PCI: Document ACPI description of PCI host bridges

Add a writeup about how PCI host bridges should be described in ACPI
using PNP0A03/PNP0A08 devices, PNP0C02 devices, and the MCFG table.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/PCI/00-INDEX

@@ -1,5 +1,7 @@
 00-INDEX
 	- this file
+acpi-info.txt
+	- info on how PCI host bridges are represented in ACPI
 MSI-HOWTO.txt
 	- the Message Signaled Interrupts (MSI) Driver Guide HOWTO and FAQ.
 PCIEBUS-HOWTO.txt

Documentation/PCI/acpi-info.txt

@@ -0,0 +1,187 @@
+		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.