Our desktop systems (X86) come with BIOS chip, which provides important hardware information required to initialize kernel. ARM, however, does not come with any such information. All hardware information has to be statically written in kernel source. This modified source code cannot boot other boards which are very different. This means once kernel is modified to boot BeagleBone, it can not boot other ARM boards. Modified kernel has to be maintained by the vendor. They are responsible to bring all Mainline kernel changes to their modified kernel. To address this problem, new hardware description specification was introduced. These hardware description data structures are called Device Trees. These hardware description files are provided at boot time to the kernel by the bootloader and kernel uses them to initialize itself. Code for all ARM boards can exist in kernel and you specify your board details while booting via device tree files. BeagleBone kernel team went a step further and modified kernel to allow dynamically loading of small device tree overlay. This can help us to enable/disable hardware runtime. For more details visit http://elinux.org/BeagleBone_and_the_3.8_Kernel. We used device tree in Chapter 9, UART, I2C, and SPI Programming, to enable SPI pins while interfacing Nokia 5110 LCD. Adafruit Python library that we used has device tree files in the overlays source folder. They load runtime to change pin configuration. Cape manager deals with device tree overlays. Many prebuilt device trees are available in /lib/firmware. List of Device tree overlays loaded currently can be viewed by the following command:
cat /sys/devices/bone_capemgr.*/slots
BeagleBone Black lists HDMI and emmc in output. Device tree for HDMI and emmc are loaded at boot time. You can disable this boot time loading and free up expansion pins related to them to interface more components. You can disable HDMI by uncommenting the following line in the /boot/uEnv.txt file:
##Disable HDMI cape_disable=capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN