Applying device tree overlay

arch-armdevice-treeembeddedu-boot

I have a device based on imx233-Olinuxino-Nano.

The Arch Linux Arm distro boots and works.

However, there is a slave device on my board that is connected to SPI of CPU. It is supported by kernel, but, obviously, generic device tree for Olinuxino doesn't contain its description.

How do I make it show in /dev directory?
I know I can alter device tree in Linux kernel and recompile that device tree, but I heard that it's possible to supply device tree overlay file to alter the device tree on boot. How do I do that? The information that I found up to now is scarce and vague.

Best Answer

I don't know a lot about this but you can read de devicetree Doc in the folder bindings there are information about specific drivers, here you are an example in the Beaglebone Black,

When I was working with the Beaglebone Black, I place the dtb or dtbo files in the /lib/firmware directory and enable which one I want to load in the following files:

I don't know what bootloader are you using it, this is an example that how I do it using uboot

/boot/uboot/uEnv.txt:

optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN,BB-BONE-EMMC-2G capemgr.enable_partno=BB-UART1,BB-UART4,BB-UART5

/etc/default/capemgr:

CAPE=BBBIO-EHRPWM,BBBIO-EHRPWM-OTHER

Related Solutions

Linux – Is it possible to get the information for a device tree using /sys of a running kernel

/proc/device-tree or /sys/firmware/devicetree/base

/proc/device-tree is a symlink to /sys/firmware/devicetree/base and the kernel documentation says userland should stick to /proc/device-tree:

Userspace must not use the /sys/firmware/devicetree/base path directly, but instead should follow /proc/device-tree symlink. It is possible that the absolute path will change in the future, but the symlink is the stable ABI.

You can then access dts properties from files:

 hexdump /sys/firmware/devicetree/base/apb-pclk/clock-frequency

The output format for integers is binary, so hexdump is needed.

dtc -I fs

Get a full device tree from the filesystem:

sudo apt-get install device-tree-compiler
dtc -I fs -O dts /sys/firmware/devicetree/base

outputs the dts to stdout.

dtc in Buildroot

Buildroot has a BR2_PACKAGE_DTC=y config to put dtc inside the root filesystem.

QEMU -machine dumpdtb

If you are running Linux inside QEMU, QEMU automatically generates the DTBs if you don't give it explicitly with -dtb, and so it is also able to dump it directly with:

qemu-system-aarch64 -machine virt -cpu cortex-a57 -machine dumpdtb=dtb.dtb

as mentioned at: https://lists.gnu.org/archive/html/qemu-discuss/2017-02/msg00051.html

Tested with this QEMU + Buildroot setup on the Linux kernel v4.19 arm64.

Thanks to Harry Tsai for pointing out the kernel documentation that says that /proc/device-tree is preferred for userland.

Getting U-Boot to load dtb from FIT image

The answer to this exact problem comes from understanding that U-Boot tries to be extremely flexible and this can lead to some confusion at times. Looking at the provided environment we can see that we have the bootcmd (which is executed when the boot delay runs out) boils down to:

bootm ${loadaddr} - ${fdtaddr}

And this means that we look at ${loadaddr} for our image, no where for a ramdisk and ${fdtaddr} for the device tree to use. In the case of a legacy-style uImage this makes sense as the ramdisk and device tree are not (likely) to be contained within the file. A FIT image however has all of this included, and offers lots of extra useful features (which the poster wishes to use). What happens is that after picking out the device tree included in the FIT image, U-Boot then parses the rest of the arguments and looks at ${fdtaddr} for the device tree to use. If bootm_cmd was set to simply:

bootm ${loadaddr}

instead, it would work as expected.

Best Answer

Related Solutions

Linux – Is it possible to get the information for a device tree using /sys of a running kernel

Getting U-Boot to load dtb from FIT image

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