powerpc/pci: Add config option for using all 256 PCI buses

By default on PPC32 PCI bus numbers are unique across all PCI domains.
So a system could have only 256 PCI buses independently of available PCI
domains.

This is due to filling DT property pci-OF-bus-map which does not support
a multi-domain setup.

On all powerpc platforms except chrp and powermac there is no DT
property pci-OF-bus-map anymore and therefore it is possible on
non-chrp/powermac platforms to avoid this limitation of maximum number
of 256 PCI buses in a system even on multi-domain setup.

But avoiding this limitation would mean that all PCI and PCIe devices
would be present on completely different BDF addresses as every PCI
domain starts numbering PCI bueses from zero (instead of the last bus
number of previous enumerated PCI domain). Such change could break
existing software which expects fixed PCI bus numbers.

So add a new config option CONFIG_PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT which
enables this change. By default it is disabled. It causes the initial
value of hose->first_busno to be zero.

Signed-off-by: Pali Rohár <pali@kernel.org>
[mpe: Minor change log wording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220706104308.5390-6-pali@kernel.org

arch/powerpc/Kconfig

@@ -375,6 +375,17 @@ config PPC_DCR
 	depends on PPC_DCR_NATIVE || PPC_DCR_MMIO
 	default y
 
+config PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT
+	depends on PPC32
+	depends on !PPC_PMAC && !PPC_CHRP
+	bool "Assign PCI bus numbers from zero individually for each PCI domain"
+	help
+	  By default on PPC32 were PCI bus numbers unique across all PCI domains.
+	  So system could have only 256 PCI buses independently of available
+	  PCI domains. When this option is enabled then PCI bus numbers are
+	  PCI domain dependent and each PCI controller on own domain can have
+	  256 PCI buses, like it is on other Linux architectures.
+
 config PPC_OF_PLATFORM_PCI
 	bool
 	depends on PCI

arch/powerpc/kernel/pci_32.c

@@ -239,7 +239,9 @@ void pcibios_setup_phb_io_space(struct pci_controller *hose)
 static int __init pcibios_init(void)
 {
 	struct pci_controller *hose, *tmp;
+#ifndef CONFIG_PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT
 	int next_busno = 0;
+#endif
 
 	printk(KERN_INFO "PCI: Probing PCI hardware\n");
 
@@ -248,13 +250,17 @@ static int __init pcibios_init(void)
 
 	/* Scan all of the recorded PCI controllers.  */
 	list_for_each_entry_safe(hose, tmp, &hose_list, list_node) {
+#ifndef CONFIG_PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT
 		if (pci_assign_all_buses)
 			hose->first_busno = next_busno;
+#endif
 		hose->last_busno = 0xff;
 		pcibios_scan_phb(hose);
 		pci_bus_add_devices(hose->bus);
+#ifndef CONFIG_PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT
 		if (pci_assign_all_buses || next_busno <= hose->last_busno)
 			next_busno = hose->last_busno + pcibios_assign_bus_offset;
+#endif
 	}
 
 #if defined(CONFIG_PPC_PMAC) || defined(CONFIG_PPC_CHRP)