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

[Arnd Bergmann <arnd@arndb.de>]

On Wed, Mar 22, 2017 at 4:04 PM, Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
> Hi Bjorn, Arnd,
>
> On Thu, Mar 16, 2017 at 04:12:43PM -0500, Bjorn Helgaas wrote:
>> [+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@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/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.
>
> I would like to respin shortly so I have to make a decision on the
> interface, either:
>
> (1) I keep it a PCI only interface (which means I can even move it to
>     include/linux/pci.h and arch*/include/asm/pci.h to override it)
>
> or
>
> (2) We add it as a generic ioremap_* interface (I am not sure though
>     that's really useful other than to map PCI config space, actually
>     the reasoning behind the naming was to limit its usage to PCI
>     config space mappings)
>
> I take it as Bjorn is keener on (2), just running a final check before
> putting v2 together to make progress.

I'm fine with either way.

       Arnd