path: root/Documentation/drivers/dt_entries.md

# 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. While no
boards in the tree currently implement this, it has been used in downstream
forks without issue for some time now.

### 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 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)