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>
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