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diff --git a/Documentation/PCI/acpi-info.rst b/Documentation/PCI/acpi-info.rst new file mode 100644 index 000000000..060217081 --- /dev/null +++ b/Documentation/PCI/acpi-info.rst @@ -0,0 +1,192 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================================== +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 the ACPI namespace must describe each host bridge, the +method for accessing PCI config space below it, the address space windows +the host bridge forwards to PCI (using _CRS), and the routing of legacy +INTx interrupts (using _PRT). + +PCI devices, which are below the host bridge, generally do not need to be +described via ACPI. The OS can discover them via the standard PCI +enumeration mechanism, using config accesses to discover and identify +devices and read and size their BARs. However, ACPI may describe PCI +devices if it provides power management or hotplug functionality for them +or if the device has INTx interrupts connected by platform interrupt +controllers and a _PRT is needed to describe those connections. + +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 registers of the host bridge +itself that are not forwarded to PCI. The host 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 host bridge registers also include ECAM +space, since it is consumed by the host 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. |