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. 193