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.. SPDX-License-Identifier: GPL-2.0
.. _kernelparameters:
The kernel's command-line parameters
====================================
The following is a consolidated list of the kernel parameters as implemented
by the __setup(), early_param(), core_param() and module_param() macros
and sorted into English Dictionary order (defined as ignoring all
punctuation and sorting digits before letters in a case insensitive
manner), and with descriptions where known.
The kernel parses parameters from the kernel command line up to "``--``";
if it doesn't recognize a parameter and it doesn't contain a '.', the
parameter gets passed to init: parameters with '=' go into init's
environment, others are passed as command line arguments to init.
Everything after "``--``" is passed as an argument to init.
Module parameters can be specified in two ways: via the kernel command
line with a module name prefix, or via modprobe, e.g.::
(kernel command line) usbcore.blinkenlights=1
(modprobe command line) modprobe usbcore blinkenlights=1
Parameters for modules which are built into the kernel need to be
specified on the kernel command line. modprobe looks through the
kernel command line (/proc/cmdline) and collects module parameters
when it loads a module, so the kernel command line can be used for
loadable modules too.
This document may not be entirely up to date and comprehensive. The command
"modinfo -p ${modulename}" shows a current list of all parameters of a loadable
module. Loadable modules, after being loaded into the running kernel, also
reveal their parameters in /sys/module/${modulename}/parameters/. Some of these
parameters may be changed at runtime by the command
``echo -n ${value} > /sys/module/${modulename}/parameters/${parm}``.
Special handling
----------------
Hyphens (dashes) and underscores are equivalent in parameter names, so::
log_buf_len=1M print-fatal-signals=1
can also be entered as::
log-buf-len=1M print_fatal_signals=1
Double-quotes can be used to protect spaces in values, e.g.::
param="spaces in here"
cpu lists
~~~~~~~~~
Some kernel parameters take a list of CPUs as a value, e.g. isolcpus,
nohz_full, irqaffinity, rcu_nocbs. The format of this list is:
<cpu number>,...,<cpu number>
or
<cpu number>-<cpu number>
(must be a positive range in ascending order)
or a mixture
<cpu number>,...,<cpu number>-<cpu number>
Note that for the special case of a range one can split the range into equal
sized groups and for each group use some amount from the beginning of that
group:
<cpu number>-<cpu number>:<used size>/<group size>
For example one can add to the command line following parameter:
isolcpus=1,2,10-20,100-2000:2/25
where the final item represents CPUs 100,101,125,126,150,151,...
The value "N" can be used to represent the numerically last CPU on the system,
i.e "foo_cpus=16-N" would be equivalent to "16-31" on a 32 core system.
Keep in mind that "N" is dynamic, so if system changes cause the bitmap width
to change, such as less cores in the CPU list, then N and any ranges using N
will also change. Use the same on a small 4 core system, and "16-N" becomes
"16-3" and now the same boot input will be flagged as invalid (start > end).
The special case-tolerant group name "all" has a meaning of selecting all CPUs,
so that "nohz_full=all" is the equivalent of "nohz_full=0-N".
The semantics of "N" and "all" is supported on a level of bitmaps and holds for
all users of bitmap_parselist().
Metric suffixes
~~~~~~~~~~~~~~~
The [KMG] suffix is commonly described after a number of kernel
parameter values. 'K', 'M', 'G', 'T', 'P', and 'E' suffixes are allowed.
These letters represent the _binary_ multipliers 'Kilo', 'Mega', 'Giga',
'Tera', 'Peta', and 'Exa', equaling 2^10, 2^20, 2^30, 2^40, 2^50, and
2^60 bytes respectively. Such letter suffixes can also be entirely omitted.
Kernel Build Options
--------------------
The parameters listed below are only valid if certain kernel build options
were enabled and if respective hardware is present. This list should be kept
in alphabetical order. The text in square brackets at the beginning
of each description states the restrictions within which a parameter
is applicable.
Parameters denoted with BOOT are actually interpreted by the boot
loader, and have no meaning to the kernel directly.
Do not modify the syntax of boot loader parameters without extreme
need or coordination with <Documentation/arch/x86/boot.rst>.
There are also arch-specific kernel-parameters not documented here.
Note that ALL kernel parameters listed below are CASE SENSITIVE, and that
a trailing = on the name of any parameter states that the parameter will
be entered as an environment variable, whereas its absence indicates that
it will appear as a kernel argument readable via /proc/cmdline by programs
running once the system is up.
The number of kernel parameters is not limited, but the length of the
complete command line (parameters including spaces etc.) is limited to
a fixed number of characters. This limit depends on the architecture
and is between 256 and 4096 characters. It is defined in the file
./include/uapi/asm-generic/setup.h as COMMAND_LINE_SIZE.
.. include:: kernel-parameters.txt
:literal:
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