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kernel-parameters.txt
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accept_memory= [MM]
Format: { eager | lazy }
default: lazy
By default, unaccepted memory is accepted lazily to
avoid prolonged boot times. The lazy option will add
some runtime overhead until all memory is eventually
accepted. In most cases the overhead is negligible.
For some workloads or for debugging purposes
accept_memory=eager can be used to accept all memory
at once during boot.
acpi= [HW,ACPI,X86,ARM64,RISCV64,EARLY]
Advanced Configuration and Power Interface
Format: { force | on | off | strict | noirq | rsdt |
copy_dsdt | nospcr }
force -- enable ACPI if default was off
on -- enable ACPI but allow fallback to DT [arm64,riscv64]
off -- disable ACPI if default was on
noirq -- do not use ACPI for IRQ routing
strict -- Be less tolerant of platforms that are not
strictly ACPI specification compliant.
rsdt -- prefer RSDT over (default) XSDT
copy_dsdt -- copy DSDT to memory
nocmcff -- Disable firmware first mode for corrected
errors. This disables parsing the HEST CMC error
source to check if firmware has set the FF flag. This
may result in duplicate corrected error reports.
nospcr -- disable console in ACPI SPCR table as
default _serial_ console on ARM64
For ARM64, ONLY "acpi=off", "acpi=on", "acpi=force" or
"acpi=nospcr" are available
For RISCV64, ONLY "acpi=off", "acpi=on" or "acpi=force"
are available
See also Documentation/power/runtime_pm.rst, pci=noacpi
acpi_apic_instance= [ACPI,IOAPIC,EARLY]
Format: <int>
2: use 2nd APIC table, if available
1,0: use 1st APIC table
default: 0
acpi_backlight= [HW,ACPI]
{ vendor | video | native | none }
If set to vendor, prefer vendor-specific driver
(e.g. thinkpad_acpi, sony_acpi, etc.) instead
of the ACPI video.ko driver.
If set to video, use the ACPI video.ko driver.
If set to native, use the device's native backlight mode.
If set to none, disable the ACPI backlight interface.
acpi_force_32bit_fadt_addr [ACPI,EARLY]
force FADT to use 32 bit addresses rather than the
64 bit X_* addresses. Some firmware have broken 64
bit addresses for force ACPI ignore these and use
the older legacy 32 bit addresses.
acpica_no_return_repair [HW, ACPI]
Disable AML predefined validation mechanism
This mechanism can repair the evaluation result to make
the return objects more ACPI specification compliant.
This option is useful for developers to identify the
root cause of an AML interpreter issue when the issue
has something to do with the repair mechanism.
acpi.debug_layer= [HW,ACPI,ACPI_DEBUG]
acpi.debug_level= [HW,ACPI,ACPI_DEBUG]
Format: <int>
CONFIG_ACPI_DEBUG must be enabled to produce any ACPI
debug output. Bits in debug_layer correspond to a
_COMPONENT in an ACPI source file, e.g.,
#define _COMPONENT ACPI_EVENTS
Bits in debug_level correspond to a level in
ACPI_DEBUG_PRINT statements, e.g.,
ACPI_DEBUG_PRINT((ACPI_DB_INFO, ...
The debug_level mask defaults to "info". See
Documentation/firmware-guide/acpi/debug.rst for more information about
debug layers and levels.
Enable processor driver info messages:
acpi.debug_layer=0x20000000
Enable AML "Debug" output, i.e., stores to the Debug
object while interpreting AML:
acpi.debug_layer=0xffffffff acpi.debug_level=0x2
Enable all messages related to ACPI hardware:
acpi.debug_layer=0x2 acpi.debug_level=0xffffffff
Some values produce so much output that the system is
unusable. The "log_buf_len" parameter may be useful
if you need to capture more output.
acpi_enforce_resources= [ACPI]
{ strict | lax | no }
Check for resource conflicts between native drivers
and ACPI OperationRegions (SystemIO and SystemMemory
only). IO ports and memory declared in ACPI might be
used by the ACPI subsystem in arbitrary AML code and
can interfere with legacy drivers.
strict (default): access to resources claimed by ACPI
is denied; legacy drivers trying to access reserved
resources will fail to bind to device using them.
lax: access to resources claimed by ACPI is allowed;
legacy drivers trying to access reserved resources
will bind successfully but a warning message is logged.
no: ACPI OperationRegions are not marked as reserved,
no further checks are performed.
acpi_force_table_verification [HW,ACPI,EARLY]
Enable table checksum verification during early stage.
By default, this is disabled due to x86 early mapping
size limitation.
acpi_irq_balance [HW,ACPI]
ACPI will balance active IRQs
default in APIC mode
acpi_irq_nobalance [HW,ACPI]
ACPI will not move active IRQs (default)
default in PIC mode
acpi_irq_isa= [HW,ACPI] If irq_balance, mark listed IRQs used by ISA
Format: <irq>,<irq>...
acpi_irq_pci= [HW,ACPI] If irq_balance, clear listed IRQs for
use by PCI
Format: <irq>,<irq>...
acpi_mask_gpe= [HW,ACPI]
Due to the existence of _Lxx/_Exx, some GPEs triggered
by unsupported hardware/firmware features can result in
GPE floodings that cannot be automatically disabled by
the GPE dispatcher.
This facility can be used to prevent such uncontrolled
GPE floodings.
Format: <byte> or <bitmap-list>
acpi_no_auto_serialize [HW,ACPI]
Disable auto-serialization of AML methods
AML control methods that contain the opcodes to create
named objects will be marked as "Serialized" by the
auto-serialization feature.
This feature is enabled by default.
This option allows to turn off the feature.
acpi_no_memhotplug [ACPI] Disable memory hotplug. Useful for kdump
kernels.
acpi_no_static_ssdt [HW,ACPI,EARLY]
Disable installation of static SSDTs at early boot time
By default, SSDTs contained in the RSDT/XSDT will be
installed automatically and they will appear under
/sys/firmware/acpi/tables.
This option turns off this feature.
Note that specifying this option does not affect
dynamic table installation which will install SSDT
tables to /sys/firmware/acpi/tables/dynamic.
acpi_no_watchdog [HW,ACPI,WDT]
Ignore the ACPI-based watchdog interface (WDAT) and let
a native driver control the watchdog device instead.
acpi_rsdp= [ACPI,EFI,KEXEC,EARLY]
Pass the RSDP address to the kernel, mostly used
on machines running EFI runtime service to boot the
second kernel for kdump.
acpi_os_name= [HW,ACPI] Tell ACPI BIOS the name of the OS
Format: To spoof as Windows 98: ="Microsoft Windows"
acpi_rev_override [ACPI] Override the _REV object to return 5 (instead
of 2 which is mandated by ACPI 6) as the supported ACPI
specification revision (when using this switch, it may
be necessary to carry out a cold reboot _twice_ in a
row to make it take effect on the platform firmware).
acpi_osi= [HW,ACPI] Modify list of supported OS interface strings
acpi_osi="string1" # add string1
acpi_osi="!string2" # remove string2
acpi_osi=!* # remove all strings
acpi_osi=! # disable all built-in OS vendor
strings
acpi_osi=!! # enable all built-in OS vendor
strings
acpi_osi= # disable all strings
'acpi_osi=!' can be used in combination with single or
multiple 'acpi_osi="string1"' to support specific OS
vendor string(s). Note that such command can only
affect the default state of the OS vendor strings, thus
it cannot affect the default state of the feature group
strings and the current state of the OS vendor strings,
specifying it multiple times through kernel command line
is meaningless. This command is useful when one do not
care about the state of the feature group strings which
should be controlled by the OSPM.
Examples:
1. 'acpi_osi=! acpi_osi="Windows 2000"' is equivalent
to 'acpi_osi="Windows 2000" acpi_osi=!', they all
can make '_OSI("Windows 2000")' TRUE.
'acpi_osi=' cannot be used in combination with other
'acpi_osi=' command lines, the _OSI method will not
exist in the ACPI namespace. NOTE that such command can
only affect the _OSI support state, thus specifying it
multiple times through kernel command line is also
meaningless.
Examples:
1. 'acpi_osi=' can make 'CondRefOf(_OSI, Local1)'
FALSE.
'acpi_osi=!*' can be used in combination with single or
multiple 'acpi_osi="string1"' to support specific
string(s). Note that such command can affect the
current state of both the OS vendor strings and the
feature group strings, thus specifying it multiple times
through kernel command line is meaningful. But it may
still not able to affect the final state of a string if
there are quirks related to this string. This command
is useful when one want to control the state of the
feature group strings to debug BIOS issues related to
the OSPM features.
Examples:
1. 'acpi_osi="Module Device" acpi_osi=!*' can make
'_OSI("Module Device")' FALSE.
2. 'acpi_osi=!* acpi_osi="Module Device"' can make
'_OSI("Module Device")' TRUE.
3. 'acpi_osi=! acpi_osi=!* acpi_osi="Windows 2000"' is
equivalent to
'acpi_osi=!* acpi_osi=! acpi_osi="Windows 2000"'
and
'acpi_osi=!* acpi_osi="Windows 2000" acpi_osi=!',
they all will make '_OSI("Windows 2000")' TRUE.
acpi_pm_good [X86]
Override the pmtimer bug detection: force the kernel
to assume that this machine's pmtimer latches its value
and always returns good values.
acpi_sci= [HW,ACPI,EARLY] ACPI System Control Interrupt trigger mode
Format: { level | edge | high | low }
acpi_skip_timer_override [HW,ACPI,EARLY]
Recognize and ignore IRQ0/pin2 Interrupt Override.
For broken nForce2 BIOS resulting in XT-PIC timer.
acpi_sleep= [HW,ACPI] Sleep options
Format: { s3_bios, s3_mode, s3_beep, s4_hwsig,
s4_nohwsig, old_ordering, nonvs,
sci_force_enable, nobl }
See Documentation/power/video.rst for information on
s3_bios and s3_mode.
s3_beep is for debugging; it makes the PC's speaker beep
as soon as the kernel's real-mode entry point is called.
s4_hwsig causes the kernel to check the ACPI hardware
signature during resume from hibernation, and gracefully
refuse to resume if it has changed. This complies with
the ACPI specification but not with reality, since
Windows does not do this and many laptops do change it
on docking. So the default behaviour is to allow resume
and simply warn when the signature changes, unless the
s4_hwsig option is enabled.
s4_nohwsig prevents ACPI hardware signature from being
used (or even warned about) during resume.
old_ordering causes the ACPI 1.0 ordering of the _PTS
control method, with respect to putting devices into
low power states, to be enforced (the ACPI 2.0 ordering
of _PTS is used by default).
nonvs prevents the kernel from saving/restoring the
ACPI NVS memory during suspend/hibernation and resume.
sci_force_enable causes the kernel to set SCI_EN directly
on resume from S1/S3 (which is against the ACPI spec,
but some broken systems don't work without it).
nobl causes the internal blacklist of systems known to
behave incorrectly in some ways with respect to system
suspend and resume to be ignored (use wisely).
acpi_use_timer_override [HW,ACPI,EARLY]
Use timer override. For some broken Nvidia NF5 boards
that require a timer override, but don't have HPET
add_efi_memmap [EFI,X86,EARLY] Include EFI memory map in
kernel's map of available physical RAM.
agp= [AGP]
{ off | try_unsupported }
off: disable AGP support
try_unsupported: try to drive unsupported chipsets
(may crash computer or cause data corruption)
ALSA [HW,ALSA]
See Documentation/sound/alsa-configuration.rst
alignment= [KNL,ARM]
Allow the default userspace alignment fault handler
behaviour to be specified. Bit 0 enables warnings,
bit 1 enables fixups, and bit 2 sends a segfault.
align_va_addr= [X86-64]
Align virtual addresses by clearing slice [14:12] when
allocating a VMA at process creation time. This option
gives you up to 3% performance improvement on AMD F15h
machines (where it is enabled by default) for a
CPU-intensive style benchmark, and it can vary highly in
a microbenchmark depending on workload and compiler.
32: only for 32-bit processes
64: only for 64-bit processes
on: enable for both 32- and 64-bit processes
off: disable for both 32- and 64-bit processes
alloc_snapshot [FTRACE]
Allocate the ftrace snapshot buffer on boot up when the
main buffer is allocated. This is handy if debugging
and you need to use tracing_snapshot() on boot up, and
do not want to use tracing_snapshot_alloc() as it needs
to be done where GFP_KERNEL allocations are allowed.
allow_mismatched_32bit_el0 [ARM64,EARLY]
Allow execve() of 32-bit applications and setting of the
PER_LINUX32 personality on systems where only a strict
subset of the CPUs support 32-bit EL0. When this
parameter is present, the set of CPUs supporting 32-bit
EL0 is indicated by /sys/devices/system/cpu/aarch32_el0
and hot-unplug operations may be restricted.
See Documentation/arch/arm64/asymmetric-32bit.rst for more
information.
amd_iommu= [HW,X86-64]
Pass parameters to the AMD IOMMU driver in the system.
Possible values are:
fullflush - Deprecated, equivalent to iommu.strict=1
off - do not initialize any AMD IOMMU found in
the system
force_isolation - Force device isolation for all
devices. The IOMMU driver is not
allowed anymore to lift isolation
requirements as needed. This option
does not override iommu=pt
force_enable - Force enable the IOMMU on platforms known
to be buggy with IOMMU enabled. Use this
option with care.
pgtbl_v1 - Use v1 page table for DMA-API (Default).
pgtbl_v2 - Use v2 page table for DMA-API.
irtcachedis - Disable Interrupt Remapping Table (IRT) caching.
nohugepages - Limit page-sizes used for v1 page-tables
to 4 KiB.
v2_pgsizes_only - Limit page-sizes used for v1 page-tables
to 4KiB/2Mib/1GiB.
amd_iommu_dump= [HW,X86-64]
Enable AMD IOMMU driver option to dump the ACPI table
for AMD IOMMU. With this option enabled, AMD IOMMU
driver will print ACPI tables for AMD IOMMU during
IOMMU initialization.
amd_iommu_intr= [HW,X86-64]
Specifies one of the following AMD IOMMU interrupt
remapping modes:
legacy - Use legacy interrupt remapping mode.
vapic - Use virtual APIC mode, which allows IOMMU
to inject interrupts directly into guest.
This mode requires kvm-amd.avic=1.
(Default when IOMMU HW support is present.)
amd_pstate= [X86,EARLY]
disable
Do not enable amd_pstate as the default
scaling driver for the supported processors
passive
Use amd_pstate with passive mode as a scaling driver.
In this mode autonomous selection is disabled.
Driver requests a desired performance level and platform
tries to match the same performance level if it is
satisfied by guaranteed performance level.
active
Use amd_pstate_epp driver instance as the scaling driver,
driver provides a hint to the hardware if software wants
to bias toward performance (0x0) or energy efficiency (0xff)
to the CPPC firmware. then CPPC power algorithm will
calculate the runtime workload and adjust the realtime cores
frequency.
guided
Activate guided autonomous mode. Driver requests minimum and
maximum performance level and the platform autonomously
selects a performance level in this range and appropriate
to the current workload.
amd_prefcore=
[X86]
disable
Disable amd-pstate preferred core.
amijoy.map= [HW,JOY] Amiga joystick support
Map of devices attached to JOY0DAT and JOY1DAT
Format: <a>,<b>
See also Documentation/input/joydev/joystick.rst
analog.map= [HW,JOY] Analog joystick and gamepad support
Specifies type or capabilities of an analog joystick
connected to one of 16 gameports
Format: <type1>,<type2>,..<type16>
apc= [HW,SPARC]
Power management functions (SPARCstation-4/5 + deriv.)
Format: noidle
Disable APC CPU standby support. SPARCstation-Fox does
not play well with APC CPU idle - disable it if you have
APC and your system crashes randomly.
apic [APIC,X86-64] Use IO-APIC. Default.
apic= [APIC,X86,EARLY] Advanced Programmable Interrupt Controller
Change the output verbosity while booting
Format: { quiet (default) | verbose | debug }
Change the amount of debugging information output
when initialising the APIC and IO-APIC components.
apic_extnmi= [APIC,X86,EARLY] External NMI delivery setting
Format: { bsp (default) | all | none }
bsp: External NMI is delivered only to CPU 0
all: External NMIs are broadcast to all CPUs as a
backup of CPU 0
none: External NMI is masked for all CPUs. This is
useful so that a dump capture kernel won't be
shot down by NMI
apicpmtimer Do APIC timer calibration using the pmtimer. Implies
apicmaintimer. Useful when your PIT timer is totally
broken.
autoconf= [IPV6]
See Documentation/networking/ipv6.rst.
apm= [APM] Advanced Power Management
See header of arch/x86/kernel/apm_32.c.
apparmor= [APPARMOR] Disable or enable AppArmor at boot time
Format: { "0" | "1" }
See security/apparmor/Kconfig help text
0 -- disable.
1 -- enable.
Default value is set via kernel config option.
arcrimi= [HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards
Format: <io>,<irq>,<nodeID>
arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of
32 bit applications.
arm64.nobti [ARM64] Unconditionally disable Branch Target
Identification support
arm64.nogcs [ARM64] Unconditionally disable Guarded Control Stack
support
arm64.nomops [ARM64] Unconditionally disable Memory Copy and Memory
Set instructions support
arm64.nomte [ARM64] Unconditionally disable Memory Tagging Extension
support
arm64.nopauth [ARM64] Unconditionally disable Pointer Authentication
support
arm64.nosme [ARM64] Unconditionally disable Scalable Matrix
Extension support
arm64.nosve [ARM64] Unconditionally disable Scalable Vector
Extension support
ataflop= [HW,M68k]
atarimouse= [HW,MOUSE] Atari Mouse
atkbd.extra= [HW] Enable extra LEDs and keys on IBM RapidAccess,
EzKey and similar keyboards
atkbd.reset= [HW] Reset keyboard during initialization
atkbd.set= [HW] Select keyboard code set
Format: <int> (2 = AT (default), 3 = PS/2)
atkbd.scroll= [HW] Enable scroll wheel on MS Office and similar
keyboards
atkbd.softraw= [HW] Choose between synthetic and real raw mode
Format: <bool> (0 = real, 1 = synthetic (default))
atkbd.softrepeat= [HW]
Use software keyboard repeat
audit= [KNL] Enable the audit sub-system
Format: { "0" | "1" | "off" | "on" }
0 | off - kernel audit is disabled and can not be
enabled until the next reboot
unset - kernel audit is initialized but disabled and
will be fully enabled by the userspace auditd.
1 | on - kernel audit is initialized and partially
enabled, storing at most audit_backlog_limit
messages in RAM until it is fully enabled by the
userspace auditd.
Default: unset
audit_backlog_limit= [KNL] Set the audit queue size limit.
Format: <int> (must be >=0)
Default: 64
bau= [X86_UV] Enable the BAU on SGI UV. The default
behavior is to disable the BAU (i.e. bau=0).
Format: { "0" | "1" }
0 - Disable the BAU.
1 - Enable the BAU.
unset - Disable the BAU.
baycom_epp= [HW,AX25]
Format: <io>,<mode>
baycom_par= [HW,AX25] BayCom Parallel Port AX.25 Modem
Format: <io>,<mode>
See header of drivers/net/hamradio/baycom_par.c.
baycom_ser_fdx= [HW,AX25]
BayCom Serial Port AX.25 Modem (Full Duplex Mode)
Format: <io>,<irq>,<mode>[,<baud>]
See header of drivers/net/hamradio/baycom_ser_fdx.c.
baycom_ser_hdx= [HW,AX25]
BayCom Serial Port AX.25 Modem (Half Duplex Mode)
Format: <io>,<irq>,<mode>
See header of drivers/net/hamradio/baycom_ser_hdx.c.
bdev_allow_write_mounted=
Format: <bool>
Control the ability to open a mounted block device
for writing, i.e., allow / disallow writes that bypass
the FS. This was implemented as a means to prevent
fuzzers from crashing the kernel by overwriting the
metadata underneath a mounted FS without its awareness.
This also prevents destructive formatting of mounted
filesystems by naive storage tooling that don't use
O_EXCL. Default is Y and can be changed through the
Kconfig option CONFIG_BLK_DEV_WRITE_MOUNTED.
bert_disable [ACPI]
Disable BERT OS support on buggy BIOSes.
bgrt_disable [ACPI,X86,EARLY]
Disable BGRT to avoid flickering OEM logo.
blkdevparts= Manual partition parsing of block device(s) for
embedded devices based on command line input.
See Documentation/block/cmdline-partition.rst
boot_delay= [KNL,EARLY]
Milliseconds to delay each printk during boot.
Only works if CONFIG_BOOT_PRINTK_DELAY is enabled,
and you may also have to specify "lpj=". Boot_delay
values larger than 10 seconds (10000) are assumed
erroneous and ignored.
Format: integer
bootconfig [KNL,EARLY]
Extended command line options can be added to an initrd
and this will cause the kernel to look for it.
See Documentation/admin-guide/bootconfig.rst
bttv.card= [HW,V4L] bttv (bt848 + bt878 based grabber cards)
bttv.radio= Most important insmod options are available as
kernel args too.
bttv.pll= See Documentation/admin-guide/media/bttv.rst
bttv.tuner=
bulk_remove=off [PPC] This parameter disables the use of the pSeries
firmware feature for flushing multiple hpte entries
at a time.
c101= [NET] Moxa C101 synchronous serial card
cachesize= [BUGS=X86-32] Override level 2 CPU cache size detection.
Sometimes CPU hardware bugs make them report the cache
size incorrectly. The kernel will attempt work arounds
to fix known problems, but for some CPUs it is not
possible to determine what the correct size should be.
This option provides an override for these situations.
carrier_timeout=
[NET] Specifies amount of time (in seconds) that
the kernel should wait for a network carrier. By default
it waits 120 seconds.
ca_keys= [KEYS] This parameter identifies a specific key(s) on
the system trusted keyring to be used for certificate
trust validation.
format: { id:<keyid> | builtin }
cca= [MIPS,EARLY] Override the kernel pages' cache coherency
algorithm. Accepted values range from 0 to 7
inclusive. See arch/mips/include/asm/pgtable-bits.h
for platform specific values (SB1, Loongson3 and
others).
ccw_timeout_log [S390]
See Documentation/arch/s390/common_io.rst for details.
cgroup_disable= [KNL] Disable a particular controller or optional feature
Format: {name of the controller(s) or feature(s) to disable}
The effects of cgroup_disable=foo are:
- foo isn't auto-mounted if you mount all cgroups in
a single hierarchy
- foo isn't visible as an individually mountable
subsystem
- if foo is an optional feature then the feature is
disabled and corresponding cgroup files are not
created
{Currently only "memory" controller deal with this and
cut the overhead, others just disable the usage. So
only cgroup_disable=memory is actually worthy}
Specifying "pressure" disables per-cgroup pressure
stall information accounting feature
cgroup_no_v1= [KNL] Disable cgroup controllers and named hierarchies in v1
Format: { { controller | "all" | "named" }
[,{ controller | "all" | "named" }...] }
Like cgroup_disable, but only applies to cgroup v1;
the blacklisted controllers remain available in cgroup2.
"all" blacklists all controllers and "named" disables
named mounts. Specifying both "all" and "named" disables
all v1 hierarchies.
cgroup_favordynmods= [KNL] Enable or Disable favordynmods.
Format: { "true" | "false" }
Defaults to the value of CONFIG_CGROUP_FAVOR_DYNMODS.
cgroup.memory= [KNL] Pass options to the cgroup memory controller.
Format: <string>
nosocket -- Disable socket memory accounting.
nokmem -- Disable kernel memory accounting.
nobpf -- Disable BPF memory accounting.
checkreqprot= [SELINUX] Set initial checkreqprot flag value.
Format: { "0" | "1" }
See security/selinux/Kconfig help text.
0 -- check protection applied by kernel (includes
any implied execute protection).
1 -- check protection requested by application.
Default value is set via a kernel config option.
Value can be changed at runtime via
/sys/fs/selinux/checkreqprot.
Setting checkreqprot to 1 is deprecated.
cio_ignore= [S390]
See Documentation/arch/s390/common_io.rst for details.
clearcpuid=X[,X...] [X86]
Disable CPUID feature X for the kernel. See
arch/x86/include/asm/cpufeatures.h for the valid bit
numbers X. Note the Linux-specific bits are not necessarily
stable over kernel options, but the vendor-specific
ones should be.
X can also be a string as appearing in the flags: line
in /proc/cpuinfo which does not have the above
instability issue. However, not all features have names
in /proc/cpuinfo.
Note that using this option will taint your kernel.
Also note that user programs calling CPUID directly
or using the feature without checking anything
will still see it. This just prevents it from
being used by the kernel or shown in /proc/cpuinfo.
Also note the kernel might malfunction if you disable
some critical bits.
clk_ignore_unused
[CLK]
Prevents the clock framework from automatically gating
clocks that have not been explicitly enabled by a Linux
device driver but are enabled in hardware at reset or
by the bootloader/firmware. Note that this does not
force such clocks to be always-on nor does it reserve
those clocks in any way. This parameter is useful for
debug and development, but should not be needed on a
platform with proper driver support. For more
information, see Documentation/driver-api/clk.rst.
clock= [BUGS=X86-32, HW] gettimeofday clocksource override.
[Deprecated]
Forces specified clocksource (if available) to be used
when calculating gettimeofday(). If specified
clocksource is not available, it defaults to PIT.
Format: { pit | tsc | cyclone | pmtmr }
clocksource= Override the default clocksource
Format: <string>
Override the default clocksource and use the clocksource
with the name specified.
Some clocksource names to choose from, depending on
the platform:
[all] jiffies (this is the base, fallback clocksource)
[ACPI] acpi_pm
[ARM] imx_timer1,OSTS,netx_timer,mpu_timer2,
pxa_timer,timer3,32k_counter,timer0_1
[X86-32] pit,hpet,tsc;
scx200_hrt on Geode; cyclone on IBM x440
[MIPS] MIPS
[PARISC] cr16
[S390] tod
[SH] SuperH
[SPARC64] tick
[X86-64] hpet,tsc
clocksource.arm_arch_timer.evtstrm=
[ARM,ARM64,EARLY]
Format: <bool>
Enable/disable the eventstream feature of the ARM
architected timer so that code using WFE-based polling
loops can be debugged more effectively on production
systems.
clocksource.verify_n_cpus= [KNL]
Limit the number of CPUs checked for clocksources
marked with CLOCK_SOURCE_VERIFY_PERCPU that
are marked unstable due to excessive skew.
A negative value says to check all CPUs, while
zero says not to check any. Values larger than
nr_cpu_ids are silently truncated to nr_cpu_ids.
The actual CPUs are chosen randomly, with
no replacement if the same CPU is chosen twice.
clocksource-wdtest.holdoff= [KNL]
Set the time in seconds that the clocksource
watchdog test waits before commencing its tests.
Defaults to zero when built as a module and to
10 seconds when built into the kernel.
cma=nn[MG]@[start[MG][-end[MG]]]
[KNL,CMA,EARLY]
Sets the size of kernel global memory area for
contiguous memory allocations and optionally the
placement constraint by the physical address range of
memory allocations. A value of 0 disables CMA
altogether. For more information, see
kernel/dma/contiguous.c
cma_pernuma=nn[MG]
[KNL,CMA,EARLY]
Sets the size of kernel per-numa memory area for
contiguous memory allocations. A value of 0 disables
per-numa CMA altogether. And If this option is not
specified, the default value is 0.
With per-numa CMA enabled, DMA users on node nid will
first try to allocate buffer from the pernuma area
which is located in node nid, if the allocation fails,
they will fallback to the global default memory area.
numa_cma=<node>:nn[MG][,<node>:nn[MG]]
[KNL,CMA,EARLY]
Sets the size of kernel numa memory area for
contiguous memory allocations. It will reserve CMA
area for the specified node.
With numa CMA enabled, DMA users on node nid will
first try to allocate buffer from the numa area
which is located in node nid, if the allocation fails,
they will fallback to the global default memory area.
cmo_free_hint= [PPC] Format: { yes | no }
Specify whether pages are marked as being inactive
when they are freed. This is used in CMO environments
to determine OS memory pressure for page stealing by
a hypervisor.
Default: yes
coherent_pool=nn[KMG] [ARM,KNL,EARLY]
Sets the size of memory pool for coherent, atomic dma
allocations, by default set to 256K.
com20020= [HW,NET] ARCnet - COM20020 chipset
Format:
<io>[,<irq>[,<nodeID>[,<backplane>[,<ckp>[,<timeout>]]]]]
com90io= [HW,NET] ARCnet - COM90xx chipset (IO-mapped buffers)
Format: <io>[,<irq>]
com90xx= [HW,NET]
ARCnet - COM90xx chipset (memory-mapped buffers)
Format: <io>[,<irq>[,<memstart>]]
condev= [HW,S390] console device
conmode=
con3215_drop= [S390,EARLY] 3215 console drop mode.
Format: y|n|Y|N|1|0
When set to true, drop data on the 3215 console when
the console buffer is full. In this case the
operator using a 3270 terminal emulator (for example
x3270) does not have to enter the clear key for the
console output to advance and the kernel to continue.
This leads to a much faster boot time when a 3270
terminal emulator is active. If no 3270 terminal
emulator is used, this parameter has no effect.
console= [KNL] Output console device and options.
tty<n> Use the virtual console device <n>.
ttyS<n>[,options]
ttyUSB0[,options]
Use the specified serial port. The options are of
the form "bbbbpnf", where "bbbb" is the baud rate,
"p" is parity ("n", "o", or "e"), "n" is number of
bits, and "f" is flow control ("r" for RTS or
omit it). Default is "9600n8".
See Documentation/admin-guide/serial-console.rst for more
information. See
Documentation/networking/netconsole.rst for an
alternative.
<DEVNAME>:<n>.<n>[,options]
Use the specified serial port on the serial core bus.
The addressing uses DEVNAME of the physical serial port
device, followed by the serial core controller instance,
and the serial port instance. The options are the same
as documented for the ttyS addressing above.
The mapping of the serial ports to the tty instances
can be viewed with:
$ ls -d /sys/bus/serial-base/devices/*:*.*/tty/*
/sys/bus/serial-base/devices/00:04:0.0/tty/ttyS0
In the above example, the console can be addressed with
console=00:04:0.0. Note that a console addressed this
way will only get added when the related device driver
is ready. The use of an earlycon parameter in addition to
the console may be desired for console output early on.
uart[8250],io,<addr>[,options]
uart[8250],mmio,<addr>[,options]
uart[8250],mmio16,<addr>[,options]
uart[8250],mmio32,<addr>[,options]
uart[8250],0x<addr>[,options]
Start an early, polled-mode console on the 8250/16550
UART at the specified I/O port or MMIO address,
switching to the matching ttyS device later.
MMIO inter-register address stride is either 8-bit
(mmio), 16-bit (mmio16), or 32-bit (mmio32).
If none of [io|mmio|mmio16|mmio32], <addr> is assumed
to be equivalent to 'mmio'. 'options' are specified in
the same format described for ttyS above; if unspecified,
the h/w is not re-initialized.
hvc<n> Use the hypervisor console device <n>. This is for
both Xen and PowerPC hypervisors.
{ null | "" }
Use to disable console output, i.e., to have kernel
console messages discarded.
This must be the only console= parameter used on the
kernel command line.
If the device connected to the port is not a TTY but a braille
device, prepend "brl," before the device type, for instance
console=brl,ttyS0
For now, only VisioBraille is supported.
console_msg_format=
[KNL] Change console messages format
default
By default we print messages on consoles in
"[time stamp] text\n" format (time stamp may not be
printed, depending on CONFIG_PRINTK_TIME or
`printk_time' param).
syslog
Switch to syslog format: "<%u>[time stamp] text\n"
IOW, each message will have a facility and loglevel
prefix. The format is similar to one used by syslog()
syscall, or to executing "dmesg -S --raw" or to reading
from /proc/kmsg.
consoleblank= [KNL] The console blank (screen saver) timeout in
seconds. A value of 0 disables the blank timer.
Defaults to 0.
coredump_filter=
[KNL] Change the default value for
/proc/<pid>/coredump_filter.
See also Documentation/filesystems/proc.rst.
coresight_cpu_debug.enable
[ARM,ARM64]
Format: <bool>
Enable/disable the CPU sampling based debugging.
0: default value, disable debugging
1: enable debugging at boot time
cpcihp_generic= [HW,PCI] Generic port I/O CompactPCI driver
Format:
<first_slot>,<last_slot>,<port>,<enum_bit>[,<debug>]
cpuidle.off=1 [CPU_IDLE]
disable the cpuidle sub-system
cpuidle.governor=
[CPU_IDLE] Name of the cpuidle governor to use.
cpufreq.off=1 [CPU_FREQ]
disable the cpufreq sub-system
cpufreq.default_governor=
[CPU_FREQ] Name of the default cpufreq governor or
policy to use. This governor must be registered in the
kernel before the cpufreq driver probes.
cpu_init_udelay=N
[X86,EARLY] Delay for N microsec between assert and de-assert
of APIC INIT to start processors. This delay occurs
on every CPU online, such as boot, and resume from suspend.
Default: 10000
cpuhp.parallel=
[SMP] Enable/disable parallel bringup of secondary CPUs
Format: <bool>
Default is enabled if CONFIG_HOTPLUG_PARALLEL=y. Otherwise
the parameter has no effect.
crash_kexec_post_notifiers
Only jump to kdump kernel after running the panic
notifiers and dumping kmsg. This option increases
the risks of a kdump failure, since some panic
notifiers can make the crashed kernel more unstable.
In configurations where kdump may not be reliable,
running the panic notifiers could allow collecting
more data on dmesg, like stack traces from other CPUS
or extra data dumped by panic_print. Note that some
configurations enable this option unconditionally,
like Hyper-V, PowerPC (fadump) and AMD SEV-SNP.
crashkernel=size[KMG][@offset[KMG]]
[KNL,EARLY] Using kexec, Linux can switch to a 'crash kernel'
upon panic. This parameter reserves the physical
memory region [offset, offset + size] for that kernel
image. If '@offset' is omitted, then a suitable offset
is selected automatically.
[KNL, X86-64, ARM64, RISCV, LoongArch] Select a region
under 4G first, and fall back to reserve region above
4G when '@offset' hasn't been specified.
See Documentation/admin-guide/kdump/kdump.rst for further details.
crashkernel=range1:size1[,range2:size2,...][@offset]
[KNL] Same as above, but depends on the memory
in the running system. The syntax of range is
start-[end] where start and end are both
a memory unit (amount[KMG]). See also
Documentation/admin-guide/kdump/kdump.rst for an example.
crashkernel=size[KMG],high
[KNL, X86-64, ARM64, RISCV, LoongArch] range could be
above 4G.
Allow kernel to allocate physical memory region from top,
so could be above 4G if system have more than 4G ram
installed. Otherwise memory region will be allocated
below 4G, if available.
It will be ignored if crashkernel=X is specified.
crashkernel=size[KMG],low
[KNL, X86-64, ARM64, RISCV, LoongArch] range under 4G.
When crashkernel=X,high is passed, kernel could allocate
physical memory region above 4G, that cause second kernel
crash on system that require some amount of low memory,
e.g. swiotlb requires at least 64M+32K low memory, also
enough extra low memory is needed to make sure DMA buffers
for 32-bit devices won't run out. Kernel would try to allocate
default size of memory below 4G automatically. The default
size is platform dependent.
--> x86: max(swiotlb_size_or_default() + 8MiB, 256MiB)
--> arm64: 128MiB
--> riscv: 128MiB
--> loongarch: 128MiB
This one lets the user specify own low range under 4G
for second kernel instead.
0: to disable low allocation.
It will be ignored when crashkernel=X,high is not used
or memory reserved is below 4G.
cryptomgr.notests
[KNL] Disable crypto self-tests
cs89x0_dma= [HW,NET]
Format: <dma>
cs89x0_media= [HW,NET]
Format: { rj45 | aui | bnc }
csdlock_debug= [KNL] Enable or disable debug add-ons of cross-CPU
function call handling. When switched on,
additional debug data is printed to the console
in case a hanging CPU is detected, and that
CPU is pinged again in order to try to resolve