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

[Bjorn Helgaas <bhelgaas@google.com>]

On Thu, Apr 20, 2017 at 5:51 AM, Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
> [+ Michael]
>
> On Wed, Apr 19, 2017 at 05:48:51PM +0100, 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@arm.com>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Bjorn Helgaas <bhelgaas@google.com>
>> Cc: Russell King <linux@armlinux.org.uk>
>> Cc: Catalin Marinas <catalin.marinas@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
>> +
>> +/*
>
> As an heads-up, this patch strictly depends on:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/arch/powerpc/include/asm/io.h?id=590c369e7ecc00be736be39ae0c62d1b5d563a51
>
> to go upstream first, otherwise we would break powerpc compilation
> (owing to powerpc including linux/io.h before ioremap_nocache() is
> defined in arch/powerpc/include/asm/io.h).
>
> If we want to decouple them I must drop the static inline and make
> it a #define, it is not ideal but we must be aware of this, I really
> want to prevent breakage if we go ahead with this set (and -next can
> hide the issue).

It looks like Stephen merges powerpc/next into linux-next before
pci/next, so this will probably be OK there.  I'll try to remember to
wait for the powerpc stuff to make it to Linus' tree during the merge
window before I send my pull request.

Bjorn