ACPI namespace details for ARM64

Bjorn Helgaas helgaas at
Wed Nov 9 14:05:06 PST 2016

Hi all,

We've been working through the details of getting ACPI to work on
arm64, and there have been lots of questions about what this means for
PCI.  I've outlined this for several people individually, but I'm
going to send this separately, apart from a specific patch series, to
make sure we're all on the same page.  Please correct my errors and


The basic requirement is that the ACPI namespace should describe
*everything* that consumes address space unless there's another
standard way for the OS to find it [1, 2].  For example, windows that
are forwarded to PCI by a PCI host bridge should be described via ACPI
devices, since the OS can't locate the host bridge by itself.  PCI
devices *below* the host bridge do not need to be described via ACPI,
because the resources they consume are inside the host bridge windows,
and the OS can discover them via the standard PCI enumeration
mechanism (using config accesses to read and size the BARs).

This ACPI resource description is done via _CRS methods of devices in
the ACPI namespace [2].   _CRS methods are like generalized PCI BARs:
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 won't 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 an ACPI device, 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.

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 any address space that is (1) not claimed by some
other ACPI device and (2) should not be assigned by the OS to
something else, should be claimed by a PNP0C02 _CRS method.

PCI host bridges are PNP0A03 or PNP0A08 devices.  Their _CRS should
describe all the address space they consume.  In principle, this would
be all the windows they forward down to the PCI bus, as well as the
bridge registers themselves.  The 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.  These bridge registers also
include ECAM space, since it is consumed by the bridge.

ACPI defined a Producer/Consumer bit that was intended to distinguish
the bridge apertures from the bridge registers [4, 5].  However,
BIOSes didn't use that bit correctly, and the result is that OSes have
to assume that everything in a PCI host bridge _CRS is a window.  That
leaves no way to describe the bridge registers in the PNP0A03/PNP0A08
device itself.

The workaround is 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.  For
ECAM, pci_root.c learns about the space from either MCFG or the _CBA

Note that the PCIe spec actually does require ECAM unless there's a
standard firmware interface for config access, e.g., the ia64 SAL
interface [7].  One reason is that we want a generic host bridge
driver (pci_root.c), and a generic driver requires a generic way to
access config space.

[1] ACPI 6.0, 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 6.0, 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 6.0, 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 6.0, sec
    Extended Address Space Descriptor
    General Flags: Bit [0] Consumer/Producer:
	1–This device consumes this resource
	0–This device produces and consumes this resource

[5] ACPI 6.0, 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 3.0, 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 3.0, 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.

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