How U-Boot Passes Device Tree Information to the Linux Kernel

During the boot process, U-Boot plays a crucial role in preparing the system for the Linux kernel by passing device tree information. This information is essential for the proper configuration and initialization of hardware resources. In this article, we will delve into the detailed steps and mechanisms used by U-Boot to pass device tree information to the Linux kernel.

Device Tree Blob (DTB) Creation

The device tree is a data structure used to describe the hardware layout of a computer system. U-Boot creates the Device Tree Blob (DTB) during the boot process, which contains hardware configuration information. This Blob is typically compiled from a Device Tree Source (DTS) file. The DTS file describes the hardware layout, including CPUs, memory, peripherals, and their configurations. Once the DTB is created, U-Boot stores it in memory, ready to be passed to the Linux kernel.

Loading the Device Tree Blob (DTB)

U-Boot loads the DTB into memory, typically by specifying a memory address where the DTB will be stored. This can be done in multiple ways:

U-Boot can load the DTB from a storage medium like an SD card or NAND flash. If a pre-existing DTB is already in memory, U-Boot can use it directly.

For ARM systems, U-Boot sets specific registers, such as r2 or r1, depending on the booting method, to point to the DTB's memory address before jumping to the kernel's entry point.

Booting the Linux Kernel

When U-Boot is ready to boot the Linux kernel, it uses the bootm or bootz command. This command passes the address of the loaded DTB to the kernel. The kernel reads the DTB from the specified memory address during its initialization phase, using the information provided to configure hardware devices, initialize drivers, and set up the system environment.

From a user perspective, the boot process involves the following steps:

Set the variable cmdline to the desired kernel command line. Use the fatload or similar command to read the kernel from the SD card and store it at a specific memory address, such as address 20000000. Again, use fatload to read the device tree binary (DTB) to another memory address, such as 21000000. Use the bootm command to start the boot process, passing the kernel address and the device tree address.

The following is an example of the bootm command:

code
bootm 20000000 21000000.
/code

Conclusion
In summary, U-Boot prepares the Device Tree Blob, loads it into memory, and passes its address to the Linux kernel during boot. This process allows the kernel to configure the system based on the hardware described in the DTB, ensuring that the system is set up correctly for optimal performance.
Keywords: U-Boot, Device Tree Blob, bootm command