AUTHOR: Bruce Dubbs <bdubbs@linuxfromscratch.org>
DATE: 2009-05-28
LICENSE: The MIT License
SYNOPSIS: Considerations when configuring the Linux kernel
PREREQUISITES: None
DESCRIPTION:
When building an LFS system, the last package built is the kernel itself. This is
the most complicated package to configure in LFS and is the most variable among users.
This hint is provided to help users decide how to configure their kernel.
REFERENCES: In the kernel tarball:
README
Documentation/kbuild/kconfig.txt
Documentation/cdrom/
Documentation/devices.txt
Documentation/filesystems/
Documentation/laptops/
Documentation/pcmcia/
Documentation/sound/
Documentation/networking/
Documentation/scsi/
There are many other files in the documentation tree that can give
detailed information about kernel options.
HINT:
The process of building the kernel is described in Chapter 8 of the LFS
book. However, detailed instruction on the configuration of the kernel
depend on many factors. This hint is an attempt to help the new user
through the complicated process of selecting configuration parameters.
The primary output of the configuration process is the .config file,
however even if a .config file is placed into the linux package tree, one
of the configuration processes must be run to set up other internal files
necessary for the build process to complete properly.
One rule of thumb to use in selecting configuration items is when in doubt,
use the default for that option.
1. Starting the configuration process
The configuration file is in the root of the linux directory with the name
.config. It is an ascii file with entries such as:
#
# General setup
#
CONFIG_LOCALVERSION=""
CONFIG_LOCALVERSION_AUTO=y
# CONFIG_IPC_NS is not set
a. make menuconfig
This is the primary user interface to kernel configuration. It is a
ncurses based application that generates a menu tree for a user to select
configuration items. Using the enter key and the arrow keys, you can
examine each item, including a help page.
The "Enter" key selects an option from the bottom of the page. Which item
that is selected is controlled by the left and right arrow keys.
<Select> goes down to a submenu if the selected line has a trailing ---> symbol.
<Exit> goes up a menu. If at the top menu, the program is exited.
<Help> brings up a page explaining the current option.
The current item is selected with the up and down arrow key. The space bar
toggles the selection between 'build into the kernel' annotated with a [*],
'build as a loadable module' annotated as [M], or don't build at all
displayed as [ ].
b. make xconfig
This is a Qt based GUI interface to the menu items. It utilizes a mouse to
make selections and scroll windows. Clicking on an item performs the same
functions as the spacebar in menuconfig.
c. make oldconfig
A command line interface is available to take an old configuration file and
port it to the new kernel. To use it, run:
cp user/some/old.config .config
make oldconfig
The program will prompt you for missing or changed options. The prompt will
end with something like:
Group CPU scheduler (GROUP_SCHED) [N/y/?] (NEW)
Responding with <return> takes the first (capitalized) option. In some
cases, the choice will look like:
PCI slot detection driver (ACPI_PCI_SLOT) [N/m/y/?] (NEW)
if building as a module is available. To override the default, just type
the appropriate letter.
New kernel releases often introduce new config symbols. Often more
important, new kernel releases may rename config symbols. When this
happens, using a previously working .config file and running "make
oldconfig" won't necessarily produce a working new kernel for you, so you
may find that you need to see what NEW kernel symbols have been introduced.
To see a list of new config symbols when using "make oldconfig", use
cp user/some/old.config .config
yes "" | make oldconfig > conf.new
grep "(NEW)" conf.new
d. make defconfig
Use the default symbol values from either arch/$ARCH/defconfig
or arch/$ARCH/configs/${PLATFORM}_defconfig, depending on the architecture.
For most LFS systems, $ARCH will be x86 and ${PLATFORM} will be i386 or
x86_64. These values are derived from the uname utility.
e. make help
A list of configuration targets and other utilities.
2. About modules
Most commercial distributions build as many options as possible as modules. This
allows the kernel to only load what it needs when it needs it. It also takes
a lot of space and increases boot time by trying virtually every driver it has
available. For instance an Ubuntu system has 97 MB of modules.
Allowing modules is not required. They can be turned off with a kernel configuration
option. However, if this is done, it precludes using some proprietary modules
like Nvidia or VMware drivers.
Since LFS does not use an initrd, some drivers like disk and filesystem drivers
for the root partition are required to be built into the kernel. Most often these
are just SATA (CONFIG_ATA) or PATA (CONFIG_IDE) drivers and the ext3
(CONFIG_EXT3_FS) drivers,
On an LFS system, you presumably know (or can find out with tools such as lspci)
what hardware you have on your system. I recommend that you minimize the modules
that you build. After all, you can always go back and recompile with different
options.
Loading modules can be controlled with /etc/modprobe.conf. For more details see
man 5 modprobe.conf. Additionally, modules can be loaded automatically by the
LFS modules boot script when directed by its configuration file,
/etc/sysconfig/modules.
3. Configuration sections
The main menu if the configuration programs is split out into the
following sections:
a. General setup
Provides overall Linux options.
b. Enable loadable module support
Provides the ability to load kernel modules. Sub-options
provide additional capabilities related to modules.
c. Enable the block layer
This needs to be enabled to be able to mount any disk drive.
d. Processor type and features
The defaults will set most of these properly for
your hardware, but you may want to disable options that
may not apply such as Multi-core scheduler support. You
can also set the number of CPUs that the kernel supports.
You can also set support for some specific laptop brands.
e. Power management and ACPI options
Controls ACPI (Advanced Configuration and Power Interface) or
APM (Advanced Power Management) BIOS support. These options are most
useful on laptops.
f. Bus options (PCI etc)
Generally only PCI suport is needed here on newer systems. Go
with the defaults.
g. Executable file formats / Emulations
Generally only ELF support is needed.
h. Networking support
This is where networking (including wireless) is enabled.
Netfilter (firewall) capabilities are also defined here.
The defaults are generally satisfactory.
i. Device Drivers
This is one of the most important configuration areas. If you want
the hardware to work, it has to be enabled with a driver. Check
your devices on a currently running system with `lspci -v` to
confirm what hardware you have. Enable any network or usb devices
that you may have. Video drivers and sound cards are also enabled
here.
Take your time in this section and make sure you add drivers for
all the hardware you want to use.
j. Firmware Drivers
The default is generally OK here.
k. File systems
If you want reiser, ext4, jfs, xfs, kernel automounter support, or nfs,
you need to select those capabilities here.
l. Kernel hacking
If you make changes here, you better know why.
m. Security options
The defaults are generally OK here too.
n. Cryptographic API
Specialized crytographic capabilites. The defaults are OK here.
o. Virtualization (NEW)
Allows using your Linux host to run other operating systems inside
virtual machines (guests).
p. Library routines
Various CRC routines. The defaults are generally appropriate here
unless you have special requirements.
CHANGELOG:
[2009-05-28]
Initial Release
[2009-05-29]
Fix Typos. Add pointer to /etc/sysconfig/modules. Thanks to Trent Shea.
[2010-03-18]
Add discusssion of some additional make options.