int		busnr;
+	int		domain_nr;
 	struct list_head windows;	/* resource_entry */
 	struct list_head dma_ranges;	/* dma ranges resource list */
 	u8 (*swizzle_irq)(struct pci_dev *, u8 *); /* Platform IRQ swizzler */