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 --- Documentation/PCI/00-INDEX | 2 + Documentation/PCI/acpi-info.txt | 136 +++++++++++++++++++++++++++++++++++++++ 2 files changed, 138 insertions(+) create mode 100644 Documentation/PCI/acpi-info.txt
diff --git a/Documentation/PCI/00-INDEX b/Documentation/PCI/00-INDEX index 147231f..0780280 100644 --- a/Documentation/PCI/00-INDEX +++ b/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 diff --git a/Documentation/PCI/acpi-info.txt b/Documentation/PCI/acpi-info.txt new file mode 100644 index 0000000..ccbcfda --- /dev/null +++ b/Documentation/PCI/acpi-info.txt @@ -0,0 +1,136 @@ + ACPI considerations for PCI host bridges + +The basic requirement is that the ACPI namespace should describe +*everything* that consumes address space unless there's another +standard way for the OS to find it [1, 2]. For example, windows that +are forwarded to PCI by a PCI host bridge should be described via ACPI +devices, since the OS can't locate the host bridge by itself. PCI +devices *below* the host bridge do not need to be described via ACPI, +because the resources they consume are inside the host bridge windows, +and the OS can discover them via the standard PCI enumeration +mechanism (using config accesses to read and size the BARs). + +This ACPI resource description is done via _CRS methods of devices in +the ACPI namespace [2]. _CRS methods are like generalized PCI BARs: +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 won't 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 an ACPI device, 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. + +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 any address space that is (1) not claimed by some +other ACPI device and (2) should not be assigned by the OS to +something else, should be claimed by a PNP0C02 _CRS method. + +PCI host bridges are PNP0A03 or PNP0A08 devices. Their _CRS should +describe all the address space they consume. In principle, this would +be all the windows they forward down to the PCI bus, as well as the +bridge registers themselves. The 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. These bridge registers also +include ECAM space, since it is consumed by the bridge. + +ACPI defined a Producer/Consumer bit that was intended to distinguish +the bridge apertures from the bridge registers [4, 5]. However, +BIOSes didn't use that bit correctly, and the result is that OSes have +to assume that everything in a PCI host bridge _CRS is a window. That +leaves no way to describe the bridge registers in the PNP0A03/PNP0A08 +device itself. + +The workaround is 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. For +ECAM, pci_root.c learns about the space from either MCFG or the _CBA +method. + +Note that the PCIe spec actually does require ECAM unless there's a +standard firmware interface for config access, e.g., the ia64 SAL +interface [7]. One reason is that we want a generic host bridge +driver (pci_root.c), and a generic driver requires a generic way to +access config space. + + +[1] ACPI 6.0, 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 6.0, 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 6.0, 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 6.0, sec 6.4.3.5.4: + Extended Address Space Descriptor + General Flags: Bit [0] Consumer/Producer: + 1–This device consumes this resource + 0–This device produces and consumes this resource + +[5] ACPI 6.0, 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 3.0, 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 3.0, 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.