Documentation: ACPI for ARM64

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