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diff --git a/Documentation/arm64/arm-acpi.txt b/Documentation/arm64/arm-acpi.txt new file mode 100644 index 0000000..570a4f8 --- /dev/null +++ b/Documentation/arm64/arm-acpi.txt @@ -0,0 +1,505 @@ +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 |