linux/io.h: Add pci_remap_cfgspace() interface

The PCI specifications (Rev 3.0, 3.2.5 "Transaction Ordering and Posting")
mandate non-posted configuration transactions. As further highlighted in
the PCIe specifications (4.0 - Rev0.3, "Ordering Considerations for the
Enhanced Configuration Access Mechanism"), through ECAM and ECAM-derivative
configuration mechanism, the memory mapped transactions from the host CPU
into Configuration Requests on the PCI express fabric may create ordering
problems for software because writes to memory address are typically posted
transactions (unless the architecture can enforce through virtual address
mapping non-posted write transactions behaviour) but writes to
Configuration Space are not posted on the PCI express fabric.

Current DT and ACPI host bridge controllers map PCI configuration space
(ECAM and ECAM-derivative) into the virtual address space through ioremap()
calls, that are non-cacheable device accesses on most architectures, but
may provide "bufferable" or "posted" write semantics in architecture like
eg ARM/ARM64 that allow ioremap'ed regions writes to be buffered in the bus
connecting the host CPU to the PCI fabric; this behaviour, as underlined in
the PCIe specifications, may trigger transactions ordering rules and must
be prevented.

Introduce a new generic and explicit API to create a memory mapping for
ECAM and ECAM-derivative config space area that defaults to
ioremap_nocache() (which should provide a sane default behaviour) but still
allowing architectures on which ioremap_nocache() results in posted write
transactions to override the function call with an arch specific
implementation that complies with the PCI specifications for configuration
transactions.

[bhelgaas: fold in #ifdef CONFIG_PCI wrapper]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>

include/linux/io.h

@@ -90,6 +90,27 @@ void devm_memunmap(struct device *dev, void *addr);
 
 void *__devm_memremap_pages(struct device *dev, struct resource *res);
 
+#ifdef CONFIG_PCI
+/*
+ * The PCI specifications (Rev 3.0, 3.2.5 "Transaction Ordering and
+ * Posting") mandate non-posted configuration transactions. There is
+ * no ioremap API in the kernel that can guarantee non-posted write
+ * semantics across arches so provide a default implementation for
+ * mapping PCI config space that defaults to ioremap_nocache(); arches
+ * should override it if they have memory mapping implementations that
+ * guarantee non-posted writes semantics to make the memory mapping
+ * compliant with the PCI specification.
+ */
+#ifndef pci_remap_cfgspace
+#define pci_remap_cfgspace pci_remap_cfgspace
+static inline void __iomem *pci_remap_cfgspace(phys_addr_t offset,
+					       size_t size)
+{
+	return ioremap_nocache(offset, size);
+}
+#endif
+#endif
+
 /*
  * Some systems do not have legacy ISA devices.
  * /dev/port is not a valid interface on these systems.