![]() Kernel contributors have also worked on solutions to load the DTBOs dynamically in the live system, but this solution is not mature yet. Then, at boot time, all you have to do is load the main DTB for your board, and then override it using the DTBOs corresponding to the capes which are currently mounted. For example, in /lib/firmware, the Debian image shipped by contains compiled Device Tree Overlays (DTBO) for the extension boards (also known as capes in the BeagleBoard world) they support, such as: BBORG_COMMS-00A2.dtbo This is exactly what Device Tree Overlays are about. What if you could prepare Device Tree fragments for each change to make, compile them once for all, and then, when you boot your device, load the main Device Tree and only the fragments you need, without having anything else to recompile? Any time you plug in a new device or an expansion board, or want to tweak other settings, you have to rebuild and update the full Device Tree for your board. However, this is still an inconvenient solution. It’s even possible to remove existing nodes and properties by using the /delete-node/ and /delete-property/ statements, as shown in the Device Tree Source Undocumented page. You can then use references to nodes in the original tree, using labels, and then modify the properties of such nodes or add new subnodes to them. It’s always better not to tweak files maintained by others, and find a way to override them instead.Ī second possibility to tweak the Device Tree for a board is to include it from a custom Device Tree for your own use case, using the /include/ statement from the Device Tree language, or #include and the C preprocessor. ![]() However, this is a rather bad idea because it will make it more difficult to upgrade to a later version of the kernel, possibly featuring changes to the Device Tree file you modified too. In you have such a need, a first way to do this would be to directly modify the Device Tree for your hardware in the kernel sources, and recompile it. Modifying operating system related properties, such as the Linux kernel command line or flash partitions.This is called Pin Multiplexing, and the Device Tree is the place where such multiplexing is configured. Changing the hardware blocks that are exposed on the external pins of their System on Chip.The simplest example is an I2C temperature sensor, but this can also include entire expansion boards plugged into the main board. Declaring external devices connected to board connectors.Normal users also have legitimate reasons for tweaking the device tree of their board, such as: Kernel developers and board maintainers are not the only ones who may need to make changes to the board device trees, though. You don’t need to recompile that kernel, at least when it supports your SoC and the devices on your board.ĭevice Tree Source (DTS) files are normally distributed by kernel developers inside the Linux sources, for example in arch/arm/boot/dts/am335x-boneblack.dts for the BeagleBone Black board. Therefore, when you create a new board, and want to use a standard GNU/Linux distribution on it, all you have to do is create a new Device Tree describing your new hardware, compile it, and boot the distribution’s kernel with it. The compiled Device Tree (DTB: Device Tree Binary), passed to the kernel by the bootloader at boot time, lets the kernel know which SoC and devices to initialize. That’s also the case of devices connected to a number of buses, such as I2C and SPI, that do not provide mechanisms for dynamic enumeration and identification of devices.įor a given CPU architecture (ARM, PowerPC, etc), such a description allows to have a unique kernel supporting many different systems with distinct Systems on a Chip. That’s the case of devices directly implemented on a System on a Chip, such as serial ports, Ethernet or Nand flash controllers. The Device Tree language is a way to describe hardware that is present in a system and cannot be automatically detected.
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