It is found on a real machine, in its ACPI namespace, the IPMI
OperationRegions (in the ACPI000D - ACPI power meter) are not defined under
the IPMI system interface device (the IPI0001 with KCS type returned from
_IFT control method):
  Device (PMI0)
  {
      Name (_HID, "ACPI000D")  // _HID: Hardware ID
      OperationRegion (SYSI, IPMI, 0x0600, 0x0100)
      Field (SYSI, BufferAcc, Lock, Preserve)
      {
          AccessAs (BufferAcc, 0x01),
          Offset (0x58),
          SCMD,   8,
          GCMD,   8
      }

      OperationRegion (POWR, IPMI, 0x3000, 0x0100)
      Field (POWR, BufferAcc, Lock, Preserve)
      {
          AccessAs (BufferAcc, 0x01),
          Offset (0xB3),
          GPMM,   8
      }
  }

  Device (PCI0)
  {
      Device (ISA)
      {
          Device (NIPM)
          {
              Name (_HID, EisaId ("IPI0001"))  // _HID: Hardware ID
              Method (_IFT, 0, NotSerialized)  // _IFT: IPMI Interface Type
              {
                  Return (0x01)
              }
          }
      }
  }

Current ACPI_IPMI code registers IPMI operation region handler on a
per-device basis, so for the above namespace the IPMI operation region
handler is registered only under the scope of \_SB.PCI0.ISA.NIPM.  Thus
when an IPMI operation region field of \PMI0 is accessed, there are errors
reported on such platform:
  ACPI Error: No handlers for Region [IPMI]
  ACPI Error: Region IPMI(7) has no handler
The solution is to install an IPMI operation region handler from root node
so that every object that defines IPMI OperationRegion can get an address
space handler registered.

When an IPMI operation region field is accessed, the Network Function
(0x06 for SYSI and 0x30 for POWR) and the Command (SCMD, GCMD, GPMM) are
passed to the operation region handler, there is no system interface
specified by the BIOS.  The patch tries to select one system interface by
monitoring the system interface notification.  IPMI messages passed from
the ACPI codes are sent to this selected global IPMI system interface.

The ACPI_IPMI will always select the first registered IPMI interface
with an ACPI handle (i.e., defined in the ACPI namespace).  It's hard to
determine the selection when there are multiple IPMI system interfaces
defined in the ACPI namespace. According to the IPMI specification:

  A BMC device may make available multiple system interfaces, but only one
  management controller is allowed to be 'active' BMC that provides BMC
  functionality for the system (in case of a 'partitioned' system, there
  can be only one active BMC per partition).  Only the system interface(s)
  for the active BMC allowed to respond to the 'Get Device Id' command.

According to the ipmi_si desigin:

  The ipmi_si registeration notifications can only happen after a
  successful "Get Device ID" command.

Thus it should be OK for non-partitioned systems to do such selection.
However, we do not have much knowledge on 'partitioned' systems.

References: https://bugzilla.kernel.org/show_bug.cgi?id=46741Signed-off-by: NLv Zheng <lv.zheng@intel.com>
Reviewed-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>