PCI: Document ACPI description of PCI host bridges

00-INDEX

00-INDEX

	- this file

	- this file

acpi-info.txt

	- info on how PCI host bridges are represented in ACPI

MSI-HOWTO.txt

MSI-HOWTO.txt

	- the Message Signaled Interrupts (MSI) Driver Guide HOWTO and FAQ.

	- the Message Signaled Interrupts (MSI) Driver Guide HOWTO and FAQ.

PCIEBUS-HOWTO.txt

PCIEBUS-HOWTO.txt

		ACPI considerations for PCI host bridges

The general rule is that the ACPI namespace should describe everything the

OS might use unless there's another way for the OS to find it [1, 2].

For example, there's no standard hardware mechanism for enumerating PCI

host bridges, so the ACPI namespace must describe each host bridge, the

method for accessing PCI config space below it, the address space windows

the host bridge forwards to PCI (using _CRS), and the routing of legacy

INTx interrupts (using _PRT).

PCI devices, which are below the host bridge, generally do not need to be

described via ACPI.  The OS can discover them via the standard PCI

enumeration mechanism, using config accesses to discover and identify

devices and read and size their BARs.  However, ACPI may describe PCI

devices if it provides power management or hotplug functionality for them

or if the device has INTx interrupts connected by platform interrupt

controllers and a _PRT is needed to describe those connections.

ACPI resource description is done via _CRS objects of devices in the ACPI

namespace [2].   The _CRS is like a generalized PCI BAR: the OS can read

_CRS and figure out what resource is being consumed even if it doesn't have

a driver for the device [3].  That's important because it means an old OS

can work correctly even on a system with new devices unknown to the OS.

The new devices might not do anything, but the OS can at least make sure no

resources conflict with them.

Static tables like MCFG, HPET, ECDT, etc., are *not* mechanisms for

reserving address space.  The static tables are for things the OS needs to

know early in boot, before it can parse the ACPI namespace.  If a new table

is defined, an old OS needs to operate correctly even though it ignores the

table.  _CRS allows that because it is generic and understood by the old

OS; a static table does not.

If the OS is expected to manage a non-discoverable device described via

ACPI, that device will have a specific _HID/_CID that tells the OS what

driver to bind to it, and the _CRS tells the OS and the driver where the

device's registers are.

PCI host bridges are PNP0A03 or PNP0A08 devices.  Their _CRS should

describe all the address space they consume.  This includes all the windows

they forward down to the PCI bus, as well as registers of the host bridge

itself that are not forwarded to PCI.  The host bridge registers include

things like secondary/subordinate bus registers that determine the bus

range below the bridge, window registers that describe the apertures, etc.

These are all device-specific, non-architected things, so the only way a

PNP0A03/PNP0A08 driver can manage them is via _PRS/_CRS/_SRS, which contain

the device-specific details.  The host bridge registers also include ECAM

space, since it is consumed by the host bridge.

ACPI defines a Consumer/Producer bit to distinguish the bridge registers

("Consumer") from the bridge apertures ("Producer") [4, 5], but early

BIOSes didn't use that bit correctly.  The result is that the current ACPI

spec defines Consumer/Producer only for the Extended Address Space

descriptors; the bit should be ignored in the older QWord/DWord/Word

Address Space descriptors.  Consequently, OSes have to assume all

QWord/DWord/Word descriptors are windows.

Prior to the addition of Extended Address Space descriptors, the failure of

Consumer/Producer meant there was no way to describe bridge registers in

the PNP0A03/PNP0A08 device itself.  The workaround was to describe the

bridge registers (including ECAM space) in PNP0C02 catch-all devices [6].

With the exception of ECAM, the bridge register space is device-specific

anyway, so the generic PNP0A03/PNP0A08 driver (pci_root.c) has no need to

know about it.  

New architectures should be able to use "Consumer" Extended Address Space

descriptors in the PNP0A03 device for bridge registers, including ECAM,

although a strict interpretation of [6] might prohibit this.  Old x86 and

ia64 kernels assume all address space descriptors, including "Consumer"

Extended Address Space ones, are windows, so it would not be safe to

describe bridge registers this way on those architectures.

PNP0C02 "motherboard" devices are basically a catch-all.  There's no

programming model for them other than "don't use these resources for

anything else."  So a PNP0C02 _CRS should claim any address space that is

(1) not claimed by _CRS under any other device object in the ACPI namespace

and (2) should not be assigned by the OS to something else.

The PCIe spec requires the Enhanced Configuration Access Method (ECAM)

unless there's a standard firmware interface for config access, e.g., the

ia64 SAL interface [7].  A host bridge consumes ECAM memory address space

and converts memory accesses into PCI configuration accesses.  The spec

defines the ECAM address space layout and functionality; only the base of

the address space is device-specific.  An ACPI OS learns the base address

from either the static MCFG table or a _CBA method in the PNP0A03 device.

The MCFG table must describe the ECAM space of non-hot pluggable host

bridges [8].  Since MCFG is a static table and can't be updated by hotplug,

a _CBA method in the PNP0A03 device describes the ECAM space of a

hot-pluggable host bridge [9].  Note that for both MCFG and _CBA, the base

address always corresponds to bus 0, even if the bus range below the bridge

(which is reported via _CRS) doesn't start at 0.

[1] ACPI 6.2, sec 6.1:

    For any device that is on a non-enumerable type of bus (for example, an

    ISA bus), OSPM enumerates the devices' identifier(s) and the ACPI

    system firmware must supply an _HID object ... for each device to

    enable OSPM to do that.

[2] ACPI 6.2, sec 3.7:

    The OS enumerates motherboard devices simply by reading through the

    ACPI Namespace looking for devices with hardware IDs.

    Each device enumerated by ACPI includes ACPI-defined objects in the

    ACPI Namespace that report the hardware resources the device could

    occupy [_PRS], an object that reports the resources that are currently

    used by the device [_CRS], and objects for configuring those resources

    [_SRS].  The information is used by the Plug and Play OS (OSPM) to

    configure the devices.

[3] ACPI 6.2, sec 6.2:

    OSPM uses device configuration objects to configure hardware resources

    for devices enumerated via ACPI.  Device configuration objects provide

    information about current and possible resource requirements, the

    relationship between shared resources, and methods for configuring

    hardware resources.

    When OSPM enumerates a device, it calls _PRS to determine the resource

    requirements of the device.  It may also call _CRS to find the current

    resource settings for the device.  Using this information, the Plug and

    Play system determines what resources the device should consume and

    sets those resources by calling the device’s _SRS control method.

    In ACPI, devices can consume resources (for example, legacy keyboards),

    provide resources (for example, a proprietary PCI bridge), or do both.

    Unless otherwise specified, resources for a device are assumed to be

    taken from the nearest matching resource above the device in the device

    hierarchy.

[4] ACPI 6.2, sec 6.4.3.5.1, 2, 3, 4:

    QWord/DWord/Word Address Space Descriptor (.1, .2, .3)

    General Flags: Bit [0] Ignored

    Extended Address Space Descriptor (.4)

    General Flags: Bit [0] Consumer/Producer:

	1–This device consumes this resource

	0–This device produces and consumes this resource

[5] ACPI 6.2, sec 19.6.43:

    ResourceUsage specifies whether the Memory range is consumed by

    this device (ResourceConsumer) or passed on to child devices

    (ResourceProducer).  If nothing is specified, then

    ResourceConsumer is assumed.

[6] PCI Firmware 3.2, sec 4.1.2:

    If the operating system does not natively comprehend reserving the

    MMCFG region, the MMCFG region must be reserved by firmware.  The

    address range reported in the MCFG table or by _CBA method (see Section

    4.1.3) must be reserved by declaring a motherboard resource.  For most

    systems, the motherboard resource would appear at the root of the ACPI

    namespace (under \_SB) in a node with a _HID of EISAID (PNP0C02), and

    the resources in this case should not be claimed in the root PCI bus’s

    _CRS.  The resources can optionally be returned in Int15 E820 or

    EFIGetMemoryMap as reserved memory but must always be reported through

    ACPI as a motherboard resource.

[7] PCI Express 4.0, sec 7.2.2:

    For systems that are PC-compatible, or that do not implement a

    processor-architecture-specific firmware interface standard that allows

    access to the Configuration Space, the ECAM is required as defined in

    this section.

[8] PCI Firmware 3.2, sec 4.1.2:

    The MCFG table is an ACPI table that is used to communicate the base

    addresses corresponding to the non-hot removable PCI Segment Groups

    range within a PCI Segment Group available to the operating system at

    boot. This is required for the PC-compatible systems.

    The MCFG table is only used to communicate the base addresses

    corresponding to the PCI Segment Groups available to the system at

    boot.

[9] PCI Firmware 3.2, sec 4.1.3:

    The _CBA (Memory mapped Configuration Base Address) control method is

    an optional ACPI object that returns the 64-bit memory mapped

    configuration base address for the hot plug capable host bridge. The

    base address returned by _CBA is processor-relative address. The _CBA

    control method evaluates to an Integer.

    This control method appears under a host bridge object. When the _CBA

    method appears under an active host bridge object, the operating system

    evaluates this structure to identify the memory mapped configuration

    base address corresponding to the PCI Segment Group for the bus number

    range specified in _CRS method. An ACPI name space object that contains

    the _CBA method must also contain a corresponding _SEG method.