Memory mapping of config space of pcie devices

[Pratyush Anand]

On Sun, Dec 15, 2013 at 04:18:23AM +0800, Bjorn Helgaas wrote:
> [+cc devicetree, linux-arm-kernel]
> 
> On Sat, Dec 14, 2013 at 1:05 PM, shiv prakash Agarwal
> <chhotu.shiv at gmail.com> wrote:
> > On Sun, Dec 15, 2013 at 12:29 AM, Bjorn Helgaas <bhelgaas at google.com> wrote:
> >> On Sat, Dec 14, 2013 at 11:32 AM, shiv prakash Agarwal
> >> <chhotu.shiv at gmail.com> wrote:
> >>> Hi All,
> >>>
> >>> How devices config spaces are mapped to host memory?
> >>> Is it being handled by core driver? I could not locate.
> >>
> >> The PCI core does not map config space into memory.  That's not even
> >> possible for the legacy config access methods, e.g., using I/O ports
> >> 0xcf8 and 0xcfc [1].
> >>
> >> If you're wondering about how Linux uses ECAM, that's mostly in
> >> arch/x86/pci/mmconfig*.  That code does ioremap the ECAM area into
> >> kernel virtual space, but only for access via pci_read_config_word(),
> >> pci_write_config_word(), etc.
> >>
> >> Bjorn
> >>
> >> [1] http://en.wikipedia.org/wiki/PCI_configuration_space#Software_implementation
> >
> > Thanks Bjorn for quick reply,
> >
> > Above reference says:
> > During system initialization, firmware determines the base address for
> > this “stolen” address region and communicates it to the root complex
> > and to the operating system. This communication method is
> > implementation-specific, and not defined in the PCI Express
> > specification.
> >
> > How above is implemented on ARM?
> 
> On x86, we learn the ECAM address via ACPI (the MCFG table or a host
> bridge _CBA method).  I don't know how this is done on ARM.  I could
> imagine it being done via device tree, but I really don't know.
> Unfortunately both the address discovery and the actual ECAM accesses
> are in arch-specific code.

Yes, it comes from device tree. DT sends CPU address range allocated
for cfg transfers. pci_read_config_word and pci_write_config_word
send bdf and offset. Address translation unit of your RC driver should
be programmed in such a way that it translates cpu address to the cfg
address based on bdf and offset.

You can see one such implementation in drivers/pci/host/pcie-designware.c

Regards
Pratyush

> 
> Bjorn
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