How u-boot pass parameters to Linux kernel

Linux (kernel) takes parameters when it boots up. This resembles an application started in a console with command line parameters. So we call kernel parameters "kernel command line", although kernel is not really started the same way as a command line application.

Linux kernel defines quite a few boot parameters. The kernel-parameters.txt file under .../Documentation has a complete list of all supported kernel parameters for the specific kernel version. Some examples are boot_delay, console, initrd, nfsroot, etc. These parameters affect the way the kernel boots up and behaves when it's up running.

Linux is typically booted (run) by a bootloader. For some bootloaders such as u-boot, they are linux aware and provide mechanism to pass command line to linux when it boots up the kernel. But for others that are not Linux aware and no way is available to pass command line, the kernel command line could be hardcoded in source code and compiled into the kernel image.

u-boot passes kernel command line by bootargs environment variable. All the key=value pairs in bootargs are passed over as command line to kernel. So if you want to pass certain kernel parameters, you put them in the bootargs environment variable (note this is u-boot environment variable). When u-boot starts kernel code by bootm for example, it will pass the start address and end address of the command line string, and kernel takes it from there.

There are many details involved in the process of passing command line from bootargs to kernel, and how the kernel parse, process and make use of the command line. But I think this is already enough to customize a bootable system.