SoC features such as GPIO are accessed via a reserved MMIO area,
we don't know its address but can obtain it from the BAR of
the P2SB device, that device is normally hidden so we have to
temporarily unhide it, read address and hide it back.

There are already a few users and at least one more is coming which
require an access to Primary to Sideband (P2SB) bridge in order
to get IO or MMIO BAR hidden by BIOS.

Create a library to access P2SB for x86 devices in a unified way.

Background information
======================
Note, the term "bridge" is used in the documentation and it has nothing
to do with a PCI (host) bridge as per the PCI specifications.

The P2SB is an interesting device by its nature and hardware design.
First of all, it has several devices in the hardware behind it. These
devices may or may not be represented as ACPI devices by a firmware.

It also has a hardwired (to 0s) the least significant bits of the
base address register which is represented by the only 64-bit BAR0.
It means that OS mustn't reallocate the BAR.

On top of that in some cases P2SB is represented by function 0 on PCI
slot (in terms of B:D.F) and according to the PCI specification any
other function can't be seen until function 0 is present and visible.

In the PCI configuration space of P2SB device the full 32-bit register
is allocated for the only purpose of hiding the entire P2SB device. As
per [3]:

  3.1.39 P2SB Control (P2SBC)—Offset E0h

  Hide Device (HIDE): When this bit is set, the P2SB will return 1s on
  any PCI Configuration Read on IOSF-P. All other transactions including
  PCI Configuration Writes on IOSF-P are unaffected by this. This does
  not affect reads performed on the IOSF-SB interface.

This doesn't prevent MMIO accesses, although preventing the OS from
assigning these addresses. The firmware on the affected platforms marks
the region as unusable (by cutting it off from the PCI host bridge
resources) as depicted in the Apollo Lake example below:

  PCI host bridge to bus 0000:00
  pci_bus 0000:00: root bus resource [io  0x0070-0x0077]
  pci_bus 0000:00: root bus resource [io  0x0000-0x006f window]
  pci_bus 0000:00: root bus resource [io  0x0078-0x0cf7 window]
  pci_bus 0000:00: root bus resource [io  0x0d00-0xffff window]
  pci_bus 0000:00: root bus resource [mem 0x7c000001-0x7fffffff window]
  pci_bus 0000:00: root bus resource [mem 0x7b800001-0x7bffffff window]
  pci_bus 0000:00: root bus resource [mem 0x80000000-0xcfffffff window]
  pci_bus 0000:00: root bus resource [mem 0xe0000000-0xefffffff window]
  pci_bus 0000:00: root bus resource [bus 00-ff]

The P2SB 16MB BAR is located at 0xd0000000-0xd0ffffff memory window.

The generic solution
====================
The generic solution for all cases when we need to access to the information
behind P2SB device is a library code where users ask for necessary resources
by demand and hence those users take care of not being run on the systems
where this access is not required.

The library provides the p2sb_bar() API to retrieve the MMIO of the BAR0 of
the device from P2SB device slot.

P2SB unconditional unhiding awareness
=====================================
Technically it's possible to unhide the P2SB device and devices on
the same PCI slot and access them at any time as needed. But there are
several potential issues with that:

 - the systems were never tested against such configuration and hence
   nobody knows what kind of bugs it may bring, especially when we talk
   about SPI NOR case which contains Intel FirmWare Image (IFWI) code
   (including BIOS) and already known to be problematic in the past for
   end users

 - the PCI by its nature is a hotpluggable bus and in case somebody
   attaches a driver to the functions of a P2SB slot device(s) the
   end user experience and system behaviour can be unpredictable

 - the kernel code would need some ugly hacks (or code looking as an
   ugly hack) under arch/x86/pci in order to enable these devices on
   only selected platforms (which may include CPU ID table followed by
   a potentially growing number of DMI strings

The future improvements
=======================
The future improvements with this code may go in order to gain some kind
of cache, if it's possible at all, to prevent unhiding and hiding many
times to take static information that may be saved once per boot.

Links
=====
[1]: https://lab.whitequark.org/notes/2017-11-08/accessing-intel-ich-pch-gpios/
[2]: https://cdrdv2.intel.com/v1/dl/getContent/332690?wapkw=332690
[3]: https://cdrdv2.intel.com/v1/dl/getContent/332691?wapkw=332691
[4]: https://medium.com/@jacksonchen_43335/bios-gpio-p2sb-70e9b829b403Signed-off-by: Jonathan Yong <jonathan.yong@intel.com>
Co-developed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Henning Schild <henning.schild@siemens.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>