Re: [PATCH 03/20] asm-generic/io.h: add PCI config space remap interface

[Bjorn Helgaas]

[+cc Luis]

On Mon, Feb 27, 2017 at 03:14:14PM +0000, Lorenzo Pieralisi wrote:
> 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@xxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> ---
>  include/asm-generic/io.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
> index 7ef015e..52dda81 100644
> --- a/include/asm-generic/io.h
> +++ b/include/asm-generic/io.h
> @@ -915,6 +915,15 @@ extern void ioport_unmap(void __iomem *p);
>  #endif /* CONFIG_GENERIC_IOMAP */
>  #endif /* CONFIG_HAS_IOPORT_MAP */
>  
> +#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);
> +}

I'm fine with this conceptually, but I think it would make more sense
if the name weren't specific to PCI or config space, e.g.,
ioremap_nopost() or something.

> +#endif
> +
>  #ifndef xlate_dev_kmem_ptr
>  #define xlate_dev_kmem_ptr xlate_dev_kmem_ptr
>  static inline void *xlate_dev_kmem_ptr(void *addr)
> -- 
> 2.10.0
>