User:NeddySeagoon/PC Boot Process

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Introduction

This page is aimed at readers configuring their own kernel, it aims to explain why some kernel settings must be set in a certain way. This understanding will help with the kernel configuration. Individual kernel settings for particular hardware are not addressed.

The Personal Computer (PC), aka the IBM PC an compatibles all follow the same boot process, from the very first 8086 up to the present day.

Other architectures do things differently but have the same problems to overcome.

Bootstrapping

At power up, the only code available to the CPU is the firmware. It doesn't matter at this level if its BIOS or EFI. It does its thing, loads the boot loader and the boot loader shows you a menu. Lets say that the boot loader is Grub but in could be any other boot loader.

All that is in RAM now is Grub. Grub can make calls to the firmware but that's it. There is no kernel yet, no initrd and no kernel filesystem tree. They all come later.

You choose the kernel entry to boot, which may include a matching initrd. Grub loads the kernel, optionally the initrd and leaves the kernel command line where the kernel will find it. Grub exits by jumping to the kernel start address.

Now we have the kernel and optional initrd in RAM and the kernel command line. If there is no initrd, its just the kernel. Without any outside help, the kernel must be able to mount the root filesystem and start the init process.

That's no quite true as the kernel cannot find the root filesystem on its own as root is not mounted, so /etc/fstab cannot be read. That's a circular dependency. The kernel parameter root=/dev/... breaks the circular dependency.

As the kernel is alone until root is mounted, all the required code to mount root must be configured as <*> for built in. Loadable modules <M> are stored in /lib/modules/`uname -r` which is on the root filesystem, which cannot be read until root is mounted. Its a very bad thing to need to read /lib/modules/ to load a module required to mount root to read /lib/modules/, Making things built in avoids that circular dependency.

Initrd

The initrd can help here. An initrd is a small root filesystem in a file that if the kernel is configured to support an initrd, it knows how to mount and read.

If there is an initrd, the kernel will mount it and use it as a temporary root filesystem. The initrd can contain whatever you like. Kernel modules, user space tools, anything you like at all. If it contains kernel modules, the initrd is tied to the kernel build because the kernel will check for bits of itself.

The use of an initrd is only compulsory if user space tools are required to make the root filesystem visible to the kernel. e.g. root on raid, where mdadm is required to start the raid. Root in a logical volume, root in LUKS ... and all the combinations. Mounting root by filesystem UUID requires the user space mount command.

Kernel configuration guidelines

  • If an option is needed to mount root, it must be configured as built in.
  • If an option is always used but not required to mount root, it can be either built in or modular.
  • If an option is never required, set it to off

Options needed to mount root

  • The partition table drivers
  • The Block layer
  • The PCIe layer
  • The SCSI layer
  • The hardware driver for the storage device holding the root filesystem
  • The root filesystem driver

Console drivers

Console output drivers

  • EFI Framebuffer, VESA Framebuffer and Simple Framebuffer. All other Framebuffer drivers must be off.
  • DRM
  • The kernel driver for your video card

Its not an error to set these options off but the console will be blank.

Console input drivers

You probably want to to be able to log in at the console.

  • evdev
  • The hardware driver for your keyboard

Other things

The list above is not intended be be exhaustive. Other options must be on too but they tend to default to on in the PC kernel.

Configuring the kernel

Warning
Using a text editor on the.config usually results in a broken kernel

All of the kernel provided configuration tools except one, provide a search function.

In menuconfig, press / and enter a search term. That's on every page of menuconfig.

However, there is a wart here. To make configuring the kernel more manageable, by default, options that cannot be selected are hidden. That is, they do not appear in menus or the search output. This is usually because something they depend on is off.

When this happens, its possible to turn on the display oy all the hidden symbols. The 'z' key is a toggle. That works everywhere that 'z' is not a shortcut. Now its possible to go to the symbol (it still cannot be selected) and read its help. The Depends on: boolean expression must be true before the dependant option can be selected.

It's possible that the above method will need to be used recursively to set the options to satisfy the Depends on: so that the option you want can be selected.

While reading the help, notice the Selects: When using a text editor on the .config file, its unlikely that the Selects: would have been chosen, resulting in an illegal .config file.