[PATCH] PCI: Add information about describing PCI in ACPI

Fri Nov 18 09:17:34 PST 2016

Hi Bjorn

Many thanks for putting this together, it really helps!

One thing below..

> -----Original Message-----
> From: linux-kernel-owner at vger.kernel.org [mailto:linux-kernel-
> owner at vger.kernel.org] On Behalf Of Bjorn Helgaas
> Sent: 17 November 2016 18:00
> To: linux-pci at vger.kernel.org
> Cc: linux-acpi at vger.kernel.org; linux-kernel at vger.kernel.org; linux-
> arm-kernel at lists.infradead.org; linaro-acpi at lists.linaro.org
> Subject: [PATCH] PCI: Add information about describing PCI in ACPI
> 
> 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 at 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.

Right so if my understanding is correct you are saying that resources
described in the MCFG table should also be declared in PNP0C02 devices
so that the PNP driver can reserve these resources.

On the other side the PCI Root bridge driver should not reserve such
resources.

Well if my understanding is correct I think we have a problem here:
http://lxr.free-electrons.com/source/drivers/pci/ecam.c#L74

As you can see pci_ecam_create() will conflict with the pnp driver
as it will try to reserve the resources from the MCFG table...

Maybe we need to rework pci_ecam_create() ?

Thanks

Gab

> +
> +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.