1 .. SPDX-License-Identifier: GPL-2.0
2
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4 ACPI considerations for PCI host bridges
5 ========================================
6
7 The general rule is that the ACPI namespace should describe everything the
8 OS might use unless there's another way for the OS to find it [1, 2].
9
10 For example, there's no standard hardware mechanism for enumerating PCI
11 host bridges, so the ACPI namespace must describe each host bridge, the
12 method for accessing PCI config space below it, the address space windows
13 the host bridge forwards to PCI (using _CRS), and the routing of legacy
14 INTx interrupts (using _PRT).
15
16 PCI devices, which are below the host bridge, generally do not need to be
17 described via ACPI. The OS can discover them via the standard PCI
18 enumeration mechanism, using config accesses to discover and identify
19 devices and read and size their BARs. However, ACPI may describe PCI
20 devices if it provides power management or hotplug functionality for them
21 or if the device has INTx interrupts connected by platform interrupt
22 controllers and a _PRT is needed to describe those connections.
23
24 ACPI resource description is done via _CRS objects of devices in the ACPI
25 namespace [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
27 a driver for the device [3]. That's important because it means an old OS
28 can work correctly even on a system with new devices unknown to the OS.
29 The new devices might not do anything, but the OS can at least make sure no
30 resources conflict with them.
31
32 Static tables like MCFG, HPET, ECDT, etc., are *not* mechanisms for
33 reserving address space. The static tables are for things the OS needs to
34 know early in boot, before it can parse the ACPI namespace. If a new table
35 is defined, an old OS needs to operate correctly even though it ignores the
36 table. _CRS allows that because it is generic and understood by the old
37 OS; a static table does not.
38
39 If the OS is expected to manage a non-discoverable device described via
40 ACPI, that device will have a specific _HID/_CID that tells the OS what
41 driver to bind to it, and the _CRS tells the OS and the driver where the
42 device's registers are.
43
44 PCI host bridges are PNP0A03 or PNP0A08 devices. Their _CRS should
45 describe all the address space they consume. This includes all the windows
46 they forward down to the PCI bus, as well as registers of the host bridge
47 itself that are not forwarded to PCI. The host bridge registers include
48 things like secondary/subordinate bus registers that determine the bus
49 range below the bridge, window registers that describe the apertures, etc.
50 These are all device-specific, non-architected things, so the only way a
51 PNP0A03/PNP0A08 driver can manage them is via _PRS/_CRS/_SRS, which contain
52 the device-specific details. The host bridge registers also include ECAM
53 space, since it is consumed by the host bridge.
54
55 ACPI defines a Consumer/Producer bit to distinguish the bridge registers
56 ("Consumer") from the bridge apertures ("Producer") [4, 5], but early
57 BIOSes didn't use that bit correctly. The result is that the current ACPI
58 spec defines Consumer/Producer only for the Extended Address Space
59 descriptors; the bit should be ignored in the older QWord/DWord/Word
60 Address Space descriptors. Consequently, OSes have to assume all
61 QWord/DWord/Word descriptors are windows.
62
63 Prior to the addition of Extended Address Space descriptors, the failure of
64 Consumer/Producer meant there was no way to describe bridge registers in
65 the PNP0A03/PNP0A08 device itself. The workaround was to describe the
66 bridge registers (including ECAM space) in PNP0C02 catch-all devices [6].
67 With the exception of ECAM, the bridge register space is device-specific
68 anyway, so the generic PNP0A03/PNP0A08 driver (pci_root.c) has no need to
69 know about it.
70
71 New architectures should be able to use "Consumer" Extended Address Space
72 descriptors in the PNP0A03 device for bridge registers, including ECAM,
73 although a strict interpretation of [6] might prohibit this. Old x86 and
74 ia64 kernels assume all address space descriptors, including "Consumer"
75 Extended Address Space ones, are windows, so it would not be safe to
76 describe bridge registers this way on those architectures.
77
78 PNP0C02 "motherboard" devices are basically a catch-all. There's no
79 programming model for them other than "don't use these resources for
80 anything 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
82 and (2) should not be assigned by the OS to something else.
83
84 The PCIe spec requires the Enhanced Configuration Access Method (ECAM)
85 unless there's a standard firmware interface for config access, e.g., the
86 ia64 SAL interface [7]. A host bridge consumes ECAM memory address space
87 and converts memory accesses into PCI configuration accesses. The spec
88 defines the ECAM address space layout and functionality; only the base of
89 the address space is device-specific. An ACPI OS learns the base address
90 from either the static MCFG table or a _CBA method in the PNP0A03 device.
91
92 The MCFG table must describe the ECAM space of non-hot pluggable host
93 bridges [8]. Since MCFG is a static table and can't be updated by hotplug,
94 a _CBA method in the PNP0A03 device describes the ECAM space of a
95 hot-pluggable host bridge [9]. Note that for both MCFG and _CBA, the base
96 address 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.
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