What is linux kernel firmware

Built-in firmware¶

Firmware can be built-in to the kernel, this means building the firmware into vmlinux directly, to enable avoiding having to look for firmware from the filesystem. Instead, firmware can be looked for inside the kernel directly. You can enable built-in firmware using the kernel configuration options:

• CONFIG_EXTRA_FIRMWARE
• CONFIG_EXTRA_FIRMWARE_DIR

This should not be confused with CONFIG_FIRMWARE_IN_KERNEL, this is for drivers which enables firmware to be built as part of the kernel build process. This option, CONFIG_FIRMWARE_IN_KERNEL, will build all firmware for all drivers enabled which ship its firmware inside the Linux kernel source tree.

There are a few reasons why you might want to consider building your firmware into the kernel with CONFIG_EXTRA_FIRMWARE though:

• Speed
• Firmware is needed for accessing the boot device, and the user doesn’t want to stuff the firmware into the boot initramfs.

Even if you have these needs there are a few reasons why you may not be able to make use of built-in firmware:

• Legalese — firmware is non-GPL compatible
• Some firmware may be optional
• Firmware upgrades are possible, therefore a new firmware would implicate a complete kernel rebuild.
• Some firmware files may be really large in size. The remote-proc subsystem is an example subsystem which deals with these sorts of firmware
• The firmware may need to be scraped out from some device specific location dynamically, an example is calibration data for for some WiFi chipsets. This calibration data can be unique per sold device.

© Copyright The kernel development community.

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Linux firmware

Linux firmware is a package distributed alongside the Linux kernel that contains firmware binary blobs necessary for partial or full functionality of certain hardware devices. These binary blobs are usually proprietary because some hardware manufacturers do not release source code necessary to build the firmware itself.

Modern graphics cards from AMD and NVIDIA almost certainly require binary blobs to be loaded for the hardware to operate correctly.

Starting at Broxton (a Skylake-based micro-architecture) Intel CPUs require binary blobs for additional low-power idle states (DMC), graphics workload scheduling on the various graphics parallel engines (GuC), and offloading some media functions from the CPU to GPU (HuC). [1]

Additionally, modern Intel Wi-Fi chipsets almost always require blobs. [2]

Contents

Installation

For security reasons, hotloading firmware into a running kernel has been shunned upon. Modern init systems such as systemd have strongly discouraged loading firmware from userspace.

Kernel

A few kernel options are important to consider when building in firmware support for certain devices in the Linux kernel:

For kernels before 4.18:

CONFIG_FIRMWARE_IN_KERNEL (DEPRECATED) Note this option has been removed as of versions v4.16 and above. [3] Enabling this option was previously necessary to build each required firmware blob specified by EXTRA_FIRMWARE into the kernel directly, where the request_firmware() function will find them without having to make a call out to userspace. On older kernels, it is necessary to enable it.

For kernels beginning with 4.18:

Firmware loading facility ( CONFIG_FW_LOADER ) This option is provided for the case where none of the in-tree modules require userspace firmware loading support, but a module built out-of-tree does. Build named firmware blobs into the kernel binary ( CONFIG_EXTRA_FIRMWARE ) This option is a string and takes the (space-separated) names of firmware files to be built into the kernel. These files will then be accessible to the kernel at runtime.

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What is linux-firmware when updating kernel?

When doing yum install kernel

It shows two packages:

What is the linux-firmware here?

1 Answer 1

Many devices have two essential software pieces that make them function in your operating system. The first is a working driver, which is the software that lets your system talk to the hardware. The second is firmware, which is usually a small piece of code that is uploaded directly to the device for it to function correctly. You can think of the firmware as a way of programming the hardware inside the device. In fact, in almost all cases firmware is treated like hardware in that it’s a black box; there’s no accompanying source code that is freely distributed with it.

While many devices can work without firmware, many more complicated ones require firmware to be properly set up, e.g. almost all modern GPUs, CPUs (microcode updates which fix bugs, erratas and vulnerabilities), Ethernet cards and WiFi adapters/phone radio modules (e.g. there’s a ton of variability in terms of properly broadcasting and receiving wireless signal and there are regional limitations as well), SCSI/RAID adapters, multimedia devices including webcams, etc.

Some people choose not to use firmware because they believe it can be used to extend hardware capabilities beyond what was initially designed/built into them, check Linux-libre So, if you have a server or a relatively old PC/laptop you can try running without firmware or even use the Linux-Libre kernel. I don’t share this point of view because modern hardware already has ROM and its circuitry is in absolute most cases «closed» sourced, so it’s not obvious whether it’s backdoor-free in the first place.

This package contains not just single firmware, it contains multiple files for various hardware devices (some devices require multiple files). Like I said, if everything works for you, you may as well never install it.

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The Linux Kernel documentation¶

This is the top level of the kernel’s documentation tree. Kernel documentation, like the kernel itself, is very much a work in progress; that is especially true as we work to integrate our many scattered documents into a coherent whole. Please note that improvements to the documentation are welcome; join the linux-doc list at vger.kernel.org if you want to help out.

Licensing documentation¶

The following describes the license of the Linux kernel source code (GPLv2), how to properly mark the license of individual files in the source tree, as well as links to the full license text.

User-oriented documentation¶

The following manuals are written for users of the kernel — those who are trying to get it to work optimally on a given system.

Firmware-related documentation¶

The following holds information on the kernel’s expectations regarding the platform firmwares.

Application-developer documentation¶

The user-space API manual gathers together documents describing aspects of the kernel interface as seen by application developers.

Introduction to kernel development¶

These manuals contain overall information about how to develop the kernel. The kernel community is quite large, with thousands of developers contributing over the course of a year. As with any large community, knowing how things are done will make the process of getting your changes merged much easier.

Kernel API documentation¶

These books get into the details of how specific kernel subsystems work from the point of view of a kernel developer. Much of the information here is taken directly from the kernel source, with supplemental material added as needed (or at least as we managed to add it — probably not all that is needed).

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Linux kernel release 5.x В¶

These are the release notes for Linux version 5. Read them carefully, as they tell you what this is all about, explain how to install the kernel, and what to do if something goes wrong.

What is Linux?В¶

Linux is a clone of the operating system Unix, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance.

It has all the features you would expect in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and multistack networking including IPv4 and IPv6.

It is distributed under the GNU General Public License v2 — see the accompanying COPYING file for more details.

On what hardware does it run?В¶

Although originally developed first for 32-bit x86-based PCs (386 or higher), today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell, IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, and ARC architectures.

Linux is easily portable to most general-purpose 32- or 64-bit architectures as long as they have a paged memory management unit (PMMU) and a port of the GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has also been ported to a number of architectures without a PMMU, although functionality is then obviously somewhat limited. Linux has also been ported to itself. You can now run the kernel as a userspace application — this is called UserMode Linux (UML).

Documentation¶

• There is a lot of documentation available both in electronic form on the Internet and in books, both Linux-specific and pertaining to general UNIX questions. I’d recommend looking into the documentation subdirectories on any Linux FTP site for the LDP (Linux Documentation Project) books. This README is not meant to be documentation on the system: there are much better sources available.
• There are various README files in the Documentation/ subdirectory: these typically contain kernel-specific installation notes for some drivers for example. Please read the Documentation/process/changes.rst file, as it contains information about the problems, which may result by upgrading your kernel.

Installing the kernel source¶

If you install the full sources, put the kernel tarball in a directory where you have permissions (e.g. your home directory) and unpack it:

Replace «X» with the version number of the latest kernel.

Do NOT use the /usr/src/linux area! This area has a (usually incomplete) set of kernel headers that are used by the library header files. They should match the library, and not get messed up by whatever the kernel-du-jour happens to be.

You can also upgrade between 5.x releases by patching. Patches are distributed in the xz format. To install by patching, get all the newer patch files, enter the top level directory of the kernel source (linux-5.x) and execute:

Replace «x» for all versions bigger than the version «x» of your current source tree, in_order, and you should be ok. You may want to remove the backup files (some-file-name

or some-file-name.orig), and make sure that there are no failed patches (some-file-name# or some-file-name.rej). If there are, either you or I have made a mistake.

Unlike patches for the 5.x kernels, patches for the 5.x.y kernels (also known as the -stable kernels) are not incremental but instead apply directly to the base 5.x kernel. For example, if your base kernel is 5.0 and you want to apply the 5.0.3 patch, you must not first apply the 5.0.1 and 5.0.2 patches. Similarly, if you are running kernel version 5.0.2 and want to jump to 5.0.3, you must first reverse the 5.0.2 patch (that is, patch -R) before applying the 5.0.3 patch. You can read more on this in Documentation/process/applying-patches.rst .

Alternatively, the script patch-kernel can be used to automate this process. It determines the current kernel version and applies any patches found:

The first argument in the command above is the location of the kernel source. Patches are applied from the current directory, but an alternative directory can be specified as the second argument.

Make sure you have no stale .o files and dependencies lying around:

You should now have the sources correctly installed.

Software requirements¶

Build directory for the kernel¶

When compiling the kernel, all output files will per default be stored together with the kernel source code. Using the option make O=output/dir allows you to specify an alternate place for the output files (including .config). Example:

To configure and build the kernel, use:

Please note: If the O=output/dir option is used, then it must be used for all invocations of make.

Configuring the kernel¶

Alternative configuration commands are:

You can find more information on using the Linux kernel config tools in Documentation/kbuild/kconfig.rst.

NOTES on make config :

• Having unnecessary drivers will make the kernel bigger, and can under some circumstances lead to problems: probing for a nonexistent controller card may confuse your other controllers.
• A kernel with math-emulation compiled in will still use the coprocessor if one is present: the math emulation will just never get used in that case. The kernel will be slightly larger, but will work on different machines regardless of whether they have a math coprocessor or not.
• The «kernel hacking» configuration details usually result in a bigger or slower kernel (or both), and can even make the kernel less stable by configuring some routines to actively try to break bad code to find kernel problems ( kmalloc() ). Thus you should probably answer ‘n’ to the questions for «development», «experimental», or «debugging» features.

Compiling the kernel¶

Make sure you have at least gcc 4.6 available. For more information, refer to Documentation/process/changes.rst .

Please note that you can still run a.out user programs with this kernel.

Do a make to create a compressed kernel image. It is also possible to do make install if you have lilo installed to suit the kernel makefiles, but you may want to check your particular lilo setup first.

To do the actual install, you have to be root, but none of the normal build should require that. Don’t take the name of root in vain.

If you configured any of the parts of the kernel as modules , you will also have to do make modules_install .

Verbose kernel compile/build output:

Normally, the kernel build system runs in a fairly quiet mode (but not totally silent). However, sometimes you or other kernel developers need to see compile, link, or other commands exactly as they are executed. For this, use «verbose» build mode. This is done by passing V=1 to the make command, e.g.:

To have the build system also tell the reason for the rebuild of each target, use V=2 . The default is V=0 .

Keep a backup kernel handy in case something goes wrong. This is especially true for the development releases, since each new release contains new code which has not been debugged. Make sure you keep a backup of the modules corresponding to that kernel, as well. If you are installing a new kernel with the same version number as your working kernel, make a backup of your modules directory before you do a make modules_install .

Alternatively, before compiling, use the kernel config option «LOCALVERSION» to append a unique suffix to the regular kernel version. LOCALVERSION can be set in the «General Setup» menu.

In order to boot your new kernel, you’ll need to copy the kernel image (e.g. . /linux/arch/x86/boot/bzImage after compilation) to the place where your regular bootable kernel is found.

Booting a kernel directly from a floppy without the assistance of a bootloader such as LILO, is no longer supported.

If you boot Linux from the hard drive, chances are you use LILO, which uses the kernel image as specified in the file /etc/lilo.conf. The kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or /boot/bzImage. To use the new kernel, save a copy of the old image and copy the new image over the old one. Then, you MUST RERUN LILO to update the loading map! If you don’t, you won’t be able to boot the new kernel image.

Reinstalling LILO is usually a matter of running /sbin/lilo. You may wish to edit /etc/lilo.conf to specify an entry for your old kernel image (say, /vmlinux.old) in case the new one does not work. See the LILO docs for more information.

After reinstalling LILO, you should be all set. Shutdown the system, reboot, and enjoy!

If you ever need to change the default root device, video mode, ramdisk size, etc. in the kernel image, use the rdev program (or alternatively the LILO boot options when appropriate). No need to recompile the kernel to change these parameters.

Reboot with the new kernel and enjoy.

If something goes wrong¶

If you have problems that seem to be due to kernel bugs, please check the file MAINTAINERS to see if there is a particular person associated with the part of the kernel that you are having trouble with. If there isn’t anyone listed there, then the second best thing is to mail them to me (torvalds @ linux-foundation . org), and possibly to any other relevant mailing-list or to the newsgroup.

In all bug-reports, please tell what kernel you are talking about, how to duplicate the problem, and what your setup is (use your common sense). If the problem is new, tell me so, and if the problem is old, please try to tell me when you first noticed it.

If the bug results in a message like:

or similar kernel debugging information on your screen or in your system log, please duplicate it exactly. The dump may look incomprehensible to you, but it does contain information that may help debugging the problem. The text above the dump is also important: it tells something about why the kernel dumped code (in the above example, it’s due to a bad kernel pointer). More information on making sense of the dump is in Documentation/admin-guide/bug-hunting.rst

If you compiled the kernel with CONFIG_KALLSYMS you can send the dump as is, otherwise you will have to use the ksymoops program to make sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred). This utility can be downloaded from https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ . Alternatively, you can do the dump lookup by hand:

In debugging dumps like the above, it helps enormously if you can look up what the EIP value means. The hex value as such doesn’t help me or anybody else very much: it will depend on your particular kernel setup. What you should do is take the hex value from the EIP line (ignore the 0010: ), and look it up in the kernel namelist to see which kernel function contains the offending address.

To find out the kernel function name, you’ll need to find the system binary associated with the kernel that exhibited the symptom. This is the file ‘linux/vmlinux’. To extract the namelist and match it against the EIP from the kernel crash, do:

This will give you a list of kernel addresses sorted in ascending order, from which it is simple to find the function that contains the offending address. Note that the address given by the kernel debugging messages will not necessarily match exactly with the function addresses (in fact, that is very unlikely), so you can’t just ‘grep’ the list: the list will, however, give you the starting point of each kernel function, so by looking for the function that has a starting address lower than the one you are searching for but is followed by a function with a higher address you will find the one you want. In fact, it may be a good idea to include a bit of «context» in your problem report, giving a few lines around the interesting one.

If you for some reason cannot do the above (you have a pre-compiled kernel image or similar), telling me as much about your setup as possible will help. Please read the admin-guide/reporting-bugs.rst document for details.

Alternatively, you can use gdb on a running kernel. (read-only; i.e. you cannot change values or set break points.) To do this, first compile the kernel with -g; edit arch/x86/Makefile appropriately, then do a make clean . You’ll also need to enable CONFIG_PROC_FS (via make config ).

After you’ve rebooted with the new kernel, do gdb vmlinux /proc/kcore . You can now use all the usual gdb commands. The command to look up the point where your system crashed is l *0xXXXXXXXX . (Replace the XXXes with the EIP value.)

gdb’ing a non-running kernel currently fails because gdb (wrongly) disregards the starting offset for which the kernel is compiled.

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