Documentation/admin-guide/kernel-parameters.rst

   1 The kernel's command-line parameters
   2 ====================================
   3
   4 The following is a consolidated list of the kernel parameters as
   5 implemented by the __setup(), core_param() and module_param() macros
   6 and sorted into English Dictionary order (defined as ignoring all
   7 punctuation and sorting digits before letters in a case insensitive
   8 manner), and with descriptions where known.
   9
  10 The kernel parses parameters from the kernel command line up to "--";
  11 if it doesn't recognize a parameter and it doesn't contain a '.', the
  12 parameter gets passed to init: parameters with '=' go into init's
  13 environment, others are passed as command line arguments to init.
  14 Everything after "--" is passed as an argument to init.
  15
  16 Module parameters can be specified in two ways: via the kernel command
  17 line with a module name prefix, or via modprobe, e.g.::
  18
  19         (kernel command line) usbcore.blinkenlights=1
  20         (modprobe command line) modprobe usbcore blinkenlights=1
  21
  22 Parameters for modules which are built into the kernel need to be
  23 specified on the kernel command line.  modprobe looks through the
  24 kernel command line (/proc/cmdline) and collects module parameters
  25 when it loads a module, so the kernel command line can be used for
  26 loadable modules too.
  27
  28 Hyphens (dashes) and underscores are equivalent in parameter names, so::
  29
  30         log_buf_len=1M print-fatal-signals=1
  31
  32 can also be entered as::
  33
  34         log-buf-len=1M print_fatal_signals=1
  35
  36 Double-quotes can be used to protect spaces in values, e.g.::
  37
  38         param="spaces in here"
  39
  40 cpu lists:
  41 ----------
  42
  43 Some kernel parameters take a list of CPUs as a value, e.g.  isolcpus,
  44 nohz_full, irqaffinity, rcu_nocbs.  The format of this list is:
  45
  46         <cpu number>,...,<cpu number>
  47
  48 or
  49
  50         <cpu number>-<cpu number>
  51         (must be a positive range in ascending order)
  52
  53 or a mixture
  54
  55 <cpu number>,...,<cpu number>-<cpu number>
  56
  57 Note that for the special case of a range one can split the range into equal
  58 sized groups and for each group use some amount from the beginning of that
  59 group:
  60
  61         <cpu number>-cpu number>:<used size>/<group size>
  62
  63 For example one can add to the command line following parameter:
  64
  65         isolcpus=1,2,10-20,100-2000:2/25
  66
  67 where the final item represents CPUs 100,101,125,126,150,151,...
  68
  69
  70
  71 This document may not be entirely up to date and comprehensive. The command
  72 "modinfo -p ${modulename}" shows a current list of all parameters of a loadable
  73 module. Loadable modules, after being loaded into the running kernel, also
  74 reveal their parameters in /sys/module/${modulename}/parameters/. Some of these
  75 parameters may be changed at runtime by the command
  76 ``echo -n ${value} > /sys/module/${modulename}/parameters/${parm}``.
  77
  78 The parameters listed below are only valid if certain kernel build options were
  79 enabled and if respective hardware is present. The text in square brackets at
  80 the beginning of each description states the restrictions within which a
  81 parameter is applicable::
  82
  83         ACPI    ACPI support is enabled.
  84         AGP     AGP (Accelerated Graphics Port) is enabled.
  85         ALSA    ALSA sound support is enabled.
  86         APIC    APIC support is enabled.
  87         APM     Advanced Power Management support is enabled.
  88         ARM     ARM architecture is enabled.
  89         AVR32   AVR32 architecture is enabled.
  90         AX25    Appropriate AX.25 support is enabled.
  91         BLACKFIN Blackfin architecture is enabled.
  92         CLK     Common clock infrastructure is enabled.
  93         CMA     Contiguous Memory Area support is enabled.
  94         DRM     Direct Rendering Management support is enabled.
  95         DYNAMIC_DEBUG Build in debug messages and enable them at runtime
  96         EDD     BIOS Enhanced Disk Drive Services (EDD) is enabled
  97         EFI     EFI Partitioning (GPT) is enabled
  98         EIDE    EIDE/ATAPI support is enabled.
  99         EVM     Extended Verification Module
 100         FB      The frame buffer device is enabled.
 101         FTRACE  Function tracing enabled.
 102         GCOV    GCOV profiling is enabled.
 103         HW      Appropriate hardware is enabled.
 104         IA-64   IA-64 architecture is enabled.
 105         IMA     Integrity measurement architecture is enabled.
 106         IOSCHED More than one I/O scheduler is enabled.
 107         IP_PNP  IP DHCP, BOOTP, or RARP is enabled.
 108         IPV6    IPv6 support is enabled.
 109         ISAPNP  ISA PnP code is enabled.
 110         ISDN    Appropriate ISDN support is enabled.
 111         JOY     Appropriate joystick support is enabled.
 112         KGDB    Kernel debugger support is enabled.
 113         KVM     Kernel Virtual Machine support is enabled.
 114         LIBATA  Libata driver is enabled
 115         LP      Printer support is enabled.
 116         LOOP    Loopback device support is enabled.
 117         M68k    M68k architecture is enabled.
 118                         These options have more detailed description inside of
 119                         Documentation/m68k/kernel-options.txt.
 120         MDA     MDA console support is enabled.
 121         MIPS    MIPS architecture is enabled.
 122         MOUSE   Appropriate mouse support is enabled.
 123         MSI     Message Signaled Interrupts (PCI).
 124         MTD     MTD (Memory Technology Device) support is enabled.
 125         NET     Appropriate network support is enabled.
 126         NUMA    NUMA support is enabled.
 127         NFS     Appropriate NFS support is enabled.
 128         OSS     OSS sound support is enabled.
 129         PV_OPS  A paravirtualized kernel is enabled.
 130         PARIDE  The ParIDE (parallel port IDE) subsystem is enabled.
 131         PARISC  The PA-RISC architecture is enabled.
 132         PCI     PCI bus support is enabled.
 133         PCIE    PCI Express support is enabled.
 134         PCMCIA  The PCMCIA subsystem is enabled.
 135         PNP     Plug & Play support is enabled.
 136         PPC     PowerPC architecture is enabled.
 137         PPT     Parallel port support is enabled.
 138         PS2     Appropriate PS/2 support is enabled.
 139         RAM     RAM disk support is enabled.
 140         S390    S390 architecture is enabled.
 141         SCSI    Appropriate SCSI support is enabled.
 142                         A lot of drivers have their options described inside
 143                         the Documentation/scsi/ sub-directory.
 144         SECURITY Different security models are enabled.
 145         SELINUX SELinux support is enabled.
 146         APPARMOR AppArmor support is enabled.
 147         SERIAL  Serial support is enabled.
 148         SH      SuperH architecture is enabled.
 149         SMP     The kernel is an SMP kernel.
 150         SPARC   Sparc architecture is enabled.
 151         SWSUSP  Software suspend (hibernation) is enabled.
 152         SUSPEND System suspend states are enabled.
 153         TPM     TPM drivers are enabled.
 154         TS      Appropriate touchscreen support is enabled.
 155         UMS     USB Mass Storage support is enabled.
 156         USB     USB support is enabled.
 157         USBHID  USB Human Interface Device support is enabled.
 158         V4L     Video For Linux support is enabled.
 159         VMMIO   Driver for memory mapped virtio devices is enabled.
 160         VGA     The VGA console has been enabled.
 161         VT      Virtual terminal support is enabled.
 162         WDT     Watchdog support is enabled.
 163         XT      IBM PC/XT MFM hard disk support is enabled.
 164         X86-32  X86-32, aka i386 architecture is enabled.
 165         X86-64  X86-64 architecture is enabled.
 166                         More X86-64 boot options can be found in
 167                         Documentation/x86/x86_64/boot-options.txt .
 168         X86     Either 32-bit or 64-bit x86 (same as X86-32+X86-64)
 169         X86_UV  SGI UV support is enabled.
 170         XEN     Xen support is enabled
 171
 172 In addition, the following text indicates that the option::
 173
 174         BUGS=   Relates to possible processor bugs on the said processor.
 175         KNL     Is a kernel start-up parameter.
 176         BOOT    Is a boot loader parameter.
 177
 178 Parameters denoted with BOOT are actually interpreted by the boot
 179 loader, and have no meaning to the kernel directly.
 180 Do not modify the syntax of boot loader parameters without extreme
 181 need or coordination with <Documentation/x86/boot.txt>.
 182
 183 There are also arch-specific kernel-parameters not documented here.
 184 See for example <Documentation/x86/x86_64/boot-options.txt>.
 185
 186 Note that ALL kernel parameters listed below are CASE SENSITIVE, and that
 187 a trailing = on the name of any parameter states that that parameter will
 188 be entered as an environment variable, whereas its absence indicates that
 189 it will appear as a kernel argument readable via /proc/cmdline by programs
 190 running once the system is up.
 191
 192 The number of kernel parameters is not limited, but the length of the
 193 complete command line (parameters including spaces etc.) is limited to
 194 a fixed number of characters. This limit depends on the architecture
 195 and is between 256 and 4096 characters. It is defined in the file
 196 ./include/asm/setup.h as COMMAND_LINE_SIZE.
 197
 198 Finally, the [KMG] suffix is commonly described after a number of kernel
 199 parameter values. These 'K', 'M', and 'G' letters represent the _binary_
 200 multipliers 'Kilo', 'Mega', and 'Giga', equalling 2^10, 2^20, and 2^30
 201 bytes respectively. Such letter suffixes can also be entirely omitted:
 202
 203 .. include:: kernel-parameters.txt
 204    :literal:
 205
 206 Todo
 207 ----
 208
 209         Add more DRM drivers.