Documentation/PCI/acpi-info.rst

1 .. SPDX-License-Identifier: GPL-2.0
4 ACPI considerations for PCI host bridges
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
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
19 devices and read and size their BARs.  However, ACPI may describe PCI
25 namespace [2].   The _CRS is like a generalized PCI BAR: the OS can read
36 table.  _CRS allows that because it is generic and understood by the old
39 If the OS is expected to manage a non-discoverable device described via
44 PCI host bridges are PNP0A03 or PNP0A08 devices.  Their _CRS should
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
50 These are all device-specific, non-architected things, so the only way a
52 the device-specific details.  The host bridge registers also include ECAM
53 space, since it is consumed by the host bridge.
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
72 descriptors in the PNP0A03 device for bridge registers, including ECAM,
78 PNP0C02 "motherboard" devices are basically a catch-all.  There's no
84 The PCIe spec requires the Enhanced Configuration Access Method (ECAM)
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
92 The MCFG table must describe the ECAM space of non-hot pluggable host
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
101     For any device that is on a non-enumerable type of bus (for example, an
110     Each device enumerated by ACPI includes ACPI-defined objects in the
131     provide resources (for example, a proprietary PCI bridge), or do both.
152 [6] PCI Firmware 3.2, sec 4.1.2:
159     the resources in this case should not be claimed in the root PCI bus’s
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
170 [8] PCI Firmware 3.2, sec 4.1.2:
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.
177     corresponding to the PCI Segment Groups available to the system at
180 [9] PCI Firmware 3.2, sec 4.1.3:
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
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
190     base address corresponding to the PCI Segment Group for the bus number