--- /dev/null
+.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
+.. [see the bottom of this file for redistribution information]
+
+===========================================
+How to quickly build a trimmed Linux kernel
+===========================================
+
+This guide explains how to swiftly build Linux kernels that are ideal for
+testing purposes, but perfectly fine for day-to-day use, too.
+
+The essence of the process (aka 'TL;DR')
+========================================
+
+*[If you are new to compiling Linux, ignore this TLDR and head over to the next
+section below: it contains a step-by-step guide, which is more detailed, but
+still brief and easy to follow; that guide and its accompanying reference
+section also mention alternatives, pitfalls, and additional aspects, all of
+which might be relevant for you.]*
+
+If your system uses techniques like Secure Boot, prepare it to permit starting
+self-compiled Linux kernels; install compilers and everything else needed for
+building Linux; make sure to have 12 Gigabyte free space in your home directory.
+Now run the following commands to download fresh Linux mainline sources, which
+you then use to configure, build and install your own kernel::
+
+ git clone --depth 1 -b master \
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git ~/linux/
+ cd ~/linux/
+ # Hint: if you want to apply patches, do it at this point. See below for details.
+ # Hint: it's recommended to tag your build at this point. See below for details.
+ yes "" | make localmodconfig
+ # Hint: at this point you might want to adjust the build configuration; you'll
+ # have to, if you are running Debian. See below for details.
+ make -j $(nproc --all)
+ # Note: on many commodity distributions the next command suffices, but on Arch
+ # Linux, its derivatives, and some others it does not. See below for details.
+ command -v installkernel && sudo make modules_install install
+ reboot
+
+If you later want to build a newer mainline snapshot, use these commands::
+
+ cd ~/linux/
+ git fetch --depth 1 origin
+ # Note: the next command will discard any changes you did to the code:
+ git checkout --force --detach origin/master
+ # Reminder: if you want to (re)apply patches, do it at this point.
+ # Reminder: you might want to add or modify a build tag at this point.
+ make olddefconfig
+ make -j $(nproc --all)
+ # Reminder: the next command on some distributions does not suffice.
+ command -v installkernel && sudo make modules_install install
+ reboot
+
+Step-by-step guide
+==================
+
+Compiling your own Linux kernel is easy in principle. There are various ways to
+do it. Which of them actually work and is the best depends on the circumstances.
+
+This guide describes a way perfectly suited for those who want to quickly
+install Linux from sources without being bothered by complicated details; the
+goal is to cover everything typically needed on mainstream Linux distributions
+running on commodity PC or server hardware.
+
+The described approach is great for testing purposes, for example to try a
+proposed fix or to check if a problem was already fixed in the latest codebase.
+Nonetheless, kernels built this way are also totally fine for day-to-day use
+while at the same time being easy to keep up to date.
+
+The following steps describe the important aspects of the process; a
+comprehensive reference section later explains each of them in more detail. It
+sometimes also describes alternative approaches, pitfalls, as well as errors
+that might occur at a particular point -- and how to then get things rolling
+again.
+
+..
+ Note: if you see this note, you are reading the text's source file. You
+ might want to switch to a rendered version, as it makes it a lot easier to
+ quickly look something up in the reference section and afterwards jump back
+ to where you left off. Find a the latest rendered version here:
+ https://docs.kernel.org/admin-guide/quickly-build-trimmed-linux.html
+
+.. _backup_sbs:
+
+ * Create a fresh backup and put system repair and restore tools at hand, just
+ to be prepared for the unlikely case of something going sideways.
+
+ [:ref:`details<backup>`]
+
+.. _secureboot_sbs:
+
+ * On platforms with 'Secure Boot' or similar techniques, prepare everything to
+ ensure the system will permit your self-compiled kernel to boot later. The
+ quickest and easiest way to achieve this on commodity x86 systems is to
+ disable such techniques in the BIOS setup utility; alternatively, remove
+ their restrictions through a process initiated by
+ ``mokutil --disable-validation``.
+
+ [:ref:`details<secureboot>`]
+
+.. _buildrequires_sbs:
+
+ * Install all software required to build a Linux kernel. Often you will need:
+ 'bc', 'binutils' ('ld' et al.), 'bison', 'flex', 'gcc', 'git', 'openssl',
+ 'pahole', 'perl', and the development headers for 'libelf' and 'openssl'. The
+ reference section shows how to quickly install those on various popular Linux
+ distributions.
+
+ [:ref:`details<buildrequires>`]
+
+.. _diskspace_sbs:
+
+ * Ensure to have enough free space for building and installing Linux. For the
+ latter 150 Megabyte in /lib/ and 100 in /boot/ are a safe bet. For storing
+ sources and build artifacts 12 Gigabyte in your home directory should
+ typically suffice. If you have less available, be sure to check the reference
+ section for the step that explains adjusting your kernels build
+ configuration: it mentions a trick that reduce the amount of required space
+ in /home/ to around 4 Gigabyte.
+
+ [:ref:`details<diskspace>`]
+
+.. _sources_sbs:
+
+ * Retrieve the sources of the Linux version you intend to build; then change
+ into the directory holding them, as all further commands in this guide are
+ meant to be executed from there.
+
+ *[Note: the following paragraphs describe how to retrieve the sources by
+ partially cloning the Linux stable git repository. This is called a shallow
+ clone. The reference section explains two alternatives:* :ref:`packaged
+ archives<sources_archive>` *and* :ref:`a full git clone<sources_full>` *;
+ prefer the latter, if downloading a lot of data does not bother you, as that
+ will avoid some* :ref:`peculiar characteristics of shallow clones the
+ reference section explains<sources_shallow>` *.]*
+
+ First, execute the following command to retrieve a fresh mainline codebase::
+
+ git clone --no-checkout --depth 1 -b master \
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git ~/linux/
+ cd ~/linux/
+
+ If you want to access recent mainline releases and pre-releases, deepen you
+ clone's history to the oldest mainline version you are interested in::
+
+ git fetch --shallow-exclude=v6.0 origin
+
+ In case you want to access a stable/longterm release (say v6.1.5), simply add
+ the branch holding that series; afterwards fetch the history at least up to
+ the mainline version that started the series (v6.1)::
+
+ git remote set-branches --add origin linux-6.1.y
+ git fetch --shallow-exclude=v6.0 origin
+
+ Now checkout the code you are interested in. If you just performed the
+ initial clone, you will be able to check out a fresh mainline codebase, which
+ is ideal for checking whether developers already fixed an issue::
+
+ git checkout --detach origin/master
+
+ If you deepened your clone, you instead of ``origin/master`` can specify the
+ version you deepened to (``v6.0`` above); later releases like ``v6.1`` and
+ pre-release like ``v6.2-rc1`` will work, too. Stable or longterm versions
+ like ``v6.1.5`` work just the same, if you added the appropriate
+ stable/longterm branch as described.
+
+ [:ref:`details<sources>`]
+
+.. _patching_sbs:
+
+ * In case you want to apply a kernel patch, do so now. Often a command like
+ this will do the trick::
+
+ patch -p1 < ../proposed-fix.patch
+
+ If the ``-p1`` is actually needed, depends on how the patch was created; in
+ case it does not apply thus try without it.
+
+ If you cloned the sources with git and anything goes sideways, run ``git
+ reset --hard`` to undo any changes to the sources.
+
+ [:ref:`details<patching>`]
+
+.. _tagging_sbs:
+
+ * If you patched your kernel or have one of the same version installed already,
+ better add a unique tag to the one you are about to build::
+
+ echo "-proposed_fix" > localversion
+
+ Running ``uname -r`` under your kernel later will then print something like
+ '6.1-rc4-proposed_fix'.
+
+ [:ref:`details<tagging>`]
+
+ .. _configuration_sbs:
+
+ * Create the build configuration for your kernel based on an existing
+ configuration.
+
+ If you already prepared such a '.config' file yourself, copy it to
+ ~/linux/ and run ``make olddefconfig``.
+
+ Use the same command, if your distribution or somebody else already tailored
+ your running kernel to your or your hardware's needs: the make target
+ 'olddefconfig' will then try to use that kernel's .config as base.
+
+ Using this make target is fine for everybody else, too -- but you often can
+ save a lot of time by using this command instead::
+
+ yes "" | make localmodconfig
+
+ This will try to pick your distribution's kernel as base, but then disable
+ modules for any features apparently superfluous for your setup. This will
+ reduce the compile time enormously, especially if you are running an
+ universal kernel from a commodity Linux distribution.
+
+ There is a catch: the make target 'localmodconfig' will disable kernel
+ features you have not directly or indirectly through some program utilized
+ since you booted the system. You can reduce or nearly eliminate that risk by
+ using tricks outlined in the reference section; for quick testing purposes
+ that risk is often negligible, but it is an aspect you want to keep in mind
+ in case your kernel behaves oddly.
+
+ [:ref:`details<configuration>`]
+
+.. _configmods_sbs:
+
+ * Check if you might want to or have to adjust some kernel configuration
+ options:
+
+ * Evaluate how you want to handle debug symbols. Enable them, if you later
+ might need to decode a stack trace found for example in a 'panic', 'Oops',
+ 'warning', or 'BUG'; on the other hand disable them, if you are short on
+ storage space or prefer a smaller kernel binary. See the reference section
+ for details on how to do either. If neither applies, it will likely be fine
+ to simply not bother with this. [:ref:`details<configmods_debugsymbols>`]
+
+ * Are you running Debian? Then to avoid known problems by performing
+ additional adjustments explained in the reference section.
+ [:ref:`details<configmods_distros>`].
+
+ * If you want to influence the other aspects of the configuration, do so now
+ by using make targets like 'menuconfig' or 'xconfig'.
+ [:ref:`details<configmods_individual>`].
+
+.. _build_sbs:
+
+ * Build the image and the modules of your kernel::
+
+ make -j $(nproc --all)
+
+ If you want your kernel packaged up as deb, rpm, or tar file, see the
+ reference section for alternatives.
+
+ [:ref:`details<build>`]
+
+.. _install_sbs:
+
+ * Now install your kernel::
+
+ command -v installkernel && sudo make modules_install install
+
+ Often all left for you to do afterwards is a ``reboot``, as many commodity
+ Linux distributions will then create an initramfs (also known as initrd) and
+ an entry for your kernel in your bootloader's configuration; but on some
+ distributions you have to take care of these two steps manually for reasons
+ the reference section explains.
+
+ On a few distributions like Arch Linux and its derivatives the above command
+ does nothing at all; in that case you have to manually install your kernel,
+ as outlined in the reference section.
+
+ [:ref:`details<install>`]
+
+.. _another_sbs:
+
+ * To later build another kernel you need similar steps, but sometimes slightly
+ different commands.
+
+ First, switch back into the sources tree::
+
+ cd ~/linux/
+
+ In case you want to build a version from a stable/longterm series you have
+ not used yet (say 6.2.y), tell git to track it::
+
+ git remote set-branches --add origin linux-6.2.y
+
+ Now fetch the latest upstream changes; you again need to specify the earliest
+ version you care about, as git otherwise might retrieve the entire commit
+ history::
+
+ git fetch --shallow-exclude=v6.1 origin
+
+ If you modified the sources (for example by applying a patch), you now need
+ to discard those modifications; that's because git otherwise will not be able
+ to switch to the sources of another version due to potential conflicting
+ changes::
+
+ git reset --hard
+
+ Now checkout the version you are interested in, as explained above::
+
+ git checkout --detach origin/master
+
+ At this point you might want to patch the sources again or set/modify a build
+ tag, as explained earlier; afterwards adjust the build configuration to the
+ new codebase and build your next kernel::
+
+ # reminder: if you want to apply patches, do it at this point
+ # reminder: you might want to update your build tag at this point
+ make olddefconfig
+ make -j $(nproc --all)
+
+ Install the kernel as outlined above::
+
+ command -v installkernel && sudo make modules_install install
+
+ [:ref:`details<another>`]
+
+.. _uninstall_sbs:
+
+ * Your kernel is easy to remove later, as its parts are only stored in two
+ places and clearly identifiable by the kernel's release name. Just ensure to
+ not delete the kernel you are running, as that might render your system
+ unbootable.
+
+ Start by deleting the directory holding your kernel's modules, which is named
+ after its release name -- '6.0.1-foobar' in the following example::
+
+ sudo rm -rf /lib/modules/6.0.1-foobar
+
+ Now try the following command, which on some distributions will delete all
+ other kernel files installed while also removing the kernel's entry from the
+ bootloader configuration::
+
+ command -v kernel-install && sudo kernel-install -v remove 6.0.1-foobar
+
+ If that command does not output anything or fails, see the reference section;
+ do the same if any files named '*6.0.1-foobar*' remain in /boot/.
+
+ [:ref:`details<uninstall>`]
+
+.. _submit_improvements:
+
+Did you run into trouble following any of the above steps that is not cleared up
+by the reference section below? Or do you have ideas how to improve the text?
+Then please take a moment of your time and let the maintainer of this document
+know by email (Thorsten Leemhuis <linux@leemhuis.info>), ideally while CCing the
+Linux docs mailing list (linux-doc@vger.kernel.org). Such feedback is vital to
+improve this document further, which is in everybody's interest, as it will
+enable more people to master the task described here.
+
+Reference section for the step-by-step guide
+============================================
+
+This section holds additional information for each of the steps in the above
+guide.
+
+.. _backup:
+
+Prepare for emergencies
+-----------------------
+
+ *Create a fresh backup and put system repair and restore tools at hand*
+ [:ref:`... <backup_sbs>`]
+
+Remember, you are dealing with computers, which sometimes do unexpected things
+-- especially if you fiddle with crucial parts like the kernel of an operating
+system. That's what you are about to do in this process. Hence, better prepare
+for something going sideways, even if that should not happen.
+
+[:ref:`back to step-by-step guide <backup_sbs>`]
+
+.. _secureboot:
+
+Dealing with techniques like Secure Boot
+----------------------------------------
+
+ *On platforms with 'Secure Boot' or similar techniques, prepare everything to
+ ensure the system will permit your self-compiled kernel to boot later.*
+ [:ref:`... <secureboot_sbs>`]
+
+Many modern systems allow only certain operating systems to start; they thus by
+default will reject booting self-compiled kernels.
+
+You ideally deal with this by making your platform trust your self-built kernels
+with the help of a certificate and signing. How to do that is not described
+here, as it requires various steps that would take the text too far away from
+its purpose; 'Documentation/admin-guide/module-signing.rst' and various web
+sides already explain this in more detail.
+
+Temporarily disabling solutions like Secure Boot is another way to make your own
+Linux boot. On commodity x86 systems it is possible to do this in the BIOS Setup
+utility; the steps to do so are not described here, as they greatly vary between
+machines.
+
+On mainstream x86 Linux distributions there is a third and universal option:
+disable all Secure Boot restrictions for your Linux environment. You can
+initiate this process by running ``mokutil --disable-validation``; this will
+tell you to create a one-time password, which is safe to write down. Now
+restart; right after your BIOS performed all self-tests the bootloader Shim will
+show a blue box with a message 'Press any key to perform MOK management'. Hit
+some key before the countdown exposes. This will open a menu and choose 'Change
+Secure Boot state' there. Shim's 'MokManager' will now ask you to enter three
+randomly chosen characters from the one-time password specified earlier. Once
+you provided them, confirm that you really want to disable the validation.
+Afterwards, permit MokManager to reboot the machine.
+
+[:ref:`back to step-by-step guide <secureboot_sbs>`]
+
+.. _buildrequires:
+
+Install build requirements
+--------------------------
+
+ *Install all software required to build a Linux kernel.*
+ [:ref:`...<buildrequires_sbs>`]
+
+The kernel is pretty stand-alone, but besides tools like the compiler you will
+sometimes need a few libraries to build one. How to install everything needed
+depends on your Linux distribution and the configuration of the kernel you are
+about to build.
+
+Here are a few examples what you typically need on some mainstream
+distributions:
+
+ * Debian, Ubuntu, and derivatives::
+
+ sudo apt install bc binutils bison dwarves flex gcc git make openssl \
+ pahole perl-base libssl-dev libelf-dev
+
+ * Fedora and derivatives::
+
+ sudo dnf install binutils /usr/include/{libelf.h,openssl/pkcs7.h} \
+ /usr/bin/{bc,bison,flex,gcc,git,openssl,make,perl,pahole}
+
+ * openSUSE and derivatives::
+
+ sudo zypper install bc binutils bison dwarves flex gcc git make perl-base \
+ openssl openssl-devel libelf-dev
+
+In case you wonder why these lists include openssl and its development headers:
+they are needed for the Secure Boot support, which many distributions enable in
+their kernel configuration for x86 machines.
+
+Sometimes you will need tools for compression formats like bzip2, gzip, lz4,
+lzma, lzo, xz, or zstd as well.
+
+You might need additional libraries and their development headers in case you
+perform tasks not covered in this guide. For example, zlib will be needed when
+building kernel tools from the tools/ directory; adjusting the build
+configuration with make targets like 'menuconfig' or 'xconfig' will require
+development headers for ncurses or Qt5.
+
+[:ref:`back to step-by-step guide <buildrequires_sbs>`]
+
+.. _diskspace:
+
+Space requirements
+------------------
+
+ *Ensure to have enough free space for building and installing Linux.*
+ [:ref:`... <diskspace_sbs>`]
+
+The numbers mentioned are rough estimates with a big extra charge to be on the
+safe side, so often you will need less.
+
+If you have space constraints, remember to read the reference section when you
+reach the :ref:`section about configuration adjustments' <configmods>`, as
+ensuring debug symbols are disabled will reduce the consumed disk space by quite
+a few gigabytes.
+
+[:ref:`back to step-by-step guide <diskspace_sbs>`]
+
+
+.. _sources:
+
+Download the sources
+--------------------
+
+ *Retrieve the sources of the Linux version you intend to build.*
+ [:ref:`...<sources_sbs>`]
+
+The step-by-step guide outlines how to retrieve Linux' sources using a shallow
+git clone. There is :ref:`more to tell about this method<sources_shallow>` and
+two alternate ways worth describing: :ref:`packaged archives<sources_archive>`
+and :ref:`a full git clone<sources_full>`. And the aspects ':ref:`wouldn't it
+be wiser to use a proper pre-release than the latest mainline code
+<sources_snapshot>`' and ':ref:`how to get an even fresher mainline codebase
+<sources_fresher>`' need elaboration, too.
+
+Note, to keep things simple the commands used in this guide store the build
+artifacts in the source tree. If you prefer to separate them, simply add
+something like ``O=~/linux-builddir/`` to all make calls; also adjust the path
+in all commands that add files or modify any generated (like your '.config').
+
+[:ref:`back to step-by-step guide <sources_sbs>`]
+
+.. _sources_shallow:
+
+Noteworthy characteristics of shallow clones
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The step-by-step guide uses a shallow clone, as it is the best solution for most
+of this document's target audience. There are a few aspects of this approach
+worth mentioning:
+
+ * This document in most places uses ``git fetch`` with ``--shallow-exclude=``
+ to specify the earliest version you care about (or to be precise: its git
+ tag). You alternatively can use the parameter ``--shallow-since=`` to specify
+ an absolute (say ``'2023-07-15'``) or relative (``'12 months'``) date to
+ define the depth of the history you want to download. As a second
+ alternative, you can also specify a certain depth explicitly with a parameter
+ like ``--depth=1``, unless you add branches for stable/longterm kernels.
+
+ * When running ``git fetch``, remember to always specify the oldest version,
+ the time you care about, or an explicit depth as shown in the step-by-step
+ guide. Otherwise you will risk downloading nearly the entire git history,
+ which will consume quite a bit of time and bandwidth while also stressing the
+ servers.
+
+ Note, you do not have to use the same version or date all the time. But when
+ you change it over time, git will deepen or flatten the history to the
+ specified point. That allows you to retrieve versions you initially thought
+ you did not need -- or it will discard the sources of older versions, for
+ example in case you want to free up some disk space. The latter will happen
+ automatically when using ``--shallow-since=`` or
+ ``--depth=``.
+
+ * Be warned, when deepening your clone you might encounter an error like
+ 'fatal: error in object: unshallow cafecaca0c0dacafecaca0c0dacafecaca0c0da'.
+ In that case run ``git repack -d`` and try again``
+
+ * In case you want to revert changes from a certain version (say Linux 6.3) or
+ perform a bisection (v6.2..v6.3), better tell ``git fetch`` to retrieve
+ objects up to three versions earlier (e.g. 6.0): ``git describe`` will then
+ be able to describe most commits just like it would in a full git clone.
+
+[:ref:`back to step-by-step guide <sources_sbs>`] [:ref:`back to section intro <sources>`]
+
+.. _sources_archive:
+
+Downloading the sources using a packages archive
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+People new to compiling Linux often assume downloading an archive via the
+front-page of https://kernel.org is the best approach to retrieve Linux'
+sources. It actually can be, if you are certain to build just one particular
+kernel version without changing any code. Thing is: you might be sure this will
+be the case, but in practice it often will turn out to be a wrong assumption.
+
+That's because when reporting or debugging an issue developers will often ask to
+give another version a try. They also might suggest temporarily undoing a commit
+with ``git revert`` or might provide various patches to try. Sometimes reporters
+will also be asked to use ``git bisect`` to find the change causing a problem.
+These things rely on git or are a lot easier and quicker to handle with it.
+
+A shallow clone also does not add any significant overhead. For example, when
+you use ``git clone --depth=1`` to create a shallow clone of the latest mainline
+codebase git will only retrieve a little more data than downloading the latest
+mainline pre-release (aka 'rc') via the front-page of kernel.org would.
+
+A shallow clone therefore is often the better choice. If you nevertheless want
+to use a packaged source archive, download one via kernel.org; afterwards
+extract its content to some directory and change to the subdirectory created
+during extraction. The rest of the step-by-step guide will work just fine, apart
+from things that rely on git -- but this mainly concerns the section on
+successive builds of other versions.
+
+[:ref:`back to step-by-step guide <sources_sbs>`] [:ref:`back to section intro <sources>`]
+
+.. _sources_full:
+
+Downloading the sources using a full git clone
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If downloading and storing a lot of data (~4,4 Gigabyte as of early 2023) is
+nothing that bothers you, instead of a shallow clone perform a full git clone
+instead. You then will avoid the specialties mentioned above and will have all
+versions and individual commits at hand at any time::
+
+ curl -L \
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/clone.bundle \
+ -o linux-stable.git.bundle
+ git clone clone.bundle ~/linux/
+ rm linux-stable.git.bundle
+ cd ~/linux/
+ git remote set-url origin
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
+ git fetch origin
+ git checkout --detach origin/master
+
+[:ref:`back to step-by-step guide <sources_sbs>`] [:ref:`back to section intro <sources>`]
+
+.. _sources_snapshot:
+
+Proper pre-releases (RCs) vs. latest mainline
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When cloning the sources using git and checking out origin/master, you often
+will retrieve a codebase that is somewhere between the latest and the next
+release or pre-release. This almost always is the code you want when giving
+mainline a shot: pre-releases like v6.1-rc5 are in no way special, as they do
+not get any significant extra testing before being published.
+
+There is one exception: you might want to stick to the latest mainline release
+(say v6.1) before its successor's first pre-release (v6.2-rc1) is out. That is
+because compiler errors and other problems are more likely to occur during this
+time, as mainline then is in its 'merge window': a usually two week long phase,
+in which the bulk of the changes for the next release is merged.
+
+[:ref:`back to step-by-step guide <sources_sbs>`] [:ref:`back to section intro <sources>`]
+
+.. _sources_fresher:
+
+Avoiding the mainline lag
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The explanations for both the shallow clone and the full clone both retrieve the
+code from the Linux stable git repository. That makes things simpler for this
+document's audience, as it allows easy access to both mainline and
+stable/longterm releases. This approach has just one downside:
+
+Changes merged into the mainline repository are only synced to the master branch
+of the Linux stable repository every few hours. This lag most of the time is
+not something to worry about; but in case you really need the latest code, just
+add the mainline repo as additional remote and checkout the code from there::
+
+ git remote add mainline \
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
+ git fetch mainline
+ git checkout --detach mainline/master
+
+When doing this with a shallow clone, remember to call ``git fetch`` with one
+of the parameters described earlier to limit the depth.
+
+[:ref:`back to step-by-step guide <sources_sbs>`] [:ref:`back to section intro <sources>`]
+
+.. _patching:
+
+Patch the sources (optional)
+----------------------------
+
+ *In case you want to apply a kernel patch, do so now.*
+ [:ref:`...<patching_sbs>`]
+
+This is the point where you might want to patch your kernel -- for example when
+a developer proposed a fix and asked you to check if it helps. The step-by-step
+guide already explains everything crucial here.
+
+[:ref:`back to step-by-step guide <patching_sbs>`]
+
+.. _tagging:
+
+Tagging this kernel build (optional, often wise)
+------------------------------------------------
+
+ *If you patched your kernel or already have that kernel version installed,
+ better tag your kernel by extending its release name:*
+ [:ref:`...<tagging_sbs>`]
+
+Tagging your kernel will help avoid confusion later, especially when you patched
+your kernel. Adding an individual tag will also ensure the kernel's image and
+its modules are installed in parallel to any existing kernels.
+
+There are various ways to add such a tag. The step-by-step guide realizes one by
+creating a 'localversion' file in your build directory from which the kernel
+build scripts will automatically pick up the tag. You can later change that file
+to use a different tag in subsequent builds or simply remove that file to dump
+the tag.
+
+[:ref:`back to step-by-step guide <tagging_sbs>`]
+
+.. _configuration:
+
+Define the build configuration for your kernel
+----------------------------------------------
+
+ *Create the build configuration for your kernel based on an existing
+ configuration.* [:ref:`... <configuration_sbs>`]
+
+There are various aspects for this steps that require a more careful
+explanation:
+
+Pitfalls when using another configuration file as base
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Make targets like localmodconfig and olddefconfig share a few common snares you
+want to be aware of:
+
+ * These targets will reuse a kernel build configuration in your build directory
+ (e.g. '~/linux/.config'), if one exists. In case you want to start from
+ scratch you thus need to delete it.
+
+ * The make targets try to find the configuration for your running kernel
+ automatically, but might choose poorly. A line like '# using defaults found
+ in /boot/config-6.0.7-250.fc36.x86_64' or 'using config:
+ '/boot/config-6.0.7-250.fc36.x86_64' tells you which file they picked. If
+ that is not the intended one, simply store it as '~/linux/.config'
+ before using these make targets.
+
+ * Unexpected things might happen if you try to use a config file prepared for
+ one kernel (say v6.0) on an older generation (say v5.15). In that case you
+ might want to use a configuration as base which your distribution utilized
+ when they used that or an slightly older kernel version.
+
+Influencing the configuration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The make target olddefconfig and the ``yes "" |`` used when utilizing
+localmodconfig will set any undefined build options to their default value. This
+among others will disable many kernel features that were introduced after your
+base kernel was released.
+
+If you want to set these configurations options manually, use ``oldconfig``
+instead of ``olddefconfig`` or omit the ``yes "" |`` when utilizing
+localmodconfig. Then for each undefined configuration option you will be asked
+how to proceed. In case you are unsure what to answer, simply hit 'enter' to
+apply the default value.
+
+Big pitfall when using localmodconfig
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As explained briefly in the step-by-step guide already: with localmodconfig it
+can easily happen that your self-built kernel will lack modules for tasks you
+did not perform before utilizing this make target. That's because those tasks
+require kernel modules that are normally autoloaded when you perform that task
+for the first time; if you didn't perform that task at least once before using
+localmodonfig, the latter will thus assume these modules are superfluous and
+disable them.
+
+You can try to avoid this by performing typical tasks that often will autoload
+additional kernel modules: start a VM, establish VPN connections, loop-mount a
+CD/DVD ISO, mount network shares (CIFS, NFS, ...), and connect all external
+devices (2FA keys, headsets, webcams, ...) as well as storage devices with file
+systems you otherwise do not utilize (btrfs, ext4, FAT, NTFS, XFS, ...). But it
+is hard to think of everything that might be needed -- even kernel developers
+often forget one thing or another at this point.
+
+Do not let that risk bother you, especially when compiling a kernel only for
+testing purposes: everything typically crucial will be there. And if you forget
+something important you can turn on a missing feature later and quickly run the
+commands to compile and install a better kernel.
+
+But if you plan to build and use self-built kernels regularly, you might want to
+reduce the risk by recording which modules your system loads over the course of
+a few weeks. You can automate this with `modprobed-db
+<https://github.com/graysky2/modprobed-db>`_. Afterwards use ``LSMOD=<path>`` to
+point localmodconfig to the list of modules modprobed-db noticed being used::
+
+ yes "" | make LSMOD="${HOME}"/.config/modprobed.db localmodconfig
+
+Remote building with localmodconfig
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want to use localmodconfig to build a kernel for another machine, run
+``lsmod > lsmod_foo-machine`` on it and transfer that file to your build host.
+Now point the build scripts to the file like this: ``yes "" | make
+LSMOD=~/lsmod_foo-machine localmodconfig``. Note, in this case
+you likely want to copy a base kernel configuration from the other machine over
+as well and place it as .config in your build directory.
+
+[:ref:`back to step-by-step guide <configuration_sbs>`]
+
+.. _configmods:
+
+Adjust build configuration
+--------------------------
+
+ *Check if you might want to or have to adjust some kernel configuration
+ options:*
+
+Depending on your needs you at this point might want or have to adjust some
+kernel configuration options.
+
+.. _configmods_debugsymbols:
+
+Debug symbols
+~~~~~~~~~~~~~
+
+ *Evaluate how you want to handle debug symbols.*
+ [:ref:`...<configmods_sbs>`]
+
+Most users do not need to care about this, it's often fine to leave everything
+as it is; but you should take a closer look at this, if you might need to decode
+a stack trace or want to reduce space consumption.
+
+Having debug symbols available can be important when your kernel throws a
+'panic', 'Oops', 'warning', or 'BUG' later when running, as then you will be
+able to find the exact place where the problem occurred in the code. But
+collecting and embedding the needed debug information takes time and consumes
+quite a bit of space: in late 2022 the build artifacts for a typical x86 kernel
+configured with localmodconfig consumed around 5 Gigabyte of space with debug
+symbols, but less than 1 when they were disabled. The resulting kernel image and
+the modules are bigger as well, which increases load times.
+
+Hence, if you want a small kernel and are unlikely to decode a stack trace
+later, you might want to disable debug symbols to avoid above downsides::
+
+ ./scripts/config --file .config -d DEBUG_INFO \
+ -d DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT -d DEBUG_INFO_DWARF4 \
+ -d DEBUG_INFO_DWARF5 -e CONFIG_DEBUG_INFO_NONE
+ make olddefconfig
+
+You on the other hand definitely want to enable them, if there is a decent
+chance that you need to decode a stack trace later (as explained by 'Decode
+failure messages' in Documentation/admin-guide/tainted-kernels.rst in more
+detail)::
+
+ ./scripts/config --file .config -d DEBUG_INFO_NONE -e DEBUG_KERNEL
+ -e DEBUG_INFO -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT -e KALLSYMS -e KALLSYMS_ALL
+ make olddefconfig
+
+Note, many mainstream distributions enable debug symbols in their kernel
+configurations -- make targets like localmodconfig and olddefconfig thus will
+often pick that setting up.
+
+[:ref:`back to step-by-step guide <configmods_sbs>`]
+
+.. _configmods_distros:
+
+Distro specific adjustments
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ *Are you running* [:ref:`... <configmods_sbs>`]
+
+The following sections help you to avoid build problems that are known to occur
+when following this guide on a few commodity distributions.
+
+**Debian:**
+
+ * Remove a stale reference to a certificate file that would cause your build to
+ fail::
+
+ ./scripts/config --file .config --set-str SYSTEM_TRUSTED_KEYS ''
+
+ Alternatively, download the needed certificate and make that configuration
+ option point to it, as `the Debian handbook explains in more detail
+ <https://debian-handbook.info/browse/stable/sect.kernel-compilation.html>`_
+ -- or generate your own, as explained in
+ Documentation/admin-guide/module-signing.rst.
+
+[:ref:`back to step-by-step guide <configmods_sbs>`]
+
+.. _configmods_individual:
+
+Individual adjustments
+~~~~~~~~~~~~~~~~~~~~~~
+
+ *If you want to influence the other aspects of the configuration, do so
+ now* [:ref:`... <configmods_sbs>`]
+
+You at this point can use a command like ``make menuconfig`` to enable or
+disable certain features using a text-based user interface; to use a graphical
+configuration utilize, use the make target ``xconfig`` or ``gconfig`` instead.
+All of them require development libraries from toolkits they are based on
+(ncurses, Qt5, Gtk2); an error message will tell you if something required is
+missing.
+
+[:ref:`back to step-by-step guide <configmods_sbs>`]
+
+.. _build:
+
+Build your kernel
+-----------------
+
+ *Build the image and the modules of your kernel* [:ref:`... <build_sbs>`]
+
+A lot can go wrong at this stage, but the instructions below will help you help
+yourself. Another subsection explains how to directly package your kernel up as
+deb, rpm or tar file.
+
+Dealing with build errors
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a build error occurs, it might be caused by some aspect of your machine's
+setup that often can be fixed quickly; other times though the problem lies in
+the code and can only be fixed by a developer. A close examination of the
+failure messages coupled with some research on the internet will often tell you
+which of the two it is. To perform such a investigation, restart the build
+process like this::
+
+ make V=1
+
+The ``V=1`` activates verbose output, which might be needed to see the actual
+error. To make it easier to spot, this command also omits the ``-j $(nproc
+--all)`` used earlier to utilize every CPU core in the system for the job -- but
+this parallelism also results in some clutter when failures occur.
+
+After a few seconds the build process should run into the error again. Now try
+to find the most crucial line describing the problem. Then search the internet
+for the most important and non-generic section of that line (say 4 to 8 words);
+avoid or remove anything that looks remotely system-specific, like your username
+or local path names like ``/home/username/linux/``. First try your regular
+internet search engine with that string, afterwards search Linux kernel mailing
+lists via `lore.kernel.org/all/ <https://lore.kernel.org/all/>`_.
+
+This most of the time will find something that will explain what is wrong; quite
+often one of the hits will provide a solution for your problem, too. If you
+do not find anything that matches your problem, try again from a different angle
+by modifying your search terms or using another line from the error messages.
+
+In the end, most trouble you are to run into has likely been encountered and
+reported by others already. That includes issues where the cause is not your
+system, but lies the code. If you run into one of those, you might thus find a
+solution (e.g. a patch) or workaround for your problem, too.
+
+Package your kernel up
+~~~~~~~~~~~~~~~~~~~~~~
+
+The step-by-step guide uses the default make targets (e.g. 'bzImage' and
+'modules' on x86) to build the image and the modules of your kernel, which later
+steps of the guide then install. You instead can also directly build everything
+and directly package it up by using one of the following targets:
+
+ * ``make -j $(nproc --all) bindeb-pkg`` to generate a deb package
+
+ * ``make -j $(nproc --all) binrpm-pkg`` to generate a rpm package
+
+ * ``make -j $(nproc --all) tarbz2-pkg`` to generate a bz2 compressed tarball
+
+This is just a selection of available make targets for this purpose, see
+``make help`` for others. You can also use these targets after running
+``make -j $(nproc --all)``, as they will pick up everything already built.
+
+If you employ the targets to generate deb or rpm packages, ignore the
+step-by-step guide's instructions on installing and removing your kernel;
+instead install and remove the packages using the package utility for the format
+(e.g. dpkg and rpm) or a package management utility build on top of them (apt,
+aptitude, dnf/yum, zypper, ...). Be aware that the packages generated using
+these two make targets are designed to work on various distributions utilizing
+those formats, they thus will sometimes behave differently than your
+distribution's kernel packages.
+
+[:ref:`back to step-by-step guide <build_sbs>`]
+
+.. _install:
+
+Install your kernel
+-------------------
+
+ *Now install your kernel* [:ref:`... <install_sbs>`]
+
+What you need to do after executing the command in the step-by-step guide
+depends on the existence and the implementation of an ``installkernel``
+executable. Many commodity Linux distributions ship such a kernel installer in
+``/sbin/`` that does everything needed, hence there is nothing left for you
+except rebooting. But some distributions contain an installkernel that does
+only part of the job -- and a few lack it completely and leave all the work to
+you.
+
+If ``installkernel`` is found, the kernel's build system will delegate the
+actual installation of your kernel's image and related files to this executable.
+On almost all Linux distributions it will store the image as '/boot/vmlinuz-
+<your kernel's release name>' and put a 'System.map-<your kernel's release
+name>' alongside it. Your kernel will thus be installed in parallel to any
+existing ones, unless you already have one with exactly the same release name.
+
+Installkernel on many distributions will afterwards generate an 'initramfs'
+(often also called 'initrd'), which commodity distributions rely on for booting;
+hence be sure to keep the order of the two make targets used in the step-by-step
+guide, as things will go sideways if you install your kernel's image before its
+modules. Often installkernel will then add your kernel to the bootloader
+configuration, too. You have to take care of one or both of these tasks
+yourself, if your distributions installkernel doesn't handle them.
+
+A few distributions like Arch Linux and its derivatives totally lack an
+installkernel executable. On those just install the modules using the kernel's
+build system and then install the image and the System.map file manually::
+
+ sudo make modules_install
+ sudo install -m 0600 $(make -s image_name) /boot/vmlinuz-$(make -s kernelrelease)
+ sudo install -m 0600 System.map /boot/System.map-$(make -s kernelrelease)
+
+If your distribution boots with the help of an initramfs, now generate one for
+your kernel using the tools your distribution provides for this process.
+Afterwards add your kernel to your bootloader configuration and reboot.
+
+[:ref:`back to step-by-step guide <install_sbs>`]
+
+.. _another:
+
+Another round later
+-------------------
+
+ *To later build another kernel you need similar, but sometimes slightly
+ different commands* [:ref:`... <another_sbs>`]
+
+The process to build later kernels is similar, but at some points slightly
+different. You for example do not want to use 'localmodconfig' for succeeding
+kernel builds, as you already created a trimmed down configuration you want to
+use from now on. Hence instead just use ``oldconfig`` or ``olddefconfig`` to
+adjust your build configurations to the needs of the kernel version you are
+about to build.
+
+If you created a shallow-clone with git, remember what the :ref:`section that
+explained the setup described in more detail <sources>`: you need to use a
+slightly different ``git fetch`` command and when switching to another series
+need to add an additional remote branch.
+
+[:ref:`back to step-by-step guide <another_sbs>`]
+
+.. _uninstall:
+
+Uninstall the kernel later
+--------------------------
+
+ *All parts of your installed kernel are identifiable by its release name and
+ thus easy to remove later.* [:ref:`... <uninstall_sbs>`]
+
+Do not worry installing your kernel manually and thus bypassing your
+distribution's packaging system will totally mess up your machine: all parts of
+your kernel are easy to remove later, as files are stored in two places only and
+normally identifiable by the kernel's release name.
+
+One of the two places is a directory in /lib/modules/, which holds the modules
+for each installed kernel. This directory is named after the kernel's release
+name; hence, to remove all modules for one of your kernels, simply remove its
+modules directory in /lib/modules/.
+
+The other place is /boot/, where typically one to five files will be placed
+during installation of a kernel. All of them usually contain the release name in
+their file name, but how many files and their name depends somewhat on your
+distribution's installkernel executable (:ref:`see above <install>`) and its
+initramfs generator. On some distributions the ``kernel-install`` command
+mentioned in the step-by-step guide will remove all of these files for you --
+and the entry for your kernel in the bootloader configuration at the same time,
+too. On others you have to take care of these steps yourself. The following
+command should interactively remove the two main files of a kernel with the
+release name '6.0.1-foobar'::
+
+ rm -i /boot/{System.map,vmlinuz}-6.0.1-foobar
+
+Now remove the belonging initramfs, which often will be called something like
+``/boot/initramfs-6.0.1-foobar.img`` or ``/boot/initrd.img-6.0.1-foobar``.
+Afterwards check for other files in /boot/ that have '6.0.1-foobar' in their
+name and delete them as well. Now remove the kernel from your bootloader's
+configuration.
+
+Note, be very careful with wildcards like '*' when deleting files or directories
+for kernels manually: you might accidentally remove files of a 6.0.11 kernel
+when all you want is to remove 6.0 or 6.0.1.
+
+[:ref:`back to step-by-step guide <uninstall_sbs>`]
+
+.. _faq:
+
+FAQ
+===
+
+Why does this 'how-to' not work on my system?
+---------------------------------------------
+
+As initially stated, this guide is 'designed to cover everything typically
+needed [to build a kernel] on mainstream Linux distributions running on
+commodity PC or server hardware'. The outlined approach despite this should work
+on many other setups as well. But trying to cover every possible use-case in one
+guide would defeat its purpose, as without such a focus you would need dozens or
+hundreds of constructs along the lines of 'in case you are having <insert
+machine or distro>, you at this point have to do <this and that>
+<instead|additionally>'. Each of which would make the text longer, more
+complicated, and harder to follow.
+
+That being said: this of course is a balancing act. Hence, if you think an
+additional use-case is worth describing, suggest it to the maintainers of this
+document, as :ref:`described above <submit_improvements>`.
+
+
+..
+ end-of-content
+..
+ This document is maintained by Thorsten Leemhuis <linux@leemhuis.info>. If
+ you spot a typo or small mistake, feel free to let him know directly and
+ he'll fix it. You are free to do the same in a mostly informal way if you
+ want to contribute changes to the text -- but for copyright reasons please CC
+ linux-doc@vger.kernel.org and 'sign-off' your contribution as
+ Documentation/process/submitting-patches.rst explains in the section 'Sign
+ your work - the Developer's Certificate of Origin'.
+..
+ This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
+ of the file. If you want to distribute this text under CC-BY-4.0 only,
+ please use 'The Linux kernel development community' for author attribution
+ and link this as source:
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/quickly-build-trimmed-linux.rst
+..
+ Note: Only the content of this RST file as found in the Linux kernel sources
+ is available under CC-BY-4.0, as versions of this text that were processed
+ (for example by the kernel's build system) might contain content taken from
+ files which use a more restrictive license.
+
projects / platform / kernel / linux-rpi.git / commitdiff