Date:   Wed, 4 Jul 2018 11:40:52 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Linux PCI <linux-pci@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] PCI: Document ACPI description of PCI host bridges

On Fri, Jun 29, 2018 at 10:27 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> From: Bjorn Helgaas <bhelgaas@...gle.com>
>
> Add a writeup about how PCI host bridges should be described in ACPI
> using PNP0A03/PNP0A08 devices, PNP0C02 devices, and the MCFG table.
>
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> ---
>  Documentation/PCI/00-INDEX      |    2
>  Documentation/PCI/acpi-info.txt |  183 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 185 insertions(+)
>  create mode 100644 Documentation/PCI/acpi-info.txt
>
> diff --git a/Documentation/PCI/00-INDEX b/Documentation/PCI/00-INDEX
> index 0f1d1de087f1..fc6af2957e55 100644
> --- a/Documentation/PCI/00-INDEX
> +++ b/Documentation/PCI/00-INDEX
> @@ -1,5 +1,7 @@
>  00-INDEX
>         - this file
> +acpi-info.txt
> +       - info on how PCI host bridges are represented in ACPI
>  MSI-HOWTO.txt
>         - the Message Signaled Interrupts (MSI) Driver Guide HOWTO and FAQ.
>  PCIEBUS-HOWTO.txt
> diff --git a/Documentation/PCI/acpi-info.txt b/Documentation/PCI/acpi-info.txt
> new file mode 100644
> index 000000000000..9b8e7b560b50
> --- /dev/null
> +++ b/Documentation/PCI/acpi-info.txt
> @@ -0,0 +1,183 @@
> +           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 ACPI must describe each host bridge, the method for
> +accessing PCI config space below it, the address space windows the bridge
> +forwards to PCI, and the routing of legacy INTx interrupts.
> +
> +PCI devices *below* the host bridge generally do not need to be described
> +via ACPI because the OS can discover them via the standard PCI enumeration
> +mechanism, which uses config accesses to discover and identify the device
> +and read and size its BARs.

While they can be discovered without ACPI, power management or hotplug
may depend on it.

Also things like _PRT come to mind here.

> +
> +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 bridge registers that are not

I believe you mean registers of the host bridge itself here, but it is
somewhat unclear if that applies to bridges below it too.

> +forwarded to PCI.  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.  The bridge registers also include ECAM space,
> +since it is consumed by the 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.

Apart from the minor comments above, it looks all good.  Thanks for
taking care of documenting this!

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

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