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