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

[Lorenzo Pieralisi]

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@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.

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.

Thanks !
Lorenzo