1.. SPDX-License-Identifier: GPL-2.0 2 3.. _kernelparameters: 4 5The kernel's command-line parameters 6==================================== 7 8The following is a consolidated list of the kernel parameters as implemented 9by the __setup(), early_param(), core_param() and module_param() macros 10and sorted into English Dictionary order (defined as ignoring all 11punctuation and sorting digits before letters in a case insensitive 12manner), and with descriptions where known. 13 14The kernel parses parameters from the kernel command line up to "``--``"; 15if it doesn't recognize a parameter and it doesn't contain a '.', the 16parameter gets passed to init: parameters with '=' go into init's 17environment, others are passed as command line arguments to init. 18Everything after "``--``" is passed as an argument to init. 19 20Module parameters can be specified in two ways: via the kernel command 21line with a module name prefix, or via modprobe, e.g.:: 22 23 (kernel command line) usbcore.blinkenlights=1 24 (modprobe command line) modprobe usbcore blinkenlights=1 25 26Parameters for modules which are built into the kernel need to be 27specified on the kernel command line. modprobe looks through the 28kernel command line (/proc/cmdline) and collects module parameters 29when it loads a module, so the kernel command line can be used for 30loadable modules too. 31 32This document may not be entirely up to date and comprehensive. The command 33"modinfo -p ${modulename}" shows a current list of all parameters of a loadable 34module. Loadable modules, after being loaded into the running kernel, also 35reveal their parameters in /sys/module/${modulename}/parameters/. Some of these 36parameters may be changed at runtime by the command 37``echo -n ${value} > /sys/module/${modulename}/parameters/${parm}``. 38 39Special handling 40---------------- 41 42Hyphens (dashes) and underscores are equivalent in parameter names, so:: 43 44 log_buf_len=1M print-fatal-signals=1 45 46can also be entered as:: 47 48 log-buf-len=1M print_fatal_signals=1 49 50Double-quotes can be used to protect spaces in values, e.g.:: 51 52 param="spaces in here" 53 54cpu lists 55~~~~~~~~~ 56 57Some kernel parameters take a list of CPUs as a value, e.g. isolcpus, 58nohz_full, irqaffinity, rcu_nocbs. The format of this list is: 59 60 <cpu number>,...,<cpu number> 61 62or 63 64 <cpu number>-<cpu number> 65 (must be a positive range in ascending order) 66 67or a mixture 68 69<cpu number>,...,<cpu number>-<cpu number> 70 71Note that for the special case of a range one can split the range into equal 72sized groups and for each group use some amount from the beginning of that 73group: 74 75 <cpu number>-<cpu number>:<used size>/<group size> 76 77For example one can add to the command line following parameter: 78 79 isolcpus=1,2,10-20,100-2000:2/25 80 81where the final item represents CPUs 100,101,125,126,150,151,... 82 83The value "N" can be used to represent the numerically last CPU on the system, 84i.e "foo_cpus=16-N" would be equivalent to "16-31" on a 32 core system. 85 86Keep in mind that "N" is dynamic, so if system changes cause the bitmap width 87to change, such as less cores in the CPU list, then N and any ranges using N 88will also change. Use the same on a small 4 core system, and "16-N" becomes 89"16-3" and now the same boot input will be flagged as invalid (start > end). 90 91The special case-tolerant group name "all" has a meaning of selecting all CPUs, 92so that "nohz_full=all" is the equivalent of "nohz_full=0-N". 93 94The semantics of "N" and "all" is supported on a level of bitmaps and holds for 95all users of bitmap_parselist(). 96 97Metric suffixes 98~~~~~~~~~~~~~~~ 99 100The [KMG] suffix is commonly described after a number of kernel 101parameter values. 'K', 'M', 'G', 'T', 'P', and 'E' suffixes are allowed. 102These letters represent the _binary_ multipliers 'Kilo', 'Mega', 'Giga', 103'Tera', 'Peta', and 'Exa', equaling 2^10, 2^20, 2^30, 2^40, 2^50, and 1042^60 bytes respectively. Such letter suffixes can also be entirely omitted. 105 106Kernel Build Options 107-------------------- 108 109The parameters listed below are only valid if certain kernel build options 110were enabled and if respective hardware is present. This list should be kept 111in alphabetical order. The text in square brackets at the beginning 112of each description states the restrictions within which a parameter 113is applicable. 114 115Parameters denoted with BOOT are actually interpreted by the boot 116loader, and have no meaning to the kernel directly. 117Do not modify the syntax of boot loader parameters without extreme 118need or coordination with <Documentation/arch/x86/boot.rst>. 119 120There are also arch-specific kernel-parameters not documented here. 121 122Note that ALL kernel parameters listed below are CASE SENSITIVE, and that 123a trailing = on the name of any parameter states that the parameter will 124be entered as an environment variable, whereas its absence indicates that 125it will appear as a kernel argument readable via /proc/cmdline by programs 126running once the system is up. 127 128The number of kernel parameters is not limited, but the length of the 129complete command line (parameters including spaces etc.) is limited to 130a fixed number of characters. This limit depends on the architecture 131and is between 256 and 4096 characters. It is defined in the file 132./include/uapi/asm-generic/setup.h as COMMAND_LINE_SIZE. 133 134.. include:: kernel-parameters.txt 135 :literal: 136