401 lines
17 KiB
ReStructuredText
401 lines
17 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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===================================================
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ACPI Device Tree - Representation of ACPI Namespace
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===================================================
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:Copyright: |copy| 2013, Intel Corporation
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:Author: Lv Zheng <lv.zheng@intel.com>
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:Credit: Thanks for the help from Zhang Rui <rui.zhang@intel.com> and
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Rafael J.Wysocki <rafael.j.wysocki@intel.com>.
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Abstract
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========
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The Linux ACPI subsystem converts ACPI namespace objects into a Linux
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device tree under the /sys/devices/LNXSYSTEM:00 and updates it upon
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receiving ACPI hotplug notification events. For each device object
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in this hierarchy there is a corresponding symbolic link in the
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/sys/bus/acpi/devices.
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This document illustrates the structure of the ACPI device tree.
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ACPI Definition Blocks
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======================
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The ACPI firmware sets up RSDP (Root System Description Pointer) in the
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system memory address space pointing to the XSDT (Extended System
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Description Table). The XSDT always points to the FADT (Fixed ACPI
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Description Table) using its first entry, the data within the FADT
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includes various fixed-length entries that describe fixed ACPI features
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of the hardware. The FADT contains a pointer to the DSDT
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(Differentiated System Descripition Table). The XSDT also contains
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entries pointing to possibly multiple SSDTs (Secondary System
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Description Table).
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The DSDT and SSDT data is organized in data structures called definition
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blocks that contain definitions of various objects, including ACPI
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control methods, encoded in AML (ACPI Machine Language). The data block
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of the DSDT along with the contents of SSDTs represents a hierarchical
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data structure called the ACPI namespace whose topology reflects the
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structure of the underlying hardware platform.
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The relationships between ACPI System Definition Tables described above
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are illustrated in the following diagram::
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+---------+ +-------+ +--------+ +------------------------+
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| RSDP | +->| XSDT | +->| FADT | | +-------------------+ |
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+---------+ | +-------+ | +--------+ +-|->| DSDT | |
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| Pointer | | | Entry |-+ | ...... | | | +-------------------+ |
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+---------+ | +-------+ | X_DSDT |--+ | | Definition Blocks | |
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| Pointer |-+ | ..... | | ...... | | +-------------------+ |
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+---------+ +-------+ +--------+ | +-------------------+ |
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| Entry |------------------|->| SSDT | |
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+- - - -+ | +-------------------| |
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| Entry | - - - - - - - -+ | | Definition Blocks | |
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+- - - -+ | | +-------------------+ |
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| | +- - - - - - - - - -+ |
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+-|->| SSDT | |
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| +-------------------+ |
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| | Definition Blocks | |
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| +- - - - - - - - - -+ |
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+------------------------+
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OSPM Loading |
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\|/
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+----------------+
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| ACPI Namespace |
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+----------------+
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Figure 1. ACPI Definition Blocks
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.. note:: RSDP can also contain a pointer to the RSDT (Root System
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Description Table). Platforms provide RSDT to enable
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compatibility with ACPI 1.0 operating systems. The OS is expected
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to use XSDT, if present.
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Example ACPI Namespace
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======================
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All definition blocks are loaded into a single namespace. The namespace
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is a hierarchy of objects identified by names and paths.
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The following naming conventions apply to object names in the ACPI
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namespace:
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1. All names are 32 bits long.
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2. The first byte of a name must be one of 'A' - 'Z', '_'.
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3. Each of the remaining bytes of a name must be one of 'A' - 'Z', '0'
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- '9', '_'.
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4. Names starting with '_' are reserved by the ACPI specification.
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5. The '\' symbol represents the root of the namespace (i.e. names
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prepended with '\' are relative to the namespace root).
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6. The '^' symbol represents the parent of the current namespace node
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(i.e. names prepended with '^' are relative to the parent of the
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current namespace node).
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The figure below shows an example ACPI namespace::
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+------+
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| \ | Root
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+------+
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| +------+
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+-| _PR | Scope(_PR): the processor namespace
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| +------+
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| |
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| | +------+
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| +-| CPU0 | Processor(CPU0): the first processor
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| +------+
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| +------+
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+-| _SB | Scope(_SB): the system bus namespace
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| +------+
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| |
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| | +------+
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| +-| LID0 | Device(LID0); the lid device
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| | +------+
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| | |
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| | | +------+
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| | +-| _HID | Name(_HID, "PNP0C0D"): the hardware ID
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| | | +------+
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| | |
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| | +-| _STA | Method(_STA): the status control method
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| | +------+
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| |
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| | +------+
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| +-| PCI0 | Device(PCI0); the PCI root bridge
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| +------+
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| |
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| | +------+
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| +-| _HID | Name(_HID, "PNP0A08"): the hardware ID
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| | +------+
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| | +------+
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| +-| _CID | Name(_CID, "PNP0A03"): the compatible ID
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| | +------+
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| |
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| | +------+
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| +-| RP03 | Scope(RP03): the PCI0 power scope
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| | +------+
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| | +-| PXP3 | PowerResource(PXP3): the PCI0 power resource
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| | +------+
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| |
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| | +------+
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| +-| GFX0 | Device(GFX0): the graphics adapter
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| +------+
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| | +------+
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| +-| _ADR | Name(_ADR, 0x00020000): the PCI bus address
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| | +------+
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| |
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| | +------+
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| +-| DD01 | Device(DD01): the LCD output device
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| +------+
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| +-| _BCL | Method(_BCL): the backlight control method
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| +------+
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| +------+
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+-| _TZ | Scope(_TZ): the thermal zone namespace
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| +------+
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| +-| FN00 | PowerResource(FN00): the FAN0 power resource
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| | +------+
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| | +------+
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| +-| FAN0 | Device(FAN0): the FAN0 cooling device
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| | +------+
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| | +-| _HID | Name(_HID, "PNP0A0B"): the hardware ID
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| | +------+
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| +-| TZ00 | ThermalZone(TZ00); the FAN thermal zone
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| +------+
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| +------+
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+-| _GPE | Scope(_GPE): the GPE namespace
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+------+
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Figure 2. Example ACPI Namespace
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Linux ACPI Device Objects
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=========================
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The Linux kernel's core ACPI subsystem creates struct acpi_device
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objects for ACPI namespace objects representing devices, power resources
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processors, thermal zones. Those objects are exported to user space via
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sysfs as directories in the subtree under /sys/devices/LNXSYSTM:00. The
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format of their names is <bus_id:instance>, where 'bus_id' refers to the
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ACPI namespace representation of the given object and 'instance' is used
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for distinguishing different object of the same 'bus_id' (it is
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two-digit decimal representation of an unsigned integer).
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The value of 'bus_id' depends on the type of the object whose name it is
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part of as listed in the table below::
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+---+-----------------+-------+----------+
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| | Object/Feature | Table | bus_id |
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+---+-----------------+-------+----------+
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| N | Root | xSDT | LNXSYSTM |
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+---+-----------------+-------+----------+
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| N | Device | xSDT | _HID |
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+---+-----------------+-------+----------+
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| N | Processor | xSDT | LNXCPU |
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+---+-----------------+-------+----------+
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| N | ThermalZone | xSDT | LNXTHERM |
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+---+-----------------+-------+----------+
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| N | PowerResource | xSDT | LNXPOWER |
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+---+-----------------+-------+----------+
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| N | Other Devices | xSDT | device |
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+---+-----------------+-------+----------+
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| F | PWR_BUTTON | FADT | LNXPWRBN |
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+---+-----------------+-------+----------+
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| F | SLP_BUTTON | FADT | LNXSLPBN |
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+---+-----------------+-------+----------+
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| M | Video Extension | xSDT | LNXVIDEO |
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+---+-----------------+-------+----------+
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| M | ATA Controller | xSDT | LNXIOBAY |
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+---+-----------------+-------+----------+
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| M | Docking Station | xSDT | LNXDOCK |
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+---+-----------------+-------+----------+
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Table 1. ACPI Namespace Objects Mapping
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The following rules apply when creating struct acpi_device objects on
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the basis of the contents of ACPI System Description Tables (as
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indicated by the letter in the first column and the notation in the
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second column of the table above):
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N:
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The object's source is an ACPI namespace node (as indicated by the
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named object's type in the second column). In that case the object's
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directory in sysfs will contain the 'path' attribute whose value is
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the full path to the node from the namespace root.
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F:
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The struct acpi_device object is created for a fixed hardware
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feature (as indicated by the fixed feature flag's name in the second
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column), so its sysfs directory will not contain the 'path'
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attribute.
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M:
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The struct acpi_device object is created for an ACPI namespace node
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with specific control methods (as indicated by the ACPI defined
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device's type in the second column). The 'path' attribute containing
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its namespace path will be present in its sysfs directory. For
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example, if the _BCL method is present for an ACPI namespace node, a
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struct acpi_device object with LNXVIDEO 'bus_id' will be created for
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it.
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The third column of the above table indicates which ACPI System
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Description Tables contain information used for the creation of the
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struct acpi_device objects represented by the given row (xSDT means DSDT
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or SSDT).
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The fourth column of the above table indicates the 'bus_id' generation
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rule of the struct acpi_device object:
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_HID:
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_HID in the last column of the table means that the object's bus_id
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is derived from the _HID/_CID identification objects present under
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the corresponding ACPI namespace node. The object's sysfs directory
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will then contain the 'hid' and 'modalias' attributes that can be
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used to retrieve the _HID and _CIDs of that object.
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LNXxxxxx:
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The 'modalias' attribute is also present for struct acpi_device
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objects having bus_id of the "LNXxxxxx" form (pseudo devices), in
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which cases it contains the bus_id string itself.
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device:
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'device' in the last column of the table indicates that the object's
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bus_id cannot be determined from _HID/_CID of the corresponding
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ACPI namespace node, although that object represents a device (for
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example, it may be a PCI device with _ADR defined and without _HID
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or _CID). In that case the string 'device' will be used as the
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object's bus_id.
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Linux ACPI Physical Device Glue
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===============================
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ACPI device (i.e. struct acpi_device) objects may be linked to other
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objects in the Linux' device hierarchy that represent "physical" devices
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(for example, devices on the PCI bus). If that happens, it means that
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the ACPI device object is a "companion" of a device otherwise
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represented in a different way and is used (1) to provide configuration
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information on that device which cannot be obtained by other means and
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(2) to do specific things to the device with the help of its ACPI
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control methods. One ACPI device object may be linked this way to
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multiple "physical" devices.
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If an ACPI device object is linked to a "physical" device, its sysfs
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directory contains the "physical_node" symbolic link to the sysfs
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directory of the target device object. In turn, the target device's
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sysfs directory will then contain the "firmware_node" symbolic link to
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the sysfs directory of the companion ACPI device object.
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The linking mechanism relies on device identification provided by the
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ACPI namespace. For example, if there's an ACPI namespace object
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representing a PCI device (i.e. a device object under an ACPI namespace
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object representing a PCI bridge) whose _ADR returns 0x00020000 and the
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bus number of the parent PCI bridge is 0, the sysfs directory
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representing the struct acpi_device object created for that ACPI
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namespace object will contain the 'physical_node' symbolic link to the
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/sys/devices/pci0000:00/0000:00:02:0/ sysfs directory of the
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corresponding PCI device.
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The linking mechanism is generally bus-specific. The core of its
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implementation is located in the drivers/acpi/glue.c file, but there are
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complementary parts depending on the bus types in question located
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elsewhere. For example, the PCI-specific part of it is located in
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drivers/pci/pci-acpi.c.
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Example Linux ACPI Device Tree
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=================================
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The sysfs hierarchy of struct acpi_device objects corresponding to the
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example ACPI namespace illustrated in Figure 2 with the addition of
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fixed PWR_BUTTON/SLP_BUTTON devices is shown below::
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+--------------+---+-----------------+
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| LNXSYSTEM:00 | \ | acpi:LNXSYSTEM: |
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+--------------+---+-----------------+
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| +-------------+-----+----------------+
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+-| LNXPWRBN:00 | N/A | acpi:LNXPWRBN: |
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| +-------------+-----+----------------+
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| +-------------+-----+----------------+
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+-| LNXSLPBN:00 | N/A | acpi:LNXSLPBN: |
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| +-------------+-----+----------------+
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| +-----------+------------+--------------+
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+-| LNXCPU:00 | \_PR_.CPU0 | acpi:LNXCPU: |
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| +-----------+------------+--------------+
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| +-------------+-------+----------------+
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+-| LNXSYBUS:00 | \_SB_ | acpi:LNXSYBUS: |
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| +-------------+-------+----------------+
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| |
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| | +- - - - - - - +- - - - - - +- - - - - - - -+
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| +-| PNP0C0D:00 | \_SB_.LID0 | acpi:PNP0C0D: |
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| | +- - - - - - - +- - - - - - +- - - - - - - -+
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| | +------------+------------+-----------------------+
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| +-| PNP0A08:00 | \_SB_.PCI0 | acpi:PNP0A08:PNP0A03: |
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| +------------+------------+-----------------------+
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| | +-----------+-----------------+-----+
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| +-| device:00 | \_SB_.PCI0.RP03 | N/A |
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| | +-----------+-----------------+-----+
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| | | +-------------+----------------------+----------------+
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| | +-| LNXPOWER:00 | \_SB_.PCI0.RP03.PXP3 | acpi:LNXPOWER: |
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| | +-------------+----------------------+----------------+
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| | +-------------+-----------------+----------------+
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| +-| LNXVIDEO:00 | \_SB_.PCI0.GFX0 | acpi:LNXVIDEO: |
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| +-------------+-----------------+----------------+
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| | +-----------+-----------------+-----+
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| +-| device:01 | \_SB_.PCI0.DD01 | N/A |
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| +-----------+-----------------+-----+
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| +-------------+-------+----------------+
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+-| LNXSYBUS:01 | \_TZ_ | acpi:LNXSYBUS: |
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+-------------+-------+----------------+
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| +-------------+------------+----------------+
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+-| LNXPOWER:0a | \_TZ_.FN00 | acpi:LNXPOWER: |
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| +-------------+------------+----------------+
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| +------------+------------+---------------+
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+-| PNP0C0B:00 | \_TZ_.FAN0 | acpi:PNP0C0B: |
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| +------------+------------+---------------+
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| +-------------+------------+----------------+
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+-| LNXTHERM:00 | \_TZ_.TZ00 | acpi:LNXTHERM: |
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+-------------+------------+----------------+
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Figure 3. Example Linux ACPI Device Tree
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.. note:: Each node is represented as "object/path/modalias", where:
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1. 'object' is the name of the object's directory in sysfs.
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2. 'path' is the ACPI namespace path of the corresponding
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ACPI namespace object, as returned by the object's 'path'
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sysfs attribute.
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3. 'modalias' is the value of the object's 'modalias' sysfs
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attribute (as described earlier in this document).
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.. note:: N/A indicates the device object does not have the 'path' or the
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'modalias' attribute.
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