[PATCH v4 02/21] linux/io.h: add PCI config space remap interface

[Lorenzo Pieralisi]

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.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Bjorn Helgaas <bhelgaas at google.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas at arm.com>
---
 include/linux/io.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/io.h b/include/linux/io.h
index 82ef36e..3934aba 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -91,6 +91,25 @@ void devm_memunmap(struct device *dev, void *addr);
 void *__devm_memremap_pages(struct device *dev, struct resource *res);
 
 /*
+ * 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
+
+/*
  * Some systems do not have legacy ISA devices.
  * /dev/port is not a valid interface on these systems.
  * So for those archs, <asm/io.h> should define the following symbol.
-- 
2.10.0