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+ACPI on ARMv8 Servers
+---------------------
+ACPI can be used for ARMv8 general purpose servers designed to follow
+the ARM SBSA (Server Base System Architecture) [0] and SBBR (Server
+Base Boot Requirements) [1] specifications.  Please note that the SBBR
+can be retrieved simply by visiting [1], but the SBSA is currently only
+available to those with an ARM login due to ARM IP licensing concerns.
+
+The ARMv8 kernel implements the reduced hardware model of ACPI version
+5.1 or later.  Links to the specification and all external documents
+it refers to are managed by the UEFI Forum.  The specification is
+available at http://www.uefi.org/specifications and documents referenced
+by the specification can be found via http://www.uefi.org/acpi.
+
+If an ARMv8 system does not meet the requirements of the SBSA and SBBR,
+or cannot be described using the mechanisms defined in the required ACPI
+specifications, then ACPI may not be a good fit for the hardware.
+
+While the documents mentioned above set out the requirements for building
+industry-standard ARMv8 servers, they also apply to more than one operating
+system.  The purpose of this document is to describe the interaction between
+ACPI and Linux only, on an ARMv8 system -- that is, what Linux expects of
+ACPI and what ACPI can expect of Linux.
+
+
+Why ACPI on ARM?
+----------------
+Before examining the details of the interface between ACPI and Linux, it is
+useful to understand why ACPI is being used.  Several technologies already
+exist in Linux for describing non-enumerable hardware, after all.  In this
+section we summarize a blog post [2] from Grant Likely that outlines the
+reasoning behind ACPI on ARMv8 servers.  Actually, we snitch a good portion
+of the summary text almost directly, to be honest.
+
+The short form of the rationale for ACPI on ARM is:
+
+-- ACPI’s bytecode (AML) allows the platform to encode hardware behavior,
+   while DT explicitly does not support this.  For hardware vendors, being
+   able to encode behavior is a key tool used in supporting operating
+   system releases on new hardware.
+
+-- ACPI’s OSPM defines a power management model that constrains what the
+   platform is allowed to do into a specific model, while still providing
+   flexibility in hardware design.
+
+-- In the enterprise server environment, ACPI has established bindings (such
+   as for RAS) which are currently used in production systems.  DT does not.
+   Such bindings could be defined in DT at some point, but doing so means ARM
+   and x86 would end up using completely different code paths in both firmware
+   and the kernel.
+
+-- Choosing a single interface to describe the abstraction between a platform
+   and an OS is important.  Hardware vendors would not be required to implement
+   both DT and ACPI if they want to support multiple operating systems.  And,
+   agreeing on a single interface instead of being fragmented into per OS
+   interfaces makes for better interoperability overall.
+
+-- The new ACPI governance process works well and Linux is now at the same
+   table as hardware vendors and other OS vendors.  In fact, there is no
+   longer any reason to feel that ACPI is only belongs to Windows or that
+   Linux is in any way secondary to Microsoft in this arena.  The move of
+   ACPI governance into the UEFI forum has significantly opened up the
+   specification development process, and currently, a large portion of the
+   changes being made to ACPI is being driven by Linux.
+
+Key to the use of ACPI is the support model.  For servers in general, the
+responsibility for hardware behaviour cannot solely be the domain of the
+kernel, but rather must be split between the platform and the kernel, in
+order to allow for orderly change over time.  ACPI frees the OS from needing
+to understand all the minute details of the hardware so that the OS doesn’t
+need to be ported to each and every device individually.  It allows the
+hardware vendors to take responsibility for power management behaviour without
+depending on an OS release cycle which is not under their control.
+
+ACPI is also important because hardware and OS vendors have already worked
+out the mechanisms for supporting a general purpose computing ecosystem.  The
+infrastructure is in place, the bindings are in place, and the processes are
+in place.  DT does exactly what Linux needs it to when working with vertically
+integrated devices, but there are no good processes for supporting what the
+server vendors need.  Linux could potentially get there with DT, but doing so
+really just duplicates something that already works.  ACPI already does what
+the hardware vendors need, Microsoft won’t collaborate on DT, and hardware
+vendors would still end up providing two completely separate firmware
+interfaces -- one for Linux and one for Windows.
+
+
+Kernel Compatibility
+--------------------
+One of the primary motivations for ACPI is standardization, and using that
+to provide backward compatibility for Linux kernels.  In the server market,
+software and hardware are often used for long periods.  ACPI allows the
+kernel and firmware to agree on a consistent abstraction that can be
+maintained over time, even as hardware or software change.  As long as the
+abstraction is supported, systems can be updated without necessarily having
+to replace the kernel.
+
+When a Linux driver or subsystem is first implemented using ACPI, it by
+definition ends up requiring a specific version of the ACPI specification
+-- it's baseline.  ACPI firmware must continue to work, even though it may
+not be optimal, with the earliest kernel version that first provides support
+for that baseline version of ACPI.  There may be a need for additional drivers,
+but adding new functionality (e.g., CPU power management) should not break
+older kernel versions.  Further, ACPI firmware must also work with the most
+recent version of the kernel.
+
+
+Relationship with Device Tree
+-----------------------------
+ACPI support in drivers and subsystems for ARMv8 should never be mutually
+exclusive with DT support at compile time.
+
+At boot time the kernel will only use one description method depending on
+parameters passed from the bootloader (including kernel bootargs).
+
+Regardless of whether DT or ACPI is used, the kernel must always be capable
+of booting with either scheme (in kernels with both schemes enabled at compile
+time).
+
+
+Booting using ACPI tables
+-------------------------
+The only defined method for passing ACPI tables to the kernel on ARMv8
+is via the UEFI system configuration table.  Just so it is explicit, this
+means that ACPI is only supported on platforms that boot via UEFI.
+
+When an ARMv8 system boots, it can either have DT information, ACPI tables,
+or in some very unusual cases, both.  If no command line parameters are used,
+the kernel will try to use DT for device enumeration; if there is no DT
+present, the kernel will try to use ACPI tables, but only if they are present.
+In neither is available, the kernel will not boot.  If acpi=force is used
+on the command line, the kernel will attempt to use ACPI tables first, but
+fall back to DT if there are no ACPI tables present.  The basic idea is that
+the kernel will not fail to boot unless it absolutely has no other choice.
+
+Processing of ACPI tables may be disabled by passing acpi=off on the kernel
+command line; this is the default behavior.
+
+In order for the kernel to load and use ACPI tables, the UEFI implementation
+MUST set the ACPI_20_TABLE_GUID to point to the RSDP table (the table with
+the ACPI signature "RSD PTR ").  If this pointer is incorrect and acpi=force
+is used, the kernel will disable ACPI and try to use DT to boot instead; the
+kernel has, in effect, determined that ACPI tables are not present at that
+point.
+
+If the pointer to the RSDP table is correct, the table will be mapped into
+the kernel by the ACPI core, using the address provided by UEFI.
+
+The ACPI core will then locate and map in all other ACPI tables provided by
+using the addresses in the RSDP table to find the XSDT (eXtended System
+Description Table).  The XSDT in turn provides the addresses to all other
+ACPI tables provided by the system firmware; the ACPI core will then traverse
+this table and map in the tables listed.
+
+The ACPI core will ignore any provided RSDT (Root System Description Table).
+RSDTs have been deprecated and are ignored on arm64 since they only allow
+for 32-bit addresses.
+
+Further, the ACPI core will only use the 64-bit address fields in the FADT
+(Fixed ACPI Description Table).  Any 32-bit address fields in the FADT will
+be ignored on arm64.
+
+Hardware reduced mode (see Section 4.1 of the ACPI 5.1 specification) will
+be enforced by the ACPI core on arm64.  Doing so allows the ACPI core to
+run less complex code since it no longer has to provide support for legacy
+hardware from other architectures.  Any fields that are not to be used for
+hardware reduced mode must be set to zero.
+
+For the ACPI core to operate properly, and in turn provide the information
+the kernel needs to configure devices, it expects to find the following
+tables (all section numbers refer to the ACPI 5.1 specfication):
+
+    -- RSDP (Root System Description Pointer), section 5.2.5
+
+    -- XSDT (eXtended System Description Table), section 5.2.8
+
+    -- FADT (Fixed ACPI Description Table), section 5.2.9
+
+    -- DSDT (Differentiated System Description Table), section
+       5.2.11.1
+
+    -- MADT (Multiple APIC Description Table), section 5.2.12
+
+    -- GTDT (Generic Timer Description Table), section 5.2.24
+
+    -- If PCI is supported, the MCFG (Memory mapped ConFiGuration
+       Table), section 5.2.6, specifically Table 5-31.
+
+If the above tables are not all present, the kernel may or may not be
+able to boot properly since it may not be able to configure all of the
+devices available.
+
+
+ACPI Detection
+--------------
+Drivers should determine their probe() type by checking for a null
+value for ACPI_HANDLE, or checking .of_node, or other information in
+the device structure.  This is detailed further in the "Driver
+Recommendations" section.
+
+In non-driver code, if the presence of ACPI needs to be detected at
+runtime, then check the value of acpi_disabled. If CONFIG_ACPI is not
+set, acpi_disabled will always be 1.
+
+
+Device Enumeration
+------------------
+Device descriptions in ACPI should use standard recognized ACPI interfaces.
+These may contain less information than is typically provided via a Device
+Tree description for the same device.  This is also one of the reasons that
+ACPI can be useful -- the driver takes into account that it may have less
+detailed information about the device and uses sensible defaults instead.
+If done properly in the driver, the hardware can change and improve over
+time without the driver having to change at all.
+
+Clocks provide an excellent example.  In DT, clocks need to be specified
+and the drivers need to take them into account.  In ACPI, the assumption
+is that UEFI will leave the device in a reasonable default state, including
+any clock settings.  If for some reason the driver needs to change a clock
+value, this can be done in an ACPI method; all the driver needs to do is
+invoke the method and not concern itself with what the method needs to do
+to change the clock.  Changing the hardware can then take place over time
+by changing what the ACPI method does, and not the driver.
+
+In DT, the parameters needed by the driver to set up clocks as in the example
+above are known as "bindings"; in ACPI, these are known as "Device Properties"
+and provided to a driver via the _DSD object.
+
+ACPI tables are described with a formal language called ASL, the ACPI
+Source Language (section 19 of the specification).  This means that there
+are always multiple ways to describe the same thing -- including device
+properties.  For example, device properties could use an ASL construct
+that looks like this: Name(KEY0, "value0").  An ACPI device driver would
+then retrieve the value of the property by evaluating the KEY0 object.
+However, using Name() this way has multiple problems: (1) ACPI limits
+names ("KEY0") to four characters unlike DT; (2) there is no industry
+wide registry that maintains a list of names, minimzing re-use; (3)
+there is also no registry for the definition of property values ("value0"),
+again making re-use difficult; and (4) how does one maintain backward
+compatibility as new hardware comes out?  The _DSD method was created
+to solve precisely these sorts of problems; Linux drivers should ALWAYS
+use the _DSD method for device properties and nothing else.
+
+The _DSM object (ACPI Section 9.14.1) could also be used for conveying
+device properties to a driver.  Linux drivers should only expect it to
+be used if _DSD cannot represent the data required, and there is no way
+to create a new UUID for the _DSD object.  Note that there is even less
+regulation of the use of _DSM than there is of _DSD.  Drivers that depend
+on the contents of _DSM objects will be more difficult to maintain over
+time because of this; as of this writing, the use of _DSM is the cause
+of quite a few firmware problems and is not recommended.
+
+Drivers should look for device properties in the _DSD object ONLY; the _DSD
+object is described in the ACPI specification section 6.2.5, but this only
+describes how to define the structure of an object returned via _DSD, and
+how specific data structures are defined by specific UUIDs.  Linux should
+only use the _DSD Device Properties UUID [5]:
+
+   -- UUID: daffd814-6eba-4d8c-8a91-bc9bbf4aa301
+
+   -- http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
+
+The UEFI Forum provides a mechanism for registering device properties [4]
+so that they may be used across all operating systems supporting ACPI.
+Device properties that have not been registered with the UEFI Forum should
+not be used.
+
+Before creating new device properties, check to be sure that they have not
+been defined before and either registered in the Linux kernel documentation
+as DT bindings, or the UEFI Forum as device properties.  While we do not want
+to simply move all DT bindings into ACPI device properties, we can learn from
+what has been previously defined.
+
+If it is necessary to define a new device property, or if it makes sense to
+synthesize the definition of a binding so it can be used in any firmware,
+both DT bindings and ACPI device properties for device drivers have review
+processes.  Use them both.  When the driver itself is submitted for review
+to the Linux mailing lists, the device property definitions needed must be
+submitted at the same time.  A driver that supports ACPI and uses device
+properties will not be considered complete without their definitions.  Once
+the device property has been accepted by the Linux community, it must be
+registered with the UEFI Forum [4], which will review it again for consistency
+within the registry.  This may require iteration.  The UEFI Forum, though,
+will always be the canonical site for device property definitions.
+
+It may make sense to provide notice to the UEFI Forum that there is the
+intent to register a previously unused device property name as a means of
+reserving the name for later use.  Other operating system vendors will
+also be submitting registration requests and this may help smooth the
+process.
+
+Once registration and review have been completed, the kernel provides an
+interface for looking up device properties in a manner independent of
+whether DT or ACPI is being used.  This API should be used [6]; it can
+eliminate some duplication of code paths in driver probing functions and
+discourage divergence between DT bindings and ACPI device properties.
+
+
+Programmable Power Control Resources
+------------------------------------
+Programmable power control resources include such resources as voltage/current
+providers (regulators) and clock sources.
+
+With ACPI, the kernel clock and regulator framework is not expected to be used
+at all.
+
+The kernel assumes that power control of these resources is represented with
+Power Resource Objects (ACPI section 7.1).  The ACPI core will then handle
+correctly enabling and disabling resources as they are needed.  In order to
+get that to work, ACPI assumes each device has defined D-states and that these
+can be controlled through the optional ACPI methods _PS0, _PS1, _PS2, and _PS3;
+in ACPI, _PS0 is the method to invoke to turn a device full on, and _PS3 is for
+turning a device full off.
+
+There are two options for using those Power Resources.  They can:
+
+   -- be managed in a _PSx method which gets called on entry to power
+      state Dx.
+
+   -- be declared separately as power resources with their own _ON and _OFF
+      methods.  They are then tied back to D-states for a particular device
+      via _PRx which specifies which power resources a device needs to be on
+      while in Dx.  Kernel then tracks number of devices using a power resource
+      and calls _ON/_OFF as needed.
+
+The kernel ACPI code will also assume that the _PSx methods follow the normal
+ACPI rules for such methods:
+
+   -- If either _PS0 or _PS3 is implemented, then the other method must also
+      be implemented.
+
+   -- If a device requires usage or setup of a power resource when on, the ASL
+      should organize that it is allocated/enabled using the _PS0 method.
+
+   -- Resources allocated or enabled in the _PS0 method should be disabled
+      or de-allocated in the _PS3 method.
+
+   -- Firmware will leave the resources in a reasonable state before handing
+      over control to the kernel.
+
+Such code in _PSx methods will of course be very platform specific.  But,
+this allows the driver to abstract out the interface for operating the device
+and avoid having to read special non-standard values from ACPI tables. Further,
+abstracting the use of these resources allows the hardware to change over time
+without requiring updates to the driver.
+
+
+Clocks
+------
+ACPI makes the assumption that clocks are initialized by the firmware --
+UEFI, in this case -- to some working value before control is handed over
+to the kernel.  This has implications for devices such as UARTs, or SoC-driven
+LCD displays, for example.
+
+When the kernel boots, the clocks are assumed to be set to reasonable
+working values.  If for some reason the frequency needs to change -- e.g.,
+throttling for power management -- the device driver should expect that
+process to be abstracted out into some ACPI method that can be invoked
+(please see the ACPI specification for further recommendations on standard
+methods to be expected).  The only exceptions to this are CPU clocks where
+CPPC provides a much richer interface than ACPI methods.  If the clocks
+are not set, there is no direct way for Linux to control them.
+
+If an SoC vendor wants to provide fine-grained control of the system clocks,
+they could do so by providing ACPI methods that could be invoked by Linux
+drivers.  However, this is NOT recommended and Linux drivers should NOT use
+such methods, even if they are provided.  Such methods are not currently
+standardized in the ACPI specification, and using them could tie a kernel
+to a very specific SoC, or tie an SoC to a very specific version of the
+kernel, both of which we are trying to avoid.
+
+
+Driver Recommendations
+----------------------
+DO NOT remove any DT handling when adding ACPI support for a driver.  The
+same device may be used on many different systems.
+
+DO try to structure the driver so that it is data-driven.  That is, set up
+a struct containing internal per-device state based on defaults and whatever
+else must be discovered by the driver probe function.  Then, have the rest
+of the driver operate off of the contents of that struct.  Doing so should
+allow most divergence between ACPI and DT functionality to be kept local to
+the probe function instead of being scattered throughout the driver.  For
+example:
+
+static int device_probe_dt(struct platform_device *pdev)
+{
+	/* DT specific functionality */
+	...
+}
+
+static int device_probe_acpi(struct platform_device *pdev)
+{
+	/* ACPI specific functionality */
+	...
+}
+
+static int device_probe(struct platform_device *pdev)
+{
+	...
+	struct device_node node = pdev->dev.of_node;
+	...
+
+	if (node)
+		ret = device_probe_dt(pdev);
+	else if (ACPI_HANDLE(&pdev->dev))
+		ret = device_probe_acpi(pdev);
+	else
+		/* other initialization */
+		...
+	/* Continue with any generic probe operations */
+	...
+}
+
+DO keep the MODULE_DEVICE_TABLE entries together in the driver to make it
+clear the different names the driver is probed for, both from DT and from
+ACPI:
+
+static struct of_device_id virtio_mmio_match[] = {
+        { .compatible = "virtio,mmio", },
+        { }
+};
+MODULE_DEVICE_TABLE(of, virtio_mmio_match);
+
+static const struct acpi_device_id virtio_mmio_acpi_match[] = {
+        { "LNRO0005", },
+        { }
+};
+MODULE_DEVICE_TABLE(acpi, virtio_mmio_acpi_match);
+
+
+ASWG
+----
+The ACPI specification changes regularly.  During the year 2014, for instance,
+version 5.1 was released and version 6.0 substantially completed, with most of
+the changes being driven by ARM-specific requirements.  Proposed changes are
+presented and discussed in the ASWG (ACPI Specification Working Group) which
+is a part of the UEFI Forum.
+
+Participation in this group is open to all UEFI members.  Please see
+http://www.uefi.org/workinggroup for details on group membership.
+
+It is the intent of the ARMv8 ACPI kernel code to follow the ACPI specification
+as closely as possible, and to only implement functionality that complies with
+the released standards from UEFI ASWG.  As a practical matter, there will be
+vendors that provide bad ACPI tables or violate the standards in some way.
+If this is because of errors, quirks and fixups may be necessary, but will
+be avoided if possible.  If there are features missing from ACPI that preclude
+it from being used on a platform, ECRs (Engineering Change Requests) should be
+submitted to ASWG and go through the normal approval process; for those that
+are not UEFI members, many other members of the Linux community are and would
+likely be willing to assist in submitting ECRs.
+
+
+Linux Code
+----------
+Individual items specific to Linux on ARM, contained in the the Linux
+source code, are in the list that follows:
+
+ACPI_OS_NAME		This macro defines the string to be returned when
+			an ACPI method invokes the _OS method.  On ARM64
+			systems, this macro will be "Linux" by default.
+			The command line parameter acpi_os=<string>
+			can be used to set it to some other value.  The
+			default value for other architectures is "Microsoft
+			Windows NT", for example.
+
+ACPI Objects
+------------
+Detailed expectations for ACPI tables and object are listed in the file
+Documentation/arm64/acpi_object_usage.txt.
+
+
+References
+----------
+[0] http://silver.arm.com -- document ARM-DEN-0029, or newer
+    "Server Base System Architecture", version 2.3, dated 27 Mar 2014
+
+[1] http://infocenter.arm.com/help/topic/com.arm.doc.den0044a/Server_Base_Boot_Requirements.pdf
+    Document ARM-DEN-0044A, or newer: "Server Base Boot Requirements, System
+    Software on ARM Platforms", dated 16 Aug 2014
+
+[2] http://www.secretlab.ca/archives/151, 10 Jan 2015, Copyright (c) 2015,
+    Linaro Ltd., written by Grant Likely.  A copy of the verbatim text (apart
+    from formatting) is also in Documentation/arm64/why_use_acpi.txt.
+
+[3] AMD ACPI for Seattle platform documentation:
+    http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2012/10/Seattle_ACPI_Guide.pdf
+
+[4] http://www.uefi.org/acpi -- please see the link for the "ACPI _DSD Device
+    Property Registry Instructions"
+
+[5] http://www.uefi.org/acpi -- please see the link for the "_DSD (Device
+    Specific Data) Implementation Guide"
+
+[6] Kernel code for the unified device property interface can be found in
+    include/linux/property.h and drivers/base/property.c.
+
+
+Authors
+-------
+Al Stone <al.stone@linaro.org>
+Graeme Gregory <graeme.gregory@linaro.org>
+Hanjun Guo <hanjun.guo@linaro.org>
+
+Grant Likely <grant.likely@linaro.org>, for the "Why ACPI on ARM?" section
