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Driver Devicetree Entries

Let's take a look at an example entry from src/mainboard/google/hatch/variants/hatch/overridetree.cb:

device pci 15.0 on
	chip drivers/i2c/generic
		register "hid" = ""ELAN0000""
		register "desc" = ""ELAN Touchpad""
		register "irq" = "ACPI_IRQ_WAKE_LEVEL_LOW(GPP_A21_IRQ)"
		register "detect" = "1"
		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, Level, ActiveLow, ExclusiveAndWake, ,, )
            {
                0x0000002D,
            }
        })
        Name (_S0W, ACPI_DEVICE_SLEEP_D3_HOT)  // _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, Level, ActiveLow, ExclusiveAndWake, ,, )
    {
        0x0000002D,
    }

which comes from:

    register "irq" = "ACPI_IRQ_WAKE_LEVEL_LOW(GPP_A21_IRQ)"

The GPIO pin IRQ settings control the "Level", "ActiveLow", and "ExclusiveAndWake" settings seen above (level means it is a level-triggered interrupt as opposed to edge-triggered; active low means the interrupt is triggered when the signal is low).

Note that the ACPI_IRQ_WAKE_LEVEL_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/acpi/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.

detect

The next register is:

    register "detect" = "1"

This flag tells the I2C driver that it should attempt to detect the presence of the device (using an I2C zero-byte write), and only generate a SSDT entry if the device is actually present. This alleviates the OS from having to determine if a device is present or not (ChromeOS/Linux) and prevents resource conflict/ driver issues (Windows).

Currently, the detect feature works and is hooked up for all I2C touchpads, and should be used any time a board has multiple touchpad options. I2C audio devices should also work without issue.

Touchscreens can use this feature as well, but special care is needed to implement the proper power sequencing for the device to be detected. Generally, this means driving the enable GPIO high and holding the reset GPIO low in early GPIO init (bootblock/romstage), then releasing reset in ramstage. The first mainboards in the tree to implement this are google/skyrim and google/guybrush. This feature has also been used in downstream forks without issue for some time now on several other boards.

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 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 ACPI_DEVICE_SLEEP_D3_HOT, representing D3hot.

_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 device driver entries in devicetrees end up in the SSDT table, and are generated in coreboot's ramstage (The lone exception to this rule is i2c touchpads with the 'detect' flag set; in this case, devices not present will not be added to the SSDT)

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