NVIDIA Jetson - Device Tree Overlay

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Introduction to Device Tree Overlay

This wiki is intended to be used as a reference if you want to create a Device Tree (DT) overlay for a custom hardware module. DT overlays are used to configure various hardware devices that may be attached to the system.

NVIDIA Jetson kernels use a device tree to describe the hardware present in the NVIDIA Jetson board. You can use Jetson‑IO tool to support a custom hardware module by creating a device tree overlay for the hardware module to allow optional external hardware to be described and configured.

Create a Device Tree Overlay

Jetson‑IO

NVIDIA provides the Jetson Expansion Header Tool (also known as Jetson‑IO). It is a Python script that runs on a Jetson developer kit and lets you apply a DT overlay configuration through a graphic user interface. Jetson‑IO finds the overlay file and allows you to apply it.

You can use Jetson‑IO to perform 2 tasks:

  1. Configure header pins manually: displays the expansion header configuration screen, which lets you specify which functions to enable on the header. The application creates the DT overlay and it is applied.
  2. Configure for compatible hardware: displays the compatible hardware screen, which lets you select from a list of configurations for hardware modules that can be attached to the header. You can use a pre-created DT overlay for certain hardware or create your custom DT overlay and select it from the list.

This guide uses the second option to apply a custom DT overlay.

Device Tree Overlay structure

A device tree overlay for a hardware module must define the property:

  • overlay-name: specifies a name for the hardware module.
  • jetson-header-name: specifies the expansion header to which the hardware module is associated; it must specify one of the values described in the table jetson-header-name property values.
  • compatible: indicates which combination of Jetson module and carrier board the overlay supports; t must specify one or more of the values described in the table compatible property values.
  • fragment: a DT overlay comprises a number of fragments, each of them indicates child nodes and the target.
  • target: phandle target of the overlay.
  • target-path: target path of the overlay.

Creating a Simple Device Tree Overlay

To create a simple device tree overlay to add a new custom property for Jetson Xavier NX Developer Kit and attach it to the 40‑pin expansion header, create a file named my-overlay.dts on the target platform with the following contents: 

/dts-v1/;
 
/ {
    overlay-name = "My Jetson Overlay Example";
    jetson-header-name = "Jetson 40pin Header";
    compatible = "nvidia,p3509-0000+p3668-0001";
 
    fragment@0 {
        target-path = "/";
        __overlay__ {
            my-custom-property = "This Is My Overlay";
        };
    };
};

Enter the following command to compile the DTS source file into an overlay file: 

dtc -O dtb -o my-overlay.dtbo -@ my-overlay.dts

After you copy the new overlay file to the /boot directory, the Jetson‑IO tool finds the overlay file and allows you to apply it. If you use an incorrect compatible value the DT overlay will not be listed. 

sudo cp my-overlay.dtbo /boot
sudo /opt/nvidia/jetson-io/config-by-hardware.py -l

Configuration listed from Xavier NX: 

Configurations for the following hardware modules are available:
1. Adafruit SPH0645LM4H
2. FE-PI Audio V1 and Z V2
3. My Jetson Overlay example
4. ReSpeaker 4 Mic Array

Apply the device tree overlay created 

sudo /opt/nvidia/jetson-io/config-by-hardware.py -n "My Jetson Overlay example"

To apply the changes, the board needs to be rebooted. After rebooting the board, you can read the new property defined in the device tree using the DT overlay. 

cat /proc/device-tree/my-custom-property

Output: 

This Is My Overlay

Remove overlay changes applied

The changes of the DT overlay are applied over the original DT and a new device tree is created. The name of the resulting device tree combines the original DT name and the overlay name. /boot/kernel_tegra194-p3668-all-p3509-0000-my-jetson-overlay-example.dtb

To use the new DT, Jetson‑IO creates a new entry in /boot/extlinux/extlinux.conf

This is the new entry added that is added to the /boot/extlinux/extlinux.conf file when the overlay support has been added: 

LABEL My Jetson Overlay example
	MENU LABEL My Jetson Overlay example
	LINUX /boot/Image
	FDT /boot/kernel_tegra194-p3668-all-p3509-0000-my-jetson-overlay-example.dtb
	INITRD /boot/initrd
	APPEND ${cbootargs} quiet root=/dev/mmcblk0p1 rw rootwait rootfstype=ext4 console=ttyTCU0,115200n8 console=tty0 fbcon=map:0 net.ifnames=0

To remove the new overlay support, simply delete the new entry from the /boot/extlinux/extlinux.conf file, and then reboot the board. After rebooting the board, you can confirm that the property defined in the DT overlay is no longer defined since now we are using the original device tree. 

cat /proc/device-tree/my-custom-property

Output: 

cat: /proc/device-tree/my-custom-property: No such file or directory

Device tree overlay nodes

In a device tree overlay, symbols and labels that are not defined in the device tree overlay can not be used, since they are only defined in the original device tree. To manage these cases, the device tree overlay creates extra nodes to define symbols and labels that are not defined. This extra node can be created manually, or you can use the tag /plugin/ and the device tree compiler dynamically resolves all the references to the device tree using the nodes below:

  • __overlay__: contains the body of which is added to the target node.
  • __symbols__: this node is only required for the target=<phandle> method, since it contains the information required to map from a phandle to a tree location.
  • __fixups__: contains a list of properties that map the names of unresolved symbols to the lists of paths to cells within the fragments that need patching with the phandle of the target node. This section only is created if the device tree contains /plugin/ tag.
  • __local_fixups__: holds the locations of any references to labels that exist within the overlay.

To show the device tree nodes work, we are going to create a device tree overlay to add a change to the GPIO configuration for Jetson TX2 Developer Kit.

Create a file named tx2-uart-overlay.dts on the target platform with the following contents: 

/dts-v1/;
/plugin/;
/ {
    overlay-name = "tx2_uart_overlay";
    compatible = "nvidia,p2597-0000+p3310-1000";

    fragment@0 {
        target = <&pinmux>;

        __overlay__ {
            pinctrl-names = "default";
            pinctrl-0 = <&hdr40_pinmux>;

            hdr40_pinmux: header-40pin-pinmux {
                pin8 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_tx_pt0";
                    nvidia,pull = <0x0>;
                    nvidia,tristate = <0x0>;
                    nvidia,enable-input = <0x0>;
                    nvidia,lpdr = <0x0>;
                };
                pin10 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_rx_pt1";
                    nvidia,pull = <0x2>;
                    nvidia,tristate = <0x1>;
                    nvidia,enable-input = <0x01>;
                    nvidia,lpdr = <0x0>;
                };
            };
        };
    };
};

The device tree overlays contains:

  • label: hdr40_pinmux
  • Properties that map the names of unresolved symbols: target = <&pinmux>;
  • References to labels that exist within the overlay: hdr40_pinmux

To see how the compiler dynamically resolves these labels and symbols, you can see the dtbo file generated. Compile the device tree and use fdtdump to see the device tree overlay content. 

dtc -O dtb -o tx2-uart-overlay.dtbo -@ tx2-uart-overlay.dts
fdtdump tx2-uart-overlay.dtbo

This is the fdtdump tool output, since the tag /plugin/ was added to the overlay, the nodes __symbols__, __fixups__ and __local_fixups__ are created automatically. 

/dts-v1/;
// magic:		0xd00dfeed
// totalsize:		0x394 (916)
// off_dt_struct:	0x38
// off_dt_strings:	0x2f0
// off_mem_rsvmap:	0x28
// version:		17
// last_comp_version:	16
// boot_cpuid_phys:	0x0
// size_dt_strings:	0xa4
// size_dt_struct:	0x2b8

/ {
    overlay-name = "d3_overlay_v4";
    compatible = "nvidia,p2597-0000+p3310-1000";
    fragment@0 {
        target = <0xffffffff>;
        __overlay__ {
            pinctrl-names = "default";
            pinctrl-0 = <0x00000001>;
            header-40pin-pinmux {
                phandle = <0x00000001>;
                pin8 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_tx_pt0";
                    nvidia,pull = <0x00000000>;
                    nvidia,tristate = <0x00000000>;
                    nvidia,enable-input = <0x00000000>;
                    nvidia,lpdr = <0x00000000>;
                };
                pin10 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_rx_pt1";
                    nvidia,pull = <0x00000002>;
                    nvidia,tristate = <0x00000001>;
                    nvidia,enable-input = <0x00000001>;
                    nvidia,lpdr = <0x00000000>;
                };
            };
        };
    };
    __symbols__ {
        hdr40_pinmux = "/fragment@0/__overlay__/header-40pin-pinmux";
    };
    __fixups__ {
        pinmux = "/fragment@0:target:0";
    };
    __local_fixups__ {
        fragment@0 {
            __overlay__ {
                pinctrl-0 = <0x00000000>;
            };
        };
    };
};

Notes

Example: Since ub953_0 is created and used also as a target, This will produce an error when is applied the overlay. 

    fragment@0 {
        target = <&i2c_fpdlink>; // i2c2 = "i2c_fpdlink: i2c@0"
        __overlay__ {
            ub953_0: ub953@40 {
            compatible = "d3,ub953";
            status = "disabled";
            . . .
             };
        };
    };

    fragment@1 {
        target = <&ub953_0>;
        __overlay__ {
            ov10640_0: ov10640@60 {
                status = "disabled";
                compatible = "d3,ov10640";
                . . .
	    }; /* End __overlay__ */
    }; /* End fragment@1 */

Pre-process the DT overlay file with the C preprocessor (cpp)

If you want to include kernel definition since the device tree compiler doesn't support C definition syntax you have to use C Pre-process (cpp) to create a file that can be compiled by the Device Tree compiler.

Example: This is the same previous device tree but using pinctrl tegra definitions to set GPIO configuration: 

/dts-v1/;
/plugin/;

#include <dt-bindings/pinctrl/pinctrl-tegra.h>
#define OVERLAY_VERSION "0.1"
/ {
    overlay-name = "tx2_uart_overlay";
    compatible = "nvidia,p2597-0000+p3310-1000";

    fragment@0 {
        target-path = "/";
        __overlay__ {
            d3,dts-overlay = __FILE__;
            d3,dts-overlay-version = OVERLAY_VERSION;
        };
    };
    
    fragment@1 {
        target = <&pinmux>;

        __overlay__ {
            pinctrl-names = "default";
            pinctrl-0 = <&hdr40_pinmux>;
            hdr40_pinmux: header-40pin-pinmux {
                pin8 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_tx_pt0";
                    nvidia,pull = <TEGRA_PIN_PULL_NONE>;
                    nvidia,tristate = <TEGRA_PIN_DISABLE>;
                    nvidia,enable-input = <TEGRA_PIN_DISABLE>;
                    nvidia,lpdr = <TEGRA_PIN_DISABLE>;
                };
                pin10 {
                    nvidia,function = "uarta";
                    nvidia,pins = "uart1_rx_pt1";
                    nvidia,pull = <TEGRA_PIN_PULL_UP>;
                    nvidia,tristate = <TEGRA_PIN_ENABLE>;
                    nvidia,enable-input = <TEGRA_PIN_ENABLE>;
                    nvidia,lpdr = <TEGRA_PIN_DISABLE>;
                };
            };
        };
    };
};

Use the C Pre-process (cpp) to replace Constants in the device tree overlay 

cpp -nostdinc -I kernel/kernel-4.9/include/ -undef -x assembler-with-cpp tx2-uart-overlay.dts  tx2-uart-overlay.dts.preprocessed

Parameter description:

  • -nostdinc: Do not search the standard system directories for header files.
  • -undef: Do not predefine any system-specific or GCC-specific macros.
  • -x assembler-with-cpp: Allow #tags in the code (example: #address-cells)

Applying the overlay manually

The compiled overlay can be applied to an already existing device tree blob without the Jetson IO tool by using the fdtoverlay command. This is particularly useful in platforms that don't support the Jetson IO tool, such as the production version of the Jetson Nano (with EMMC).

The ftdoverlay command takes the base device tree blob and applies the overlay on it, generating a new device tree. 

$ fdtoverlay --help
Usage: apply a number of overlays to a base blob
	fdtoverlay <options> [<overlay.dtbo> [<overlay.dtbo>]]

<type>	s=string, i=int, u=unsigned, x=hex
	Optional modifier prefix:
		hh or b=byte, h=2 byte, l=4 byte (default)

Options: -[i:o:vhV]
  -i, --input <arg>  Input base DT blob
  -o, --output <arg> Output DT blob
  -v, --verbose      Verbose messages
  -h, --help         Print this help and exit
  -V, --version      Print version and exit

The usual use case looks like the following: 

fdtoverlay -i tegra210-p3448-0002-p3449-0000-b00.dtb -o tegra210-p3448-0002-p3449-0000-b00-extended.dtb my-overlay.dtbo

In order for the new overlaid device tree to be used, a new entry in /boot/extlinux/extlinux.conf needs to be added; pointing to the proper kernel Image and DTB. As previously mentioned, the Jetson IO tool does this automatically.

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