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AT91 Device Tree Overlays and FIT image

1. Introduction

A device tree overlay is a file that can be used at runtime (by the bootloader in our case) to dynamically modify the device tree, by adding nodes to the tree and making changes to properties in the existing tree.

The FIT (Flattened Image Tree) format allows for flexibility in handling images of various types and enhances integrity protection of images with stronger checksums.

2. Build DT-Overlay

To build the overlays for a board make sure the following steps are done:

  • the environment variables ARCH and CROSS_COMPILE are set correctly
  • (optional) the environment variable KERNEL_DIR points to Linux kernel and the kernel was already built for the board. This is needed because the DT Overlay repository uses the Device Tree Compiler (dtc) from the kernel source tree. By default, KERNEL_DIR is set to a linux directory that would be under the parent directory in the directory tree: ../linux
  • (optional) the environment variable KERNEL_BUILD_DIR that points to where the Linux kernel binary and Device Tree blob, resulting of your compilation of the kernel, are located. By default, KERNEL_BUILD_DIR is set to the same directory as KERNEL_DIR. It shouldn't be changed if you have the habit of compiling your kernel within the Linux source tree

The following example shows how to build the overlays for sama5d2_xplained:

$ make sama5d2_xplained_dtbos

3. Build FIT image

To build the FIT image with overlays for a board make sure the following steps are done:

  • the environment variables ARCH and CROSS_COMPILE are set correctly
  • (optional) the environment variable KERNEL_DIR points to Linux kernel and the kernel was already built for the board. This is needed because the DT Overlay repository uses the Device Tree Compiler (dtc) from the kernel source tree. By default, KERNEL_DIR is set to a linux directory that would be under the parent directory in the directory tree: ../linux
  • (optional) the environment variable KERNEL_BUILD_DIR that points to where the Linux kernel binary and Device Tree blob, resulting of your compilation of the kernel, are located. By default, KERNEL_BUILD_DIR is set to the same directory as KERNEL_DIR. It shouldn't be changed if you have the habit of compiling your kernel within the Linux source tree.
  • mkimage is installed on the development machine and the Device Tree Compiler from Linux kernel is in the PATH environment variable

The following example shows how to build the FIT image for sama5d2_xplained:

$ make sama5d2_xplained.itb

4. Loading FIT image in u-boot

The FIT image is a placeholder that has the zImage and the base Device Tree, plus additional overlays that can be selected at boot time.

The following steps are required to boot the FIT Image from U-boot:

  • Load the FIT image like you would normally load the uImage or zImage.
  • There is no need to load additional Device Tree Blob, the FIT image includes it
  • When booting the FIT image, specify the FIT configuration to use. Several configurations can be appended to the basic configuration, which we name 'kernel_dtb'

Example:

bootm 0x24000000#kernel_dtb

This will load the FIT image from address 0x24000000 in memory and then run the configuration named 'kernel_dtb'. This configuration includes the kernel plus the base Device Tree Blob built with the kernel.

To load additional FIT configurations, just append another configuration to the command.

Example to load the image sensor controller Device Tree overlay + sensor omnivision 0v7740:

bootm 0x24000000#kernel_dtb#isc#ov7740

5. Contributing

To contribute to AT91 Device Tree Overlays you should submit the patches for review to the github pull-request facility directly. Do not forget to Cc the maintainers.

Maintainers:

Cristian Birsan [email protected]

Nicolas Ferre [email protected]

When creating patches insert the [dt-overlay-at91] tag in the subject, for example use something like:

git format-patch -s --subject-prefix='dt-overlay-at91][PATCH' <origin>