ACPI Tables
  -----------
  The expectations of individual ACPI tables are discussed in the list that
  follows.
  
  If a section number is used, it refers to a section number in the ACPI
  specification where the object is defined.  If "Signature Reserved" is used,
  the table signature (the first four bytes of the table) is the only portion
  of the table recognized by the specification, and the actual table is defined
  outside of the UEFI Forum (see Section 5.2.6 of the specification).
  
  For ACPI on arm64, tables also fall into the following categories:
  
         -- Required: DSDT, FADT, GTDT, MADT, MCFG, RSDP, SPCR, XSDT

         -- Recommended: BERT, EINJ, ERST, HEST, PCCT, SSDT

         -- Optional: BGRT, CPEP, CSRT, DBG2, DRTM, ECDT, FACS, FPDT, IORT,
            MCHI, MPST, MSCT, NFIT, PMTT, RASF, SBST, SLIT, SPMI, SRAT, STAO,
  	  TCPA, TPM2, UEFI, XENV

         -- Not supported: BOOT, DBGP, DMAR, ETDT, HPET, IBFT, IVRS, LPIT,
            MSDM, OEMx, PSDT, RSDT, SLIC, WAET, WDAT, WDRT, WPBT

  
  Table  Usage for ARMv8 Linux
  -----  ----------------------------------------------------------------
  BERT   Section 18.3 (signature == "BERT")
         == Boot Error Record Table ==
         Must be supplied if RAS support is provided by the platform.  It
         is recommended this table be supplied.
  
  BOOT   Signature Reserved (signature == "BOOT")
         == simple BOOT flag table ==
         Microsoft only table, will not be supported.
  
  BGRT   Section 5.2.22 (signature == "BGRT")
         == Boot Graphics Resource Table ==
         Optional, not currently supported, with no real use-case for an
         ARM server.
  
  CPEP   Section 5.2.18 (signature == "CPEP")
         == Corrected Platform Error Polling table ==
         Optional, not currently supported, and not recommended until such
         time as ARM-compatible hardware is available, and the specification
         suitably modified.
  
  CSRT   Signature Reserved (signature == "CSRT")
         == Core System Resources Table ==
         Optional, not currently supported.
  
  DBG2   Signature Reserved (signature == "DBG2")
         == DeBuG port table 2 ==

         License has changed and should be usable.  Optional if used instead
         of earlycon=<device> on the command line.

  
  DBGP   Signature Reserved (signature == "DBGP")
         == DeBuG Port table ==
         Microsoft only table, will not be supported.
  
  DSDT   Section 5.2.11.1 (signature == "DSDT")
         == Differentiated System Description Table ==
         A DSDT is required; see also SSDT.
  
         ACPI tables contain only one DSDT but can contain one or more SSDTs,
         which are optional.  Each SSDT can only add to the ACPI namespace,
         but cannot modify or replace anything in the DSDT.
  
  DMAR   Signature Reserved (signature == "DMAR")
         == DMA Remapping table ==
         x86 only table, will not be supported.
  
  DRTM   Signature Reserved (signature == "DRTM")
         == Dynamic Root of Trust for Measurement table ==
         Optional, not currently supported.
  
  ECDT   Section 5.2.16 (signature == "ECDT")
         == Embedded Controller Description Table ==
         Optional, not currently supported, but could be used on ARM if and
         only if one uses the GPE_BIT field to represent an IRQ number, since
         there are no GPE blocks defined in hardware reduced mode.  This would
         need to be modified in the ACPI specification.
  
  EINJ   Section 18.6 (signature == "EINJ")
         == Error Injection table ==
         This table is very useful for testing platform response to error
         conditions; it allows one to inject an error into the system as
         if it had actually occurred.  However, this table should not be
         shipped with a production system; it should be dynamically loaded
         and executed with the ACPICA tools only during testing.
  
  ERST   Section 18.5 (signature == "ERST")
         == Error Record Serialization Table ==
         On a platform supports RAS, this table must be supplied if it is not
         UEFI-based; if it is UEFI-based, this table may be supplied. When this
         table is not present, UEFI run time service will be utilized to save
         and retrieve hardware error information to and from a persistent store.
  
  ETDT   Signature Reserved (signature == "ETDT")
         == Event Timer Description Table ==
         Obsolete table, will not be supported.
  
  FACS   Section 5.2.10 (signature == "FACS")
         == Firmware ACPI Control Structure ==
         It is unlikely that this table will be terribly useful.  If it is
         provided, the Global Lock will NOT be used since it is not part of
         the hardware reduced profile, and only 64-bit address fields will
         be considered valid.
  
  FADT   Section 5.2.9 (signature == "FACP")
         == Fixed ACPI Description Table ==
         Required for arm64.
  
         The HW_REDUCED_ACPI flag must be set.  All of the fields that are
         to be ignored when HW_REDUCED_ACPI is set are expected to be set to
         zero.
  
         If an FACS table is provided, the X_FIRMWARE_CTRL field is to be
         used, not FIRMWARE_CTRL.
  
         If PSCI is used (as is recommended), make sure that ARM_BOOT_ARCH is
         filled in properly -- that the PSCI_COMPLIANT flag is set and that
         PSCI_USE_HVC is set or unset as needed (see table 5-37).
  
         For the DSDT that is also required, the X_DSDT field is to be used,
         not the DSDT field.
  
  FPDT   Section 5.2.23 (signature == "FPDT")
         == Firmware Performance Data Table ==
         Optional, not currently supported.
  
  GTDT   Section 5.2.24 (signature == "GTDT")
         == Generic Timer Description Table ==
         Required for arm64.
  
  HEST   Section 18.3.2 (signature == "HEST")
         == Hardware Error Source Table ==

         ARM-specific error sources have been defined; please use those or the
         PCI types such as type 6 (AER Root Port), 7 (AER Endpoint), or 8 (AER
         Bridge), or use type 9 (Generic Hardware Error Source).  Firmware first
         error handling is possible if and only if Trusted Firmware is being
         used on arm64.

  
         Must be supplied if RAS support is provided by the platform.  It
         is recommended this table be supplied.
  
  HPET   Signature Reserved (signature == "HPET")
         == High Precision Event timer Table ==
         x86 only table, will not be supported.
  
  IBFT   Signature Reserved (signature == "IBFT")
         == iSCSI Boot Firmware Table ==
         Microsoft defined table, support TBD.

  IORT   Signature Reserved (signature == "IORT")
         == Input Output Remapping Table ==
         arm64 only table, required in order to describe IO topology, SMMUs,
         and GIC ITSs, and how those various components are connected together,
         such as identifying which components are behind which SMMUs/ITSs.
         This table will only be required on certain SBSA platforms (e.g.,
         when using GICv3-ITS and an SMMU); on SBSA Level 0 platforms, it 
         remains optional.

  IVRS   Signature Reserved (signature == "IVRS")
         == I/O Virtualization Reporting Structure ==
         x86_64 (AMD) only table, will not be supported.
  
  LPIT   Signature Reserved (signature == "LPIT")
         == Low Power Idle Table ==

         x86 only table as of ACPI 5.1; starting with ACPI 6.0, processor
         descriptions and power states on ARM platforms should use the DSDT
         and define processor container devices (_HID ACPI0010, Section 8.4,
         and more specifically 8.4.3 and and 8.4.4).

  
  MADT   Section 5.2.12 (signature == "APIC")
         == Multiple APIC Description Table ==
         Required for arm64.  Only the GIC interrupt controller structures

         should be used (types 0xA - 0xF).

  
  MCFG   Signature Reserved (signature == "MCFG")
         == Memory-mapped ConFiGuration space ==
         If the platform supports PCI/PCIe, an MCFG table is required.
  
  MCHI   Signature Reserved (signature == "MCHI")
         == Management Controller Host Interface table ==
         Optional, not currently supported.
  
  MPST   Section 5.2.21 (signature == "MPST")
         == Memory Power State Table ==
         Optional, not currently supported.

  MSCT   Section 5.2.19 (signature == "MSCT")
         == Maximum System Characteristic Table ==
         Optional, not currently supported.

  MSDM   Signature Reserved (signature == "MSDM")
         == Microsoft Data Management table ==
         Microsoft only table, will not be supported.

  NFIT   Section 5.2.25 (signature == "NFIT")
         == NVDIMM Firmware Interface Table ==
         Optional, not currently supported.
  
  OEMx   Signature of "OEMx" only
         == OEM Specific Tables ==
         All tables starting with a signature of "OEM" are reserved for OEM
         use.  Since these are not meant to be of general use but are limited
         to very specific end users, they are not recommended for use and are
         not supported by the kernel for arm64.
  
  PCCT   Section 14.1 (signature == "PCCT)
         == Platform Communications Channel Table ==
         Recommend for use on arm64; use of PCC is recommended when using CPPC
         to control performance and power for platform processors.
  
  PMTT   Section 5.2.21.12 (signature == "PMTT")
         == Platform Memory Topology Table ==

         Optional, not currently supported.

  PSDT   Section 5.2.11.3 (signature == "PSDT")
         == Persistent System Description Table ==
         Obsolete table, will not be supported.

  RASF   Section 5.2.20 (signature == "RASF")
         == RAS Feature table ==
         Optional, not currently supported.
  
  RSDP   Section 5.2.5 (signature == "RSD PTR")
         == Root System Description PoinTeR ==
         Required for arm64.
  
  RSDT   Section 5.2.7 (signature == "RSDT")
         == Root System Description Table ==
         Since this table can only provide 32-bit addresses, it is deprecated

         on arm64, and will not be used.  If provided, it will be ignored.

  
  SBST   Section 5.2.14 (signature == "SBST")
         == Smart Battery Subsystem Table ==
         Optional, not currently supported.
  
  SLIC   Signature Reserved (signature == "SLIC")
         == Software LIcensing table ==
         Microsoft only table, will not be supported.
  
  SLIT   Section 5.2.17 (signature == "SLIT")
         == System Locality distance Information Table ==
         Optional in general, but required for NUMA systems.
  
  SPCR   Signature Reserved (signature == "SPCR")
         == Serial Port Console Redirection table ==
         Required for arm64.
  
  SPMI   Signature Reserved (signature == "SPMI")
         == Server Platform Management Interface table ==
         Optional, not currently supported.
  
  SRAT   Section 5.2.16 (signature == "SRAT")
         == System Resource Affinity Table ==
         Optional, but if used, only the GICC Affinity structures are read.

         To support arm64 NUMA, this table is required.

  
  SSDT   Section 5.2.11.2 (signature == "SSDT")
         == Secondary System Description Table ==
         These tables are a continuation of the DSDT; these are recommended
         for use with devices that can be added to a running system, but can
         also serve the purpose of dividing up device descriptions into more
         manageable pieces.
  
         An SSDT can only ADD to the ACPI namespace.  It cannot modify or
         replace existing device descriptions already in the namespace.
  
         These tables are optional, however.  ACPI tables should contain only
         one DSDT but can contain many SSDTs.

  STAO   Signature Reserved (signature == "STAO")
         == _STA Override table ==
         Optional, but only necessary in virtualized environments in order to
         hide devices from guest OSs.

  TCPA   Signature Reserved (signature == "TCPA")
         == Trusted Computing Platform Alliance table ==
         Optional, not currently supported, and may need changes to fully
         interoperate with arm64.
  
  TPM2   Signature Reserved (signature == "TPM2")
         == Trusted Platform Module 2 table ==
         Optional, not currently supported, and may need changes to fully
         interoperate with arm64.
  
  UEFI   Signature Reserved (signature == "UEFI")
         == UEFI ACPI data table ==
         Optional, not currently supported.  No known use case for arm64,
         at present.
  
  WAET   Signature Reserved (signature == "WAET")
         == Windows ACPI Emulated devices Table ==
         Microsoft only table, will not be supported.
  
  WDAT   Signature Reserved (signature == "WDAT")
         == Watch Dog Action Table ==
         Microsoft only table, will not be supported.
  
  WDRT   Signature Reserved (signature == "WDRT")
         == Watch Dog Resource Table ==
         Microsoft only table, will not be supported.
  
  WPBT   Signature Reserved (signature == "WPBT")
         == Windows Platform Binary Table ==
         Microsoft only table, will not be supported.

  XENV   Signature Reserved (signature == "XENV")
         == Xen project table ==
         Optional, used only by Xen at present.

  XSDT   Section 5.2.8 (signature == "XSDT")
         == eXtended System Description Table ==
         Required for arm64.
  
  
  ACPI Objects
  ------------

  The expectations on individual ACPI objects that are likely to be used are
  shown in the list that follows; any object not explicitly mentioned below
  should be used as needed for a particular platform or particular subsystem,
  such as power management or PCI.

  
  Name   Section         Usage for ARMv8 Linux
  ----   ------------    -------------------------------------------------

  _CCA   6.2.17          This method must be defined for all bus masters
                         on arm64 -- there are no assumptions made about
                         whether such devices are cache coherent or not.
                         The _CCA value is inherited by all descendants of
                         these devices so it does not need to be repeated.
                         Without _CCA on arm64, the kernel does not know what
                         to do about setting up DMA for the device.

                         NB: this method provides default cache coherency
                         attributes; the presence of an SMMU can be used to
                         modify that, however.  For example, a master could
                         default to non-coherent, but be made coherent with
                         the appropriate SMMU configuration (see Table 17 of
                         the IORT specification, ARM Document DEN 0049B).

  _CID   6.1.2           Use as needed, see also _HID.

  _CLS   6.1.3           Use as needed, see also _HID.

  _CPC   8.4.7.1         Use as needed, power management specific.  CPPC is
                         recommended on arm64.

  
  _CRS   6.2.2           Required on arm64.

  _CSD   8.4.2.2         Use as needed, used only in conjunction with _CST.
  
  _CST   8.4.2.1         Low power idle states (8.4.4) are recommended instead
                         of C-states.

  
  _DDN   6.1.4           This field can be used for a device name.  However,
                         it is meant for DOS device names (e.g., COM1), so be
                         careful of its use across OSes.

  _DSD   6.2.5           To be used with caution.  If this object is used, try
                         to use it within the constraints already defined by the
                         Device Properties UUID.  Only in rare circumstances
                         should it be necessary to create a new _DSD UUID.
  
                         In either case, submit the _DSD definition along with
                         any driver patches for discussion, especially when
                         device properties are used.  A driver will not be
                         considered complete without a corresponding _DSD
                         description.  Once approved by kernel maintainers,
                         the UUID or device properties must then be registered
                         with the UEFI Forum; this may cause some iteration as
                         more than one OS will be registering entries.

  _DSM   9.1.1           Do not use this method.  It is not standardized, the

                         return values are not well documented, and it is
                         currently a frequent source of error.

  \_GL   5.7.1           This object is not to be used in hardware reduced
                         mode, and therefore should not be used on arm64.
  
  _GLK   6.5.7           This object requires a global lock be defined; there
                         is no global lock on arm64 since it runs in hardware
                         reduced mode.  Hence, do not use this object on arm64.
  
  \_GPE  5.3.1           This namespace is for x86 use only.  Do not use it
                         on arm64.

  _HID   6.1.5           This is the primary object to use in device probing,
  		       though _CID and _CLS may also be used.

  
  _INI   6.5.1           Not required, but can be useful in setting up devices
                         when UEFI leaves them in a state that may not be what
                         the driver expects before it starts probing.

  _LPI   8.4.4.3         Recommended for use with processor definitions (_HID
  		       ACPI0010) on arm64.  See also _RDI.

  _MLS   6.1.7           Highly recommended for use in internationalization.

  _OFF   7.2.2           It is recommended to define this method for any device

                         that can be turned on or off.

  _ON    7.2.3           It is recommended to define this method for any device

                         that can be turned on or off.
  
  \_OS   5.7.3           This method will return "Linux" by default (this is
                         the value of the macro ACPI_OS_NAME on Linux).  The
                         command line parameter acpi_os=<string> can be used
                         to set it to some other value.
  
  _OSC   6.2.11          This method can be a global method in ACPI (i.e.,
                         \_SB._OSC), or it may be associated with a specific
                         device (e.g., \_SB.DEV0._OSC), or both.  When used
                         as a global method, only capabilities published in
                         the ACPI specification are allowed.  When used as
                         a device-specific method, the process described for
                         using _DSD MUST be used to create an _OSC definition;
                         out-of-process use of _OSC is not allowed.  That is,
                         submit the device-specific _OSC usage description as
                         part of the kernel driver submission, get it approved
                         by the kernel community, then register it with the
                         UEFI Forum.

  \_OSI  5.7.2           Deprecated on ARM64.  As far as ACPI firmware is 
  		       concerned, _OSI is not to be used to determine what 
  		       sort of system is being used or what functionality
  		       is provided.  The _OSC method is to be used instead.

  
  _PDC   8.4.1           Deprecated, do not use on arm64.
  
  \_PIC  5.8.1           The method should not be used.  On arm64, the only
                         interrupt model available is GIC.

  \_PR   5.3.1           This namespace is for x86 use only on legacy systems.
                         Do not use it on arm64.

  _PRT   6.2.13          Required as part of the definition of all PCI root
                         devices.

  _PRx   7.3.8-11        Use as needed; power management specific.  If _PR0 is

                         defined, _PR3 must also be defined.

  _PSx   7.3.2-5         Use as needed; power management specific.  If _PS0 is

                         defined, _PS3 must also be defined.  If clocks or
                         regulators need adjusting to be consistent with power
                         usage, change them in these methods.

  _RDI   8.4.4.4         Recommended for use with processor definitions (_HID
  		       ACPI0010) on arm64.  This should only be used in 
  		       conjunction with _LPI.

  
  \_REV  5.7.4           Always returns the latest version of ACPI supported.

  \_SB   5.3.1           Required on arm64; all devices must be defined in this
                         namespace.

  _SLI   6.2.15          Use is recommended when SLIT table is in use.

  
  _STA   6.3.7,          It is recommended to define this method for any device

         7.2.4           that can be turned on or off.  See also the STAO table
                         that provides overrides to hide devices in virtualized
                         environments.

  _SRS   6.2.16          Use as needed; see also _PRS.

  
  _STR   6.1.10          Recommended for conveying device names to end users;
                         this is preferred over using _DDN.
  
  _SUB   6.1.9           Use as needed; _HID or _CID are preferred.

  _SUN   6.1.11          Use as needed, but recommended.

  _SWS   7.4.3           Use as needed; power management specific; this may

                         require specification changes for use on arm64.

  _UID   6.1.12          Recommended for distinguishing devices of the same
                         class; define it if at all possible.

  
  
  ACPI Event Model
  ----------------
  Do not use GPE block devices; these are not supported in the hardware reduced
  profile used by arm64.  Since there are no GPE blocks defined for use on ARM

  platforms, ACPI events must be signaled differently.
  
  There are two options: GPIO-signaled interrupts (Section 5.6.5), and
  interrupt-signaled events (Section 5.6.9).  Interrupt-signaled events are a
  new feature in the ACPI 6.1 specification.  Either -- or both -- can be used
  on a given platform, and which to use may be dependent of limitations in any
  given SoC.  If possible, interrupt-signaled events are recommended.

  
  
  ACPI Processor Control
  ----------------------

  Section 8 of the ACPI specification changed significantly in version 6.0.
  Processors should now be defined as Device objects with _HID ACPI0007; do
  not use the deprecated Processor statement in ASL.  All multiprocessor systems
  should also define a hierarchy of processors, done with Processor Container
  Devices (see Section 8.4.3.1, _HID ACPI0010); do not use processor aggregator
  devices (Section 8.5) to describe processor topology.  Section 8.4 of the
  specification describes the semantics of these object definitions and how
  they interrelate.
  
  Most importantly, the processor hierarchy defined also defines the low power
  idle states that are available to the platform, along with the rules for
  determining which processors can be turned on or off and the circumstances
  that control that.  Without this information, the processors will run in
  whatever power state they were left in by UEFI.
  
  Note too, that the processor Device objects defined and the entries in the
  MADT for GICs are expected to be in synchronization.  The _UID of the Device
  object must correspond to processor IDs used in the MADT.
  
  It is recommended that CPPC (8.4.5) be used as the primary model for processor
  performance control on arm64.  C-states and P-states may become available at
  some point in the future, but most current design work appears to favor CPPC.

  
  Further, it is essential that the ARMv8 SoC provide a fully functional
  implementation of PSCI; this will be the only mechanism supported by ACPI

  to control CPU power state.  Booting of secondary CPUs using the ACPI
  parking protocol is possible, but discouraged, since only PSCI is supported
  for ARM servers.

  
  
  ACPI System Address Map Interfaces
  ----------------------------------
  In Section 15 of the ACPI specification, several methods are mentioned as
  possible mechanisms for conveying memory resource information to the kernel.
  For arm64, we will only support UEFI for booting with ACPI, hence the UEFI
  GetMemoryMap() boot service is the only mechanism that will be used.
  
  
  ACPI Platform Error Interfaces (APEI)
  -------------------------------------
  The APEI tables supported are described above.
  
  APEI requires the equivalent of an SCI and an NMI on ARMv8.  The SCI is used
  to notify the OSPM of errors that have occurred but can be corrected and the
  system can continue correct operation, even if possibly degraded.  The NMI is
  used to indicate fatal errors that cannot be corrected, and require immediate
  attention.
  
  Since there is no direct equivalent of the x86 SCI or NMI, arm64 handles

  these slightly differently.  The SCI is handled as a high priority interrupt;
  given that these are corrected (or correctable) errors being reported, this
  is sufficient.  The NMI is emulated as the highest priority interrupt
  possible.  This implies some caution must be used since there could be
  interrupts at higher privilege levels or even interrupts at the same priority
  as the emulated NMI.  In Linux, this should not be the case but one should
  be aware it could happen.

  
  
  ACPI Objects Not Supported on ARM64
  -----------------------------------
  While this may change in the future, there are several classes of objects
  that can be defined, but are not currently of general interest to ARM servers.

  Some of these objects have x86 equivalents, and may actually make sense in ARM
  servers.  However, there is either no hardware available at present, or there
  may not even be a non-ARM implementation yet.  Hence, they are not currently
  supported.

  The following classes of objects are not supported:

  
         -- Section 9.2: ambient light sensor devices
  
         -- Section 9.3: battery devices
  
         -- Section 9.4: lids (e.g., laptop lids)
  
         -- Section 9.8.2: IDE controllers
  
         -- Section 9.9: floppy controllers
  
         -- Section 9.10: GPE block devices
  
         -- Section 9.15: PC/AT RTC/CMOS devices
  
         -- Section 9.16: user presence detection devices
  
         -- Section 9.17: I/O APIC devices; all GICs must be enumerable via MADT
  
         -- Section 9.18: time and alarm devices (see 9.15)

         -- Section 10: power source and power meter devices
  
         -- Section 11: thermal management
  
         -- Section 12: embedded controllers interface
  
         -- Section 13: SMBus interfaces

  
  This also means that there is no support for the following objects:
  
  Name   Section                     Name   Section
  ----   ------------                ----   ------------
  _ALC   9.3.4                       _FDM   9.10.3
  _ALI   9.3.2                       _FIX   6.2.7
  _ALP   9.3.6                       _GAI   10.4.5
  _ALR   9.3.5                       _GHL   10.4.7
  _ALT   9.3.3                       _GTM   9.9.2.1.1
  _BCT   10.2.2.10                   _LID   9.5.1
  _BDN   6.5.3                       _PAI   10.4.4
  _BIF   10.2.2.1                    _PCL   10.3.2
  _BIX   10.2.2.1                    _PIF   10.3.3
  _BLT   9.2.3                       _PMC   10.4.1
  _BMA   10.2.2.4                    _PMD   10.4.8
  _BMC   10.2.2.12                   _PMM   10.4.3
  _BMD   10.2.2.11                   _PRL   10.3.4
  _BMS   10.2.2.5                    _PSR   10.3.1
  _BST   10.2.2.6                    _PTP   10.4.2
  _BTH   10.2.2.7                    _SBS   10.1.3
  _BTM   10.2.2.9                    _SHL   10.4.6
  _BTP   10.2.2.8                    _STM   9.9.2.1.1
  _DCK   6.5.2                       _UPD   9.16.1
  _EC    12.12                       _UPP   9.16.2
  _FDE   9.10.1                      _WPC   10.5.2
  _FDI   9.10.2                      _WPP   10.5.3