| 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. |
