Documentation: split devicetree driver hookup into separate page

Move the devicetree driver example into a separate page under the
drivers category, and link to it from both the devicetree page and
the drivers index page. This makes more sense from a grouping
perspective and makes the info easier to find.

Change-Id: Ic3ca80b93a0020737c7ccb5313a0877172022e1a
Signed-off-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67762
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

1 files changed, 2 insertions, 205 deletions

diff --git a/Documentation/getting_started/devicetree.md b/Documentation/getting_started/devicetree.md
index 41f590199944..824c51201a24 100644
--- a/Documentation/getting_started/devicetree.md
+++ b/Documentation/getting_started/devicetree.md
@@ -78,213 +78,10 @@ with no properties as a direct child of the SoC.
 
 ## Device drivers
 
-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 "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``.
-
-### 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, )
-```
+Platform independent device drivers are hooked up via entries in a devicetree.
+See [Driver Devicetree Entries](drivers/dt_entries.md) for more info.
 
 ## 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**