Kernel-with-device-tree
Background
The idea behind device tree is quite simple. When the kernel start he needs to initialize the driver for the device on the board. But often on embedded, device can't be discover at running time. In this case, the linux kernel has a c file that initialize the driver for the board. So the kernel must be modified/compiled for each board.
Another way is to provide a small file that describe the board to the linux kernel. When the kernel start, it reads this file (called dtb for Device Tree Binary) to know the device on the board, and initialize usefull drivers.
Some information on device tree may be found here:
- http://elinux.org/Device_Trees
- http://devicetree.org/Main_Page
- https://wiki.linaro.org/Boards/Origen/DeviceTree
- http://elinux.org/images/4/48/Experiences_With_Device_Tree_Support_Development_For_ARM-Based_SOC%27s.pdf
Compilation
To use device tree on arm both bootloader (u-boot) and kernel should be compiled with the support of device tree.
u-boot
To enable device tree on u-boot, it's quite simple, just add "#define CONFIG_OF_LIBFDT" in your configuration file (include/configs/<board>.h).
On armadeus SDK, this file is in this directory: buildroot/target/device/armadeus/<board>/<board>-u-boot-<version>.h
For example, for u-boot 2012.04 on apf27, it's: buildroot/target/devg=ice/armadeus/apf27/apf27-u-boot-2012.04.h
NOTE: on u-boot 2012.04, the command bootm with device tree is broken. If you want to use it,
you need to fix it with the following patch: http://git.denx.de/?p=u-boot.git;a=commit;h=1723997610ace497252d6f9a44ec76c06951ae43
NOTE2: it's fixed on u-boot 2012.04.01
kernel
The kernel 3.4 has already the support of some arm SoC.
- imxl (apf9328) : no
- imx27 (apf27) : yes
- imx51 (apf51) : yes
- imx28 (apf28) : no
So apf27 and apf51 can use device tree, but there isn't the dts file for armadeus board.
A dts file should be added for all board with a supported SoC (apf27 and apf51).
