1.. SPDX-License-Identifier: GPL-2.0
  2
  3========================================
  4ACPI considerations for PCI host bridges
  5========================================
  6
  7The general rule is that the ACPI namespace should describe everything the
  8OS might use unless there's another way for the OS to find it [1, 2].
  9
 10For example, there's no standard hardware mechanism for enumerating PCI
 11host bridges, so the ACPI namespace must describe each host bridge, the
 12method for accessing PCI config space below it, the address space windows
 13the host bridge forwards to PCI (using _CRS), and the routing of legacy
 14INTx interrupts (using _PRT).
 15
 16PCI devices, which are below the host bridge, generally do not need to be
 17described via ACPI.  The OS can discover them via the standard PCI
 18enumeration mechanism, using config accesses to discover and identify
 19devices and read and size their BARs.  However, ACPI may describe PCI
 20devices if it provides power management or hotplug functionality for them
 21or if the device has INTx interrupts connected by platform interrupt
 22controllers and a _PRT is needed to describe those connections.
 23
 24ACPI resource description is done via _CRS objects of devices in the ACPI
 25namespace [2].   The _CRS is like a generalized PCI BAR: the OS can read
 26_CRS and figure out what resource is being consumed even if it doesn't have
 27a driver for the device [3].  That's important because it means an old OS
 28can work correctly even on a system with new devices unknown to the OS.
 29The new devices might not do anything, but the OS can at least make sure no
 30resources conflict with them.
 31
 32Static tables like MCFG, HPET, ECDT, etc., are *not* mechanisms for
 33reserving address space.  The static tables are for things the OS needs to
 34know early in boot, before it can parse the ACPI namespace.  If a new table
 35is defined, an old OS needs to operate correctly even though it ignores the
 36table.  _CRS allows that because it is generic and understood by the old
 37OS; a static table does not.
 38
 39If the OS is expected to manage a non-discoverable device described via
 40ACPI, that device will have a specific _HID/_CID that tells the OS what
 41driver to bind to it, and the _CRS tells the OS and the driver where the
 42device's registers are.
 43
 44PCI host bridges are PNP0A03 or PNP0A08 devices.  Their _CRS should
 45describe all the address space they consume.  This includes all the windows
 46they forward down to the PCI bus, as well as registers of the host bridge
 47itself that are not forwarded to PCI.  The host bridge registers include
 48things like secondary/subordinate bus registers that determine the bus
 49range below the bridge, window registers that describe the apertures, etc.
 50These are all device-specific, non-architected things, so the only way a
 51PNP0A03/PNP0A08 driver can manage them is via _PRS/_CRS/_SRS, which contain
 52the device-specific details.  The host bridge registers also include ECAM
 53space, since it is consumed by the host bridge.
 54
 55ACPI defines a Consumer/Producer bit to distinguish the bridge registers
 56("Consumer") from the bridge apertures ("Producer") [4, 5], but early
 57BIOSes didn't use that bit correctly.  The result is that the current ACPI
 58spec defines Consumer/Producer only for the Extended Address Space
 59descriptors; the bit should be ignored in the older QWord/DWord/Word
 60Address Space descriptors.  Consequently, OSes have to assume all
 61QWord/DWord/Word descriptors are windows.
 62
 63Prior to the addition of Extended Address Space descriptors, the failure of
 64Consumer/Producer meant there was no way to describe bridge registers in
 65the PNP0A03/PNP0A08 device itself.  The workaround was to describe the
 66bridge registers (including ECAM space) in PNP0C02 catch-all devices [6].
 67With the exception of ECAM, the bridge register space is device-specific
 68anyway, so the generic PNP0A03/PNP0A08 driver (pci_root.c) has no need to
 69know about it.  
 70
 71New architectures should be able to use "Consumer" Extended Address Space
 72descriptors in the PNP0A03 device for bridge registers, including ECAM,
 73although a strict interpretation of [6] might prohibit this.  Old x86 and
 74ia64 kernels assume all address space descriptors, including "Consumer"
 75Extended Address Space ones, are windows, so it would not be safe to
 76describe bridge registers this way on those architectures.
 77
 78PNP0C02 "motherboard" devices are basically a catch-all.  There's no
 79programming model for them other than "don't use these resources for
 80anything else."  So a PNP0C02 _CRS should claim any address space that is
 81(1) not claimed by _CRS under any other device object in the ACPI namespace
 82and (2) should not be assigned by the OS to something else.
 83
 84The PCIe spec requires the Enhanced Configuration Access Method (ECAM)
 85unless there's a standard firmware interface for config access, e.g., the
 86ia64 SAL interface [7].  A host bridge consumes ECAM memory address space
 87and converts memory accesses into PCI configuration accesses.  The spec
 88defines the ECAM address space layout and functionality; only the base of
 89the address space is device-specific.  An ACPI OS learns the base address
 90from either the static MCFG table or a _CBA method in the PNP0A03 device.
 91
 92The MCFG table must describe the ECAM space of non-hot pluggable host
 93bridges [8].  Since MCFG is a static table and can't be updated by hotplug,
 94a _CBA method in the PNP0A03 device describes the ECAM space of a
 95hot-pluggable host bridge [9].  Note that for both MCFG and _CBA, the base
 96address always corresponds to bus 0, even if the bus range below the bridge
 97(which is reported via _CRS) doesn't start at 0.
 98
 99
100[1] ACPI 6.2, sec 6.1:
101    For any device that is on a non-enumerable type of bus (for example, an
102    ISA bus), OSPM enumerates the devices' identifier(s) and the ACPI
103    system firmware must supply an _HID object ... for each device to
104    enable OSPM to do that.
105
106[2] ACPI 6.2, sec 3.7:
107    The OS enumerates motherboard devices simply by reading through the
108    ACPI Namespace looking for devices with hardware IDs.
109
110    Each device enumerated by ACPI includes ACPI-defined objects in the
111    ACPI Namespace that report the hardware resources the device could
112    occupy [_PRS], an object that reports the resources that are currently
113    used by the device [_CRS], and objects for configuring those resources
114    [_SRS].  The information is used by the Plug and Play OS (OSPM) to
115    configure the devices.
116
117[3] ACPI 6.2, sec 6.2:
118    OSPM uses device configuration objects to configure hardware resources
119    for devices enumerated via ACPI.  Device configuration objects provide
120    information about current and possible resource requirements, the
121    relationship between shared resources, and methods for configuring
122    hardware resources.
123
124    When OSPM enumerates a device, it calls _PRS to determine the resource
125    requirements of the device.  It may also call _CRS to find the current
126    resource settings for the device.  Using this information, the Plug and
127    Play system determines what resources the device should consume and
128    sets those resources by calling the device’s _SRS control method.
129
130    In ACPI, devices can consume resources (for example, legacy keyboards),
131    provide resources (for example, a proprietary PCI bridge), or do both.
132    Unless otherwise specified, resources for a device are assumed to be
133    taken from the nearest matching resource above the device in the device
134    hierarchy.
135
136[4] ACPI 6.2, sec 6.4.3.5.1, 2, 3, 4:
137    QWord/DWord/Word Address Space Descriptor (.1, .2, .3)
138      General Flags: Bit [0] Ignored
139
140    Extended Address Space Descriptor (.4)
141      General Flags: Bit [0] Consumer/Producer:
142
143        * 1 – This device consumes this resource
144        * 0 – This device produces and consumes this resource
145
146[5] ACPI 6.2, sec 19.6.43:
147    ResourceUsage specifies whether the Memory range is consumed by
148    this device (ResourceConsumer) or passed on to child devices
149    (ResourceProducer).  If nothing is specified, then
150    ResourceConsumer is assumed.
151
152[6] PCI Firmware 3.2, sec 4.1.2:
153    If the operating system does not natively comprehend reserving the
154    MMCFG region, the MMCFG region must be reserved by firmware.  The
155    address range reported in the MCFG table or by _CBA method (see Section
156    4.1.3) must be reserved by declaring a motherboard resource.  For most
157    systems, the motherboard resource would appear at the root of the ACPI
158    namespace (under \_SB) in a node with a _HID of EISAID (PNP0C02), and
159    the resources in this case should not be claimed in the root PCI bus’s
160    _CRS.  The resources can optionally be returned in Int15 E820 or
161    EFIGetMemoryMap as reserved memory but must always be reported through
162    ACPI as a motherboard resource.
163
164[7] PCI Express 4.0, sec 7.2.2:
165    For systems that are PC-compatible, or that do not implement a
166    processor-architecture-specific firmware interface standard that allows
167    access to the Configuration Space, the ECAM is required as defined in
168    this section.
169
170[8] PCI Firmware 3.2, sec 4.1.2:
171    The MCFG table is an ACPI table that is used to communicate the base
172    addresses corresponding to the non-hot removable PCI Segment Groups
173    range within a PCI Segment Group available to the operating system at
174    boot. This is required for the PC-compatible systems.
175
176    The MCFG table is only used to communicate the base addresses
177    corresponding to the PCI Segment Groups available to the system at
178    boot.
179
180[9] PCI Firmware 3.2, sec 4.1.3:
181    The _CBA (Memory mapped Configuration Base Address) control method is
182    an optional ACPI object that returns the 64-bit memory mapped
183    configuration base address for the hot plug capable host bridge. The
184    base address returned by _CBA is processor-relative address. The _CBA
185    control method evaluates to an Integer.
186
187    This control method appears under a host bridge object. When the _CBA
188    method appears under an active host bridge object, the operating system
189    evaluates this structure to identify the memory mapped configuration
190    base address corresponding to the PCI Segment Group for the bus number
191    range specified in _CRS method. An ACPI name space object that contains
192    the _CBA method must also contain a corresponding _SEG method.