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dtbocfg - Device Tree Blob Overlay Configuration File System

Overview

Device Tree Overlay

Device Tree Overlay is a mechanism that enables dynamic loading/unloading of a new device tree blob on top of the kernel device tree. Device Tree Overlay was first introduced in Linux Kernel version 3.19.

dtbocfg - Device Tree Overlay Configuration File System

Linux Kernel Version 4.4.4, the latest at the time of writing (2016-04-04), supports device tree overlay, but the mechanism can only be accessible from the kernel space. A certain interface is need, when using device tree overlay from the userspace.

For example, "Transactional Device Tree & Overlays" describes an interface using ConfigFS, but this has not been merged to the upstream at the time of writing (2016-04-04).

Therefore, dtbocfg, which stands for Device Tree Blob Overlay Configuration File System, was developed to serve as a userspace API of Device Tree Overlay. Though this is a prototypical project, you can experiment with Device Tree Overlay by using dtbocfg.

Similar project

In some forked kernels such as linux-xlnx, the "ConfigFS overlay interface" is available, and provides an interface to overlay Device Tree Blob from the userspace via ConfigFS. If you use linux-xlnx or any other kernel that includes the "ConfigFS overlay interface", one may want to use this mechanism instead of dtbocfg, by turning it on by CONFIG_OF_CONFIGFS=y in the config.

For details of the "ConfigFS overlay interface", please refer configfs-overlays.txt.

Preparation

Building the Linux Kernel

This project is confirmed to work with Linux Kernel version 4.4.4.

When building a kernel, Device-Tree-Overlay option should be enabled. You can enable the option via make menu_config ---> Device Drivers ---> Device Tree and Open Firmware support ---> Device Tree overlays, or by manually addting CONFIG_OF_OVERLAY=y in .config.

Builiding dtbocfg

Clone the git repository, and run make after modifying it according to your environment.

shell% git clone https://github.com/ikwzm/dtbocfg.git
shell% cd dtbocfg
shell% make

After booting Linux on the target system, load the above-compiled device driver by doing like:

shell# insmod dtbocfg.ko
[ 1458.894102] dtbocfg_module_init
[ 1458.897231] dtbocfg_module_init: OK

If ConfigFS is not mounted yet, do so by doing like:

shell# mount -t configfs none /config

If /config/device-tree/overlays is created, it is ready to use dtbocfg.

shell# ls -la /config/device-tree/overlays/
drwxr-xr-x 2 root root 0  4  4 18:54 .
drwxr-xr-x 3 root root 0  4  4 18:54 ..
shell#

Example usage

Overlyaing uio (User I/O)

Prepare Device Tree Source

The following snippet shows an example Device Tree Source that adds uio entity.

Note: the register address, the interrupt number are just randomly picked in the snippet, and therefore you cannot actually access the created device.

/dts-v1/;
/ {
	fragment@0 {
		target-path = "/amba";
		__overlay__ {
			#address-cells = <0x1>;
			#size-cells = <0x1>;
			uio0@43c10000 {
				compatible = "generic-uio";
				reg = <0x43c10000 0x1000>;
				interrupts = <0x0 0x1d 0x4>;
			};
		};
	};
};

Create a directory in ConfigFS

To place a device tree blob overlay, make a directory under /config/device-tree/overlays. The name of the directory actually does not matter, but in this example, a directory named uio0, which corresponds to the entry in the Device Tree Source, is created.

shell# mkdir /config/device-tree/overlays/uio0

Subsequently, entries named status and dtbo will be automatically created under /config/device-tree/overlays/uio0. Although these look like standalone files, the are actually kernel attirubutes exposed by dtbocfg.

shell# ls -la /config/device-tree/overlays/uio0/
drwxr-xr-x 2 root root    0  4  4 20:08 .
drwxr-xr-x 3 root root    0  4  4 20:08 ..
-rw-r--r-- 1 root root 4096  4  4 20:09 dtbo
-rw-r--r-- 1 root root 4096  4  4 20:09 status

Writing Device Tree Blob

Write a Device Tree Blob to /config/device-tree/overlays/uio0/dtbo.

shell# dtc -I dts -O dtb -o uio0.dtbo uio0.dts
shell# cp uio0.dtbo /config/device-tree/overlays/uio0/dtbo

Adding Device Tree Blob to Device Tree

The Device Tree Blob written to dtbo can be enabled and added to the main (kernel) Device Tree by writing 1 to /config/device-tree/overlays/uio0/status. If the blob is successfully added to the kernel Device Tree, /dev/uio0 will be created, as decalred in the blob.

shell# echo 1 > /config/device-tree/overlays/uio0/status
shell# ls -la /dev/uio*
crw------- 1 root root 247, 0  4  4 20:17 /dev/uio0

Removing Device Tree Blob from Device Tree

The added Device Tree Blob can be removed from the kernel Device Tree by writing 0 to /config/device-tree/overlays/uio0/status.

shell# echo 0 > /config/device-tree/overlays/uio0/status

The same can be achieved by removing /config/device-tree/overlays/uio0 too.

shell# rmdir /config/device-tree/overlays/uio0

Overlaying udmabuf

This is another overlaying exmple involving udmabuf.

For details about udmabuf, see https://github.com/ikwzm/udmabuf.

Prepare Device Tree Source

A Device Tree Source like below should be prepared:

/dts-v1/;

/ {
	fragment@0 {
		target-path = "/amba";
		__overlay__ {
			udmabuf4 {
				compatible = "ikwzm,udmabuf-0.10.a";
				minor-number = <4>;
				size = <0x00400000>;
			};
		};
	};
};

Create a directory in ConfigFS

To place a device tree blob overlay, make a directory under /config/device-tree/overlays. Again, the name of the directory actually does not matter, and in this example, a directory named udmabuf4 is created.

shell# mkdir /config/device-tree/overlays/udmabuf4

Load udmabuf device driver

Load the udmabuf device driver if not automatically loaded up on boot.

shell# insmod udmabuf.ko

Writing Device Tree Blob

A compiled Device Tree Blob should be written to /config/device-tree/overlays/udmabuf4/dtbo. In this example, an output from Device Tree Compiler (dtc) is directly written to dtbo.

shell# dtc -I dts -O dtb -o /config/device-tree/overlays/udmabuf4/dtbo udmabuf4.dts

Adding Device Tree Blob to Device Tree

Similarly to the above example, the Device Tree Blob can be added to the kernel Device Tree by writing 1 to /config/device-tree/overlays/udmabuf4/status. If the udmabuf device driver is already loaded, /dev/udmabuf4 will be created as declared in the Device Tree Blob.

shell# echo 1 > /config/device-tree/overlays/udmabuf4/status
[ 7256.806725] udmabuf amba:udmabuf4: driver probe start.
[ 7256.827450] udmabuf udmabuf4: driver installed
[ 7256.831818] udmabuf udmabuf4: major number   = 246
[ 7256.836631] udmabuf udmabuf4: minor number   = 4
[ 7256.841192] udmabuf udmabuf4: phys address   = 0x1f500000
[ 7256.846604] udmabuf udmabuf4: buffer size    = 4194304
[ 7256.851694] udmabuf amba:udmabuf4: driver installed.
shell# ls -la /dev/udmabuf*
crw------- 1 root root 247, 0  4  4 20:30 /dev/udmabuf4

Removing Device Tree Blob from Device Tree

The added Device Tree Blob can be removed by writing 0 to /config/device-tree/overlays/udmabuf4/status.

shell# echo 0 > /config/device-tree/overlays/udmabuf4/status
[ 7440.383899] udmabuf udmabuf4: driver uninstalled
[ 7440.389533] udmabuf amba:udmabuf4: driver unloaded

Removing the /config/device-tree/overlays/udmabuf4 directory would also do the same.

shell# rmdir /config/device-tree/overlays/udmabuf4/
[ 7473.117564] udmabuf udmabuf4: driver uninstalled
[ 7473.123364] udmabuf amba:udmabuf4: driver unloaded