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diff --git a/Documentation/acpi/devicetree.md b/Documentation/acpi/devicetree.md new file mode 100644 index 0000000000..556c9668f6 --- /dev/null +++ b/Documentation/acpi/devicetree.md @@ -0,0 +1,234 @@ +# Adding new devices to a device tree + +## Introduction + +ACPI exposes a platform-independent interface for operating systems to perform +power management and other platform-level functions. Some operating systems +also use ACPI to enumerate devices that are not immediately discoverable, such +as those behind I2C or SPI busses (in contrast to PCI). This document discusses +the way that coreboot uses the concept of a "device tree" to generate ACPI +tables for usage by the operating system. + +## Devicetree and overridetree (if applicable) + +For mainboards that are organized around a "reference board" or "baseboard" +model (see ``src/mainboard/google/octopus`` or ``hatch`` for examples), there is +typically a devicetree.cb file that all boards share, and any differences for a +specific board ("variant") are captured in the overridetree.cb file. Any +settings changed in the overridetree take precedence over those in the main +devicetree. Note, not all mainboards will have the devicetree/overridetree +distinction, and may only have a devicetree.cb file. Or you can always just +write the ASL (ACPI Source Language) code yourself. + +## Device drivers + +Let's take a look at an example entry from +``src/mainboard/google/hatch/variant/hatch/overridetree.cb``: + +``` +device pci 15.0 on + chip drivers/i2c/generic + register "hid" = ""ELAN0000"" + register "desc" = ""ELAN Touchpad"" + register "irq" = "ACPI_IRQ_WAKE_EDGE_LOW(GPP_A21_IRQ)" + register "wake" = "GPE0_DW0_21" + device i2c 15 on end + end +end # I2C #0 +``` + +When this entry is processed during ramstage, it will create a device in the +ACPI SSDT table (all devices in devicetrees end up in the SSDT table). The ACPI +generation routines in coreboot actually generate the raw bytecode that +represents the device's structure, but looking at ASL code is easier to +understand; see below for what the disassembled bytecode looks like: + +``` +Scope (\_SB.PCI0.I2C0) +{ + Device (D015) + { + Name (_HID, "ELAN0000") // _HID: Hardware ID + Name (_UID, Zero) // _UID: Unique ID + Name (_DDN, "ELAN Touchpad") // _DDN: DOS Device Name + Method (_STA, 0, NotSerialized) // _STA: Status + { + Return (0x0F) + } + Name (_CRS, ResourceTemplate () // _CRS: Current Resource Settings + { + I2cSerialBusV2 (0x0015, ControllerInitiated, 400000, + AddressingMode7Bit, "\\_SB.PCI0.I2C0", + 0x00, ResourceConsumer, , Exclusive, ) + Interrupt (ResourceConsumer, Edge, ActiveLow, ExclusiveAndWake, ,, ) + { + 0x0000002D, + } + }) + Name (_S0W, 0x04) // _S0W: S0 Device Wake State + Name (_PRW, Package (0x02) // _PRW: Power Resources for Wake + { + 0x15, // GPE #21 + 0x03 // Sleep state S3 + }) + } +} +``` + +You can see it generates _HID, _UID, _DDN, _STA, _CRS, _S0W, and _PRW +names/methods in the Device's scope. + +## Utilizing a device driver + +The device driver must be enabled for your build. There will be a CONFIG option +in the Kconfig file in the directory that the driver is in (e.g., +``src/drivers/i2c/generic`` contains a Kconfig file; the option here is named +CONFIG_DRIVERS_I2C_GENERIC). The config option will need to be added to your +mainboard's Kconfig file (e.g., ``src/mainboard/google/hatch/Kconfig``) in order +to be compiled into your build. + +## Diving into the above example: + +Let's take a look at how the devicetree language corresponds to the generated +ASL. + +First, note this: + +``` + chip drivers/i2c/generic +``` + +This means that the device driver we're using has a corresponding structure, +located at ``src/drivers/i2c/generic/chip.h``, named **struct +drivers_i2c_generic_config** and it contains many properties you can specify to +be included in the ACPI table. + +### hid + +``` + register "hid" = ""ELAN0000"" +``` + +This corresponds to **const char *hid** in the struct. In the ACPI ASL, it +translates to: + +``` + Name (_HID, "ELAN0000") // _HID: Hardware ID +``` + +under the device. **This property is used to match the device to its driver +during enumeration in the OS.** + +### desc + +``` + register "desc" = ""ELAN Touchpad"" +``` + +corresponds to **const char *desc** and in ASL: + +``` + Name (_DDN, "ELAN Touchpad") // _DDN: DOS Device Name +``` + +### irq + +It also adds the interrupt, + +``` + Interrupt (ResourceConsumer, Edge, ActiveLow, ExclusiveAndWake, ,, ) + { + 0x0000002D, + } +``` + +which comes from: + +``` + register "irq" = "ACPI_IRQ_WAKE_EDGE_LOW(GPP_A21_IRQ)" +``` + +The GPIO pin IRQ settings control the "Edge", "ActiveLow", and +"ExclusiveAndWake" settings seen above (edge means it is an edge-triggered +interrupt as opposed to level-triggered; active low means the interrupt is +triggered on a falling edge). + +Note that the ACPI_IRQ_WAKE_EDGE_LOW macro informs the platform that the GPIO +will be routed through SCI (ACPI's System Control Interrupt) for use as a wake +source. Also note that the IRQ names are SoC-specific, and you will need to +find the names in your SoC's header file. The ACPI_* macros are defined in +``src/arch/x86/include/arch/acpi_device.h``. + +Using a GPIO as an IRQ requires that it is configured in coreboot correctly. +This is often done in a mainboard-specific file named ``gpio.c``. + +### wake + +The last register is: + +``` + register "wake" = "GPE0_DW0_21" +``` + +which indicates that the method of waking the system using the touchpad will be +through a GPE, #21 associated with DW0, which is set up in devicetree.cb from +this example. The "21" indicates GPP_X21, where GPP_X is mapped onto DW0 +elsewhere in the devicetree. + +The last bit of the definition of that device includes: + +``` + device i2c 15 on end +``` + +which means it's an I2C device, with 7-bit address 0x15, and the device is "on", +meaning it will be exposed in the ACPI table. The PCI device that the +controller is located in determines which I2C bus the device is expected to be +found on. In this example, this is I2C bus 0. This also determines the ACPI +"Scope" that the device names and methods will live under, in this case +"\_SB.PCI0.I2C0". + +## Other auto-generated names + +(see [ACPI specification +6.3](https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf) +for more details on ACPI methods) + +### _S0W (S0 Device Wake State) +_S0W indicates the deepest S0 sleep state this device can wake itself from, +which in this case is 4, representing _D3cold_. + +### _PRW (Power Resources for Wake) +_PRW indicates the power resources and events required for wake. There are no +dependent power resources, but the GPE (GPE0_DW0_21) is mentioned here (0x15), +as well as the deepest sleep state supporting waking the system (3), which is +S3. + +### _STA (Status) +The _STA method is generated automatically, and its values, 0xF, indicates the +following: + + Bit [0] – Set if the device is present. + Bit [1] – Set if the device is enabled and decoding its resources. + Bit [2] – Set if the device should be shown in the UI. + Bit [3] – Set if the device is functioning properly (cleared if device failed its diagnostics). + +### _CRS (Current resource settings) +The _CRS method is generated automatically, as the driver knows it is an I2C +controller, and so specifies how to configure the controller for proper +operation with the touchpad. + +``` +Name (_CRS, ResourceTemplate () // _CRS: Current Resource Settings +{ + I2cSerialBusV2 (0x0015, ControllerInitiated, 400000, + AddressingMode7Bit, "\\_SB.PCI0.I2C0", + 0x00, ResourceConsumer, , Exclusive, ) +``` + +## Notes + + - **All fields that are left unspecified in the devicetree are initialized to + zero.** + - **All devices in devicetrees end up in the SSDT table, and are generated in + coreboot's ramstage** diff --git a/Documentation/index.md b/Documentation/index.md index a2c2878ddb..50141b50bf 100644 --- a/Documentation/index.md +++ b/Documentation/index.md @@ -172,6 +172,7 @@ Contents: * [Intel IFD Binary Extraction](Binary_Extraction.md) * [Dealing with Untrusted Input in SMM](technotes/2017-02-dealing-with-untrusted-input-in-smm.md) * [GPIO toggling in ACPI AML](acpi/gpio.md) +* [Adding devices to a device tree](acpi/devicetree.md) * [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md) * [Display panel-specific documentation](gfx/display-panel.md) * [Architecture-specific documentation](arch/index.md) |