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

armdevice-treelinuxlinux-kernelreverse-engineering

Commonly for arm systems, device trees supply hardware information to the kernel (Linux). These device trees exist as dts (device tree source) files that are compiled and loaded to the kernel. Problem is that I do not have access to such a dts file, not even to a dtb file.

I have access to /sys and /proc on the machine and I wanted to ask if that would allow me to "guess the correct values" to be used in a dts?

Also potential answer could highlight additionally the aspect if the answer to this question also depends on whether the device tree interface was used in the first place (i.e. a dtb was created and provided to the kernel) instead of some more hacking "we simply divert from vanilla and patch the kernel so as to solve the device information problem for our kernel only"-solution?

Best Answer

/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.

Related Solutions

Linux – Is a device tree blob tied to a specific linux kernel version

The Device Tree is supposed to be a stable ABI so a device tree written for any version of the kernel should work with any following kernel version.

However, for practical reasons, this is quite often not the case. You can have a look at the following presentation from Thomas, explaining why: http://free-electrons.com/pub/conferences/2015/elc/petazzoni-dt-as-stable-abi-fairy-tale/petazzoni-dt-as-stable-abi-fairy-tale.pdf

Video: https://www.youtube.com/watch?v=rPRqIS9q6CY

How does kernel know which device tree to load

Determining which device tree blob (DTB) file, which is compiled from the device tree source (DTS), to load is typically handled by the bootloader.

Many device manufacturers will customize the bootloader to add manufacturer-specific code for automating the task of determining which DTB file to load for that hardware.

An example for U-Boot, a commonly used bootloader, that loads a zImage kernel image and the DTB fdt_file from a FAT partition on a SATA device. These are loaded to memory addresses specified by loadaddr and fdt_addr respectively, and then handed off to U-Boot's bootz command.

fatload sata 0:1 ${loadaddr} ${image};
fatload sata 0:1 ${fdt_addr} ${fdt_file};
bootz ${loadaddr} - ${fdt_addr};

U-Boot's bootz usage:

bootz [addr [initrd[:size]] [fdt]]
    - boot Linux zImage stored in memory
        The argument 'initrd' is optional and specifies the address
        of the initrd in memory. The optional argument ':size' allows
        specifying the size of RAW initrd.
        When booting a Linux kernel which requires a flat device-tree
        a third argument is required which is the address of the
        device-tree blob. To boot that kernel without an initrd image,
        use a '-' for the second argument. If you do not pass a third
        a bd_info struct will be passed instead

For specifics regarding how U-Boot handles the fdt_file, you can reference the source code for bootm_find_images, as well as boot_get_fdt.

You can also try to glean some information from the U-Boot manual: https://www.denx.de/wiki/view/DULG/UBootCmdFDT

Best Answer

Related Solutions

Linux – Is a device tree blob tied to a specific linux kernel version

How does kernel know which device tree to load

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