How does U-Boot communicate with Linux kernel?

Question

I'm reading the book and it tells that:

After U-Boot loads Linux kernel, the kernel will claim all the resources of U-Boot

What does this mean? Does it mean that all data structures that allocated in U-Boot will be discarded?

For example: during U-Boot, PCIE and Network Device will be initialized. After booting Linux kernel, will the PCIE and Network Device data structure be discarded? Will the Linux kernel do PCIE and NEtwork initialize again? Or U-Boot will transfer some data to kernel?

Answer 1

It depends on your CPU architecture how the communication happens, but it is usually via a special place in RAM, flash or the filesystem. No data structures are transferred, they would be meaningless to the kernel and the memory space will be different between the two. Uboot generally passes boot parameters like what type of hardware is present, what memory to use for something, or which type of mode to use for a specific driver. So yes, the kernel will re-initialize the hardware. The exception may be some of the low level CPU specifics which the kernel may expect uboot or a BIOS to have setup already.

Answer 2

Depending on your architecture, there may be different mechanism for the u-boot to communicate with the Linux kernel. Actually there may be some structures defined by u-boot which are transferred to and used by the kernel using ATAGS . The address in which these structure are passed is stored in r2 register on ARM. They convey information such as available RAM size and location, kernel command line, ...

Note that on some architectures (like ARM again) we have support for device-tree which intends for defining the hardware in which the kernel is going to be run as well as kernel command line, memory and other thins. Such description is usually created during kernel compile time, loaded into the memory by the u-boot and in case of ARM architecture, its address is transferred through r2 register.

The interesting thing about this (regarding your question) is that u-boot can change this device-tree structure before passing it to the kernel through device tree overlay mechanism. So this is a (relatively) new way of u-boot/kernel communication. Note that device-tree is not supported on some architectures.

And at the end, yes, the hardware is reinitialized by the kernel even in they have already initialized by the u-boot except for memory controller and some other very low level initialization, AFAIK.