Documentation/admin-guide/kernel-parameters.rst

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