different kernel boot command-line reference options, which were written by the
kernel developers and released under the GPL.
There are three ways to pass options to the kernel and thus control its behavior:
• When building the kernel. Most of this book discusses these options.
• When starting the kernel. Usually, parameters are passed to the kernel when
it is invoked from a boot file such as the GRUB or LILO configuration file.
• At runtime, by writing to files in the /proc and /sys directories.
This chapter describes the second method of passing options. The chapter breaks
the boot time options into different logical sections. A number of architecturespecific
and individual driver options are not listed here. For a complete list of all
known options, please see the file Documentation/kernel-parameters.txt in the
kernel source tree and the individual architecture-specific documentation files.
Not all of the listed options are always available. Most are associated with
subsystems and work only if the kernel is configured with those subsystems built
in. They also depend on the presence of the hardware with which they are
associated.
All of these parameters are case-sensitive.
Module-Specific Options
In addition to the options listed in this chapter, parameters for modules that are
built in to the kernel can also be passed on the command line. (Dynamically
loaded modules, of course, are not in memory when the kernel boots and therefore
cannot be passed as parameters at boot time.) The syntax for passing
parameters consists of the module name followed by a dot (.) and the parameter.
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For example, the usbcore module accepts the parameter blinkenlights to display
flashing lights on all supported USB 2.0 hubs (don’t ever say the kernel developers
don’t have a sense of humor). To set this parameter when loading the
module dynamically, you would enter:
$ modprobe usbcore blinkenlights=1
But if the usbcore module is built into the kernel, you achieve the same effect by
invoking the kernel with the following option:
usbcore.blinkenlights=1
Most module options for modules that are built into the kernel can also be
changed at runtime by writing to files in the subdirectory named after the module
under the /sys/module directory. Thus, the blinkenlights option is represented by
the file /sys/module/usbcore/blinkenlights.
Console Options
These options deal with the console or kernel log, where kernel debugging and
error information are displayed.
console Output console device and options.
console=Options
ttyn
Use the virtual console device n.
ttySn[,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 omitted).
Default is 9600n8.
See the file Documentation/serial-console.txt for more information
on how to use a serial console. If you wish to have access
to the kernel console information and do not have a serial
port, see the netconsole command-line option.
uart,io,addr[,options], uart,mmio,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
specified ttyS device later. The options are the same as for ttyS
shown earlier.
netconsole Output console data across the network.
netconsole=[src-port]@[src-ip]/[dev],[target-port]@target-ip/[targetmac-
address]
Send kernel console data across the network using UDPpackets to
another machine. Options are:
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src-port
Source port for the UDP packets. The default value is 6665.
src-ip
Source IP address of the interface to use.
dev
Network interface to use. eth0 is an example. The network
interface can also run normal network traffic, because the
netconsole data is not intrusive and should cause no slowdown
in other network operations.
target-port
Port that the logging agent will use. The default value is 6666.
target-ip
IP address for the logging agent.
target-mac-address
Ethernet MAC address for the logging agent.
To listen to this data, the remote machine can use the syslogd
program, or run the netcat program as follows:
netcat -u -l -p port
For more background on how to use this option, see the file Documentation/
networking/netconsole.txt.
debug Enable kernel debugging.
Cause the kernel log level to be set to the debug level, so that all
debug messages will be printed to the console at boot time.
quiet Disable all log messages.
Set the default kernel log level to KERN_WARNING (4), which
suppresses all messages during boot except extremely serious ones.
(Log levels are defined under the loglevel parameter.)
earlyprintk Show early boot messages.
earlyprintk=[vga|serial][,ttySn[,baudrate]][,keep]
Show kernel log messages that precede the initialization of the
traditional console. These messages are typically never seen on the
console unless you use this option. Enabling this can be very useful
for tracking down hardware issues. Currently, the option can
specify either the VGA device or the serial port, but not both at the
same time. Also, only the ttyS0 or ttyS1 serial devices will work.
Interaction with the standard serial driver is not very good, and the
VGA output will eventually be overwritten by the real console.
Append ,keep in order not to disable the messages shown by this
option when the real kernel console is initialized and takes over the
system.
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loglevel Set the default console log level.
loglevel=level
Specify the initial console log level. Any log messages with levels
less than this (that is, of higher priority) will be printed to the
console, whereas any messages with levels equal to or greater than
this will not be displayed.
The console log level can also be changed by the klogd program, or
by writing the specified level to the /proc/sys/kernel/printk file.
The kernel log levels are:
0 (KERN_EMERG)
The system is unusable.
1 (KERN_ALERT)
Actions that must be taken care of immediately.
2 (KERN_CRIT)
Critical conditions.
3 (KERN_ERR)
Noncritical error conditions.
4 (KERN_WARNING)
Warning conditions that should be taken care of.
5 (KERN_NOTICE)
Normal, but significant events.
6 (KERN_INFO)
Informational messages that require no action.
7 (KERN_DEBUG)
Kernel debugging messages, output by the kernel if the developer
enabled debugging at compile time.
log_buf_len Set the size of the kernel log buffer.
log_buf_len=n[KMG]
Set the size of the kernel’s internal log buffer. n must be a power of
2, if not, it will be rounded up to be a power of 2. This value can
also be changed by the CONFIG_LOG_BUF_SHIFT kernel configuration
value.
initcall_debug Debug the initcall functions in the kernel.
Cause the kernel to trace all functions that are called by the kernel
during initialization of the system as the kernel boots. This option
is useful for determining where the kernel is dying during startup.
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kstack How many words of the stack to print in kernel oopses.
kstack=n
Specify how many words from the kernel stack should be printed in
the kernel oops dumps. n is an integer value.
time Show timing data on every kernel log message.
Cause the kernel to prefix every kernel log message with a
timestamp.
Interrupt Options
Interrupts are a complex aspect of kernel behavior. The boot time options deal
mostly with the interface between the kernel and the hardware that handles interrupts,
such as the Intel chip’s Advanced Programmable Interrupt Controller
(APIC).
apic Change the verbosity of the APIC subsystem when booting.
apic=[quiet|verbose|debug]
Control how much information the APIC subsystem generates
when booting the kernel. The default is quiet.
noapic Do not use any IOAPICs.
Prevent the kernel from using any of the IOAPICs that might be
present in the system.
lapic Enable the local APIC.
Cause the kernel to enable the local APIC even if the BIOS had
disabled it.
nolapic Do not use the local APIC.
Tell the kernel not to use the local APIC.
noirqbalance Disable kernel IRQ balancing.
Disable all of the built-in kernel IRQ balancing logic.
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irqfixup Basic fix to interrupt problems.
When an interrupt is not handled, search all known interrupt
handlers for it. This is intended to get systems with badly broken
firmware running.
irqpoll Extended fix to interrupt problems.
When an interrupt is not handled, search all known interrupt
handlers for it and also check all handlers on each timer interrupt.
This is intended to get systems with badly broken firmware
running.
noirqdebug Disable unhandled interrupt detection.
By default, the kernel attempts to detect and disable unhandled
interrupt sources because they can cause problems with the responsiveness
of the rest of the kernel if left unchecked. This option
disables this logic.
Memory Options
The kernel handles memory in many different chunks and categories for different
purposes. These options allow you to tweak the sizes and settings.
highmem Specify the size of the highmem memory zone.
highmem=n
Force the highmem memory zone to have an exact size of n bytes.
This will work even on boxes that have no highmem zones by
default. It can also reduce the size of the highmem zone for
machines with a lot of memory.
hugepages Set the number of hugetlb pages.
hugepages=n
The hugetlb feature lets you configure Linux to use 4 MB pages,
one thousand times the default size. If Linux is configured this way,
this options sets the maximum number of hugetlb pages to be n.
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ihash_entries Set the number of inode hash buckets.
ihash_entries=n
Override the default number of hash buckets for the kernel’s inode
cache. Recommended only for kernel experts.
max_addr Ignore memory.
max_addr=n
Cause the kernel to ignore all physical memory greater than or
equal to the physical address n.
mem Force memory usage.
mem=n[KMG]
Set the specific ammount of memory used by the kernel. When
used with the memmap= option, physical address space collisions can
be avoided. Without the memmap= option, this option could cause
PCI devices to be placed at addresses that belong to unused RAM.
n specifies the amount of memory to force and is measured in units
of kilobytes (K), megabytes (M), or gigabytes (G).
mem Disable the use of 4 MB pages for kernel memory.
mem=nopentium
Disable the use of huge (4 MB) pages for kernel memory.
memmap Enable setting of an exact E820 memory map.
memmap=exactmap
Use a specific memory map. The exactmap lines can be constructed
based on BIOS output or other requirements.
memmap Force specific memory to be used.
memmap=n[KMG]@start[KMG]
Force the kernel to use a specific memory region. n is the size of the
memory location, and start is the start location in memory of the
range. Units can be kilobytes (K), megabytes (M), or gigabytes (G).
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noexec Enable or disable nonexecutable mappings.
noexec=[on|off]
Enable or disable the kernel’s ability to map sections of memory as
nonexecutable. By default, the mapping is enabled (on).
reserve Reserve some I/O memory.
reserve=n[KMG]
Force the kernel to ignore some of the I/O memory areas.
vmalloc Force the vmalloc area to have a specific size.
vmalloc=n[KMG]
Force vmalloc to have the exact size specified by n. This can be
used to increase the minimum size of the vmalloc area (which is
128 MB on the x86 processor). It can also be used to decrease the
size and leave more room for directly mapped kernel RAM.
norandmaps Do not use address space randomization.
By default, the kernel randomizes the address space of all programs
when they are started. This option disables this feature. It is equivalent
to writing 0 to the file /proc/sys/kernel/randomize_va_space.
vdso Enable or disable the VDSO mapping.
vdso=[0|1]
Disable (0) or enable (1) the VDSO (Virtual Dynamic Shared
Object) mapping option. By default, it is enabled.
Suspend Options
These options change the way the kernel handles suspension for power-saving
purposes.
resume Specify the partition device for the suspend image.
resume=suspend_device
Tell the kernel which disk device contains the suspended kernel
image. If the data on the image is a valid kernel image created by
the software suspend subsystem, it will be loaded into memory and
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the kernel will run it instead of continuing on with the normal boot
process. suspend_device is the kernel device name, which might be
different from what userspace thinks the device name is, so be
careful with this option.
noresume Disable resume.
Disable the resume functionality of the kernel. Any swap partitions
that were being used to hold system images to which the kernel
could be restored will revert back to available swap space.
CPU Options
These options control a wide range of behavior regarding timing, processor use in
multiprocessor systems, and other processor issues.
cachesize Override level 2 CPU cache size detection.
cachesize=n
Sometimes CPU hardware bugs make them report the cache size
incorrectly. The kernel will attempt to work around and fix known
problems with most CPUs, but for some CPUs it is not possible to
determine what the correct size should be. This option provides an
override for these situations. n is measured in bytes.
lpj Set the loops per jiffy.
lpg=n
Specify the loops per jiffy that should be used by the kernel, and
thus have the kernel avoid the time-consuming boot-time autodetection
of this value. If n is 0, the value will be autodetected as
usual.
On SMPsystems, this value will be set on all CPUs,
which might cause problems if the different CPUs need
different settings. An incorrect value will cause incorrect
delays in the kernel, which can lead to unpredictable I/O
errors and other breakage. Although unlikely, in extreme
cases this might damage your hardware.
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nmi_watchdog Set the NMI watchdog value.
nmi_watchdog=[0|1|2|3]
This is a debugging feature that allows the user to override the
default nonmaskable interrupt (NMI) watchdog value. 0 specifies
that no NMI watchdog should be used. 1 specifies that the APIC
should be used if present. 2 specifies that the local APIC should be
used if present. 3 means that the NMI watchdog is invalid, so do
not use it.
no387 Always use the 387 emulation library.
Always use the 387 math emulation library, even if a 387 math
coprocessor is present in the system.
nofxsr Disable x86 floating-point save and restore.
Disable the x86 floating-point extended register save and restore.
The kernel will save only legacy floating-point registers on a task
switch.
no-hlt Do not use the HLT instruction.
This option is available because the HLT instruction does not work
correctly for some x86 processors. This option tells the kernel not
to use the instruction.
mce Enable the machine check exception feature.
Some processors can check for machine errors (usually errors in the
hardware). This option turns this subsystem on, if it has been built
into the kernel configuration.
nomce Disable the machine check exception feature.
This option turns the subsystem off.
nosep Disable x86 SYSENTER/SYSEXIT support.
Disable x86 SYSENTER/SYSEXIT support in the kernel. This can
cause some system calls to take longer.
nosmp Run as a single-processor machine.
Tell an SMPkernel to act as a uniprocessor kernel, even on a multiprocessor
machine.
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notsc Disable the time stamp counter.
Disable the timestamp counter hardware in the system, if present.
max_cpus Maximum number of CPUs to use.
maxcpus=n
Specify the maximum number of processors that a SMPkernel
should use, even if there are more processors present in the system.
Scheduler Options
These options tweak the parameters used to make scheduling decisions. Most
depend on an intimate understanding of how scheduling works in Linux.
isolcpus Isolate CPUs from the kernel scheduler.
isolcpus=cpu_number[,cpu_number,...]
Remove the specified CPUs, as defined by the cpu_number values,
from the general kernel SMPbalancing and scheduler algroithms.
The only way to move a process onto or off an “isolated” CPU is
via the CPU affinity syscalls. cpu_number begins at 0, so the
maximum value is one less than the number of CPUs on the
system.
This option is the preferred way to isolate CPUs. The alternative,
manually setting the CPU mask of all tasks in the system, can cause
problems and suboptimal load-balancer performance.
migration_cost Override the default scheduler migrations costs.
migration_cost=level-1-useconds[level-2-useconds...]
This is a debugging option that overrides the default scheduler
migration cost matrix. The numbers specified by level-N-useconds
are indexed by the “CPU domain distance” and are measured in
microseconds.
An example of this option is migration_cost=1000,2000,3000 for a
SMT NUMA machine. It sets up an intra-core migration cost of 1
ms, another inter-core migration cost of 2 ms, and another internode
migration cost of 3 ms.
Incorrect values can severely degrade scheduler performance,
so this option should be used only for scheduler
development, never for production environments.
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migration_
debug
Verbosity of migration cost autodetection.
migration_debug=[0|1|2]
Set the migration cost debug level. If 0 is specified, no extra
messages will be printed to the kernel log. This is the default value.
1 prints some information on how the matrix is determined. 2 is
very verbose and is useful only if you use a serial console, as the
amount of information will overflow the kernel log buffer.
migration_
factor
Multiply or divide the migration costs.
migration_factor=percent
Modify the default migration costs by the specified percent. This is
a debugging option that can be used to proportionally increase or
decrease the autodetected migration costs for all entries of the
migration matrix. For example, migration_factor=150 increases
migration costs by 50 percent, so the scheduler will be less eager to
migrate cache-hot tasks. migration_factor=80 decreases migration
costs by 20 percent, thus making the scheduler more eager to
migrate tasks.
Incorrect values can severely degrade scheduler performance,
so this option should be used only for scheduler
development, never for production environments.
Ramdisk Options
These options control how the storage of information in memory used to imitate
disks (ramdisks) is done, including init ramdisks that hold information necessary
at some stages of booting.
initrd Location of initial ramdisk.
initrd=filename
Specify where the initial ramdisk for the kernel boot is located.
load_ramdisk Load a kernel ramdisk from a floppy.
load_ramdisk=n
If n is set to 1, a ramdisk is loaded by the kernel at boot time from
the floppy drive.
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noinitrd Do not use any initrd.
Do not load any initial ramdisk, even if it is configured in other
options passed to the kernel.
prompt_
ramdisk
Prompt for the list of ramdisks.
prompt_ramdisk=1
Prompt the user for the initial ramdisk before attempting to read it
from the floppy drive.
ramdisk_
blocksize
Blocksize of the ramdisk.
ramdisk_blocksize=n
Tell the ramdisk driver how many bytes to use per block. The
default size is 1,024.
ramdisk_size Size of the ramdisk.
ramdisk_size=n
Specify the size of the initial ramdisk in kilobytes. The default size
is 4,096 (4 MB). This option should be used instead of the older
ramdisk command-line option.
Root Disk Options
These options control how the kernel finds and handles the filesystem that
contains the root filesystem.
ro Mount the root device read-only on boot.
The default for the kernel is to mount the root device as read-only
at boot time. This option ensures that this is the mode the kernel
uses. It overrides the rw command-line option, if it had been specified
earlier on the boot command line.
root Specify the root filesystem to boot from.
root=device
Tell the kernel which disk device the root filesystem image is on.
device can be specified in one of the following ways:
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nnnn
A device number in hexadecimal represents the major and
minor number of the device in the internal format that the
kernel expects. This method is not recommended unless you
have access to kernel internals.
/dev/nfs
Use the NFS disk specified by the nfsroot boot option as the
root disk.
/dev/
Use the kernel disk name specified by
disk.
/dev/
Use the kernel disk name specified by
partition specified by
/dev/
Use the kernel disk name specified by
partition specified by
same as above, but is needed when
digit.
rootdelay Time to delay before attempting to mount the root filesystem.
rootdelay=n
Wait n seconds before trying to mount the root filesystem. This can
be useful if the root filesystem is on a USB or FireWire device, as
those disk devices take a bit longer to be discovered by the kernel.
rootflags The root filesystem mount options.
rootflags=options
Mount options that the kernel should use in mounting the root filesystem.
The options value depend on the filesystem type; see the
documentation for the individual types for details on what is valid.
rootfstype The root filesystem type.
rootfstype=type
Try to mount the root filesystem as this type of filesystem. For
instance, rootfstype=ext3.
rw Mount the root device read-write on boot.
The default for the kernel is to mount the root device as read-only
at boot time. This option mounts the root device as read-write
instead.
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Init Options
The init process is the first to be started by the kernel and is the ancestor of all
other processes. These options control which program is run and how it is run.
init Program to run at init time.
init=filename
Run the specified binary as the init process instead of the default
/sbin/init program.
rdinit Run the init process from the ramdisk.
rdinit=full_path_name
Run the program specified by full_path_name as the init process.
This file must be on the kernel ramdisk instead of on the root
filesystem.
S Run init in single-user mode.
The default for the kernel is to run init in multi-user mode. This
option runs init in single-user mode instead.
kexec Options
The kexec subsystem is a specialized rebooting feature that allows a fast reboot
and is usually combined with the kdump facility that enables the previous kernel’s
memory to be dumped to a safe place for analysis at a later time. These options
modify the kexec subsystem’s parameters.
crashkernel Reserve a portion of physical memory for kexec to use.
crashkernel=n[KMG]@start[KMG]
The kexec subsystem likes to have a portion of physical memory
reserved for it. This option reserves that memory from the rest of
the kernel and will switch to use it if the kernel panics. n specifies
the amount of memory to reserve, and start specifies the location
for this memory chunk. Both are measured in units of kilobytes (K),
megabytes (M), or gigabytes (G).
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elfcorehdr Start of the kernel core image ELF header.
elfcorhdr=n
The kernel, like every Linux executable, is stored in ELF format.
This option specifies the physical address where the kernel core
image’s ELF header starts. This is used by kexec to find the kernel
when booting the secondary kernel image.
RCU Options
Read Copy Update (RCU) is a portion of the kernel that handles mutual exclusion
for a variety of subsystems in a lockless manner. There are a number of
options that can be used to tune RCU in different ways:
rcu.blimit RCU batch limit.
rcu.blimit=n
Set the maximum number of finished RCU callbacks to process in
one batch.
rcu.qhimark RCU queue high level.
rcu.qhimark=n
Batch limiting is disabled when the number of queued RCU callbacks
rises above n.
rcu.qlowmark RCU queue low level.
rcu.qlowmark=n
Batch limiting is re-enabled when the number of queued RCU callbacks
falls below n.
rcu.rsinterval RCU callback queue length.
rcu.rsinterval=n
Set the number of additional RCU callbacks that should be queued
before forcing a reschedule on all CPUs.
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ACPI Options
These options control parameters that the Advanced Configuration and Power
Interface (ACPI) subsystem can use.
acpi ACPI subsystem options.
acpi=[force|off|noirq|ht|strict]
This is the main option for the Advanced Configuration and Power
Interface (ACPI). Values are:
force
Force ACPI to be enabled. Can be used to override the kernel
configuration option that disabled it.
off
Disable ACPI. Can be used to override the kernel configuration
option that enabled it.
noirq
Prevent ACPI from being used for IRQ routing.
ht
Run only enough of the ACPI layer to enable HyperThreading
on processors that are capable of it.
strict
Make the ACPI layer be less tolerant of platforms that are not
fully compliant with the ACPI specification.
acpi_sleep ACPI sleep options.
acpi_sleep=[s3_bios],[s3_mode]
During S3 resume (which happens after the machine has been
suspended to RAM), hardware needs to be reinitialized properly.
For most devices this is simple, except for video cards, which are
normally initialized by the BIOS. The kernel does not have enough
information to restore the video device, because that information is
in the BIOS and not accessable at all. This option lets the kernel try
to use the ACPI subsystem to restore the video card in two different
ways.
See the file Documentation/power/video.txt for more information on
this option and how to find the proper value for your type of
hardware.
acpi_sci ACPI System Control Interrupt trigger mode.
acpi_sci=[level|edge|high|low]
Set the ACPI System Control Interrupt trigger mode.
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acpi_irq_
balance
Enable ACPI IRQ balance.
Cause ACPI to balance the active IRQs. This is the default option
when operating in APIC mode.
acpi_irq_
nobalance
Disable ACPI IRQ balance.
Cause ACPI not to move the active IRQs. This is the default option
when operating in PIC mode.
acpi_irq_isa Mark the listed IRQs as used by ISA.
acpi_irq_isa=irq[,irq...]
If the IRQ balance option is enabled, mark the listed IRQs as used
by the ISA subsystem.
acpi_irq_pci Mark the listed IRQs as used by PCI.
acpi_irq_pci=irq[,[irq...]
If the IRQ balance option is enabled, mark the listed IRQs as used
by the PCI subsystem.
acpi_os_name Fake the operating system name to ACPI.
acpi_os_name=name
Tell the ACPI BIOS that the name of the running operating system
is name. This can be useful to spoof the BIOS into thinking that
Windows is running instead of Linux, which can help solve some
ACPI issues for older BIOSes. As an example, use the string
Microsoft 2001 to spoof the BIOS into thinking that Windows 2001
is running on the machine.
acpi_osi Disable the _OSI ACPI method.
acpi_osi=[n]
This is actually a binary option despite the integer value. If n is
absent, ACPI will disable the _OSI method. If n is present, _OSI
will not be disabled.
acpi_serialize Force serialization of AML methods.
Force the serialization of ACPI Machine Language methods.
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acpi_skip_
timer_override
Skip interrupt override issues.
Allow the ACPI layer to recognize and ignore IRQ0/pin2 interrupt
override issues for broken nForce2 BIOSes that result in the XTPIC
timer acting up.
acpi_dbg_layer ACPI debug layer.
acpi_dbg_layer=n
Set the ACPI debug layers. n is an integer in which each bit indicates
a different ACPI debug layer. After the system has booted, the
debug layers can be set via the /proc/acpi/debug_layer file.
acpi_fake_ecdt ECDT workaround.
If present, this allows ACPI to workaround BIOS failures when it
lacks an Embedded Controller Description Table.
acpi_generic_
hotkey
Use generic ACPI hotkey driver.
This allows the ACPI consolidated generic hotkey driver to override
the platform-specific driver if one is present.
acpi_pm_good Override pmtimer bug detection.
Force the kernel to assume that the machine’s pmtimer latches its
value and always returns good values.
ec_intr ACPI Embedded Controller interrupt mode.
ec_intr=n
Specify the ACPI embedded controller interrupt mode. If n is 0,
polling mode will be used, otherwise interrupt mode will be used.
Interrupt mode is the default.
memmap Mark specific memory as ACPI data.
memmap=n[KMG]#start[KMG]
Marks a specific location and range of memory as ACPI data. n is
the size of the memory location and start is the start location in
memory of the range. Both are measured in units of kilobytes (K),
megabytes (M), or gigabytes (G).
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memmap Mark specific memory as reserved.
memmap=n[KMG]$start[KMG]
This marks a specific location and range of memory as reserved. n
is the size of the memory location and start is the start location in
memory of the range.
pnpacpi Turn Plug and Play ACPI off.
pnpacpi=off
Disable the Plug and Play ACPI functionality.
processor.max_
cstate
Limit the processor to a maximum C-state.
processor.max_cstate=n
Limit the processor to a maximum C-state, no matter what the
ACPI tables say it can support. n is a valid C-state value. A value of
9 overrides any DMI blacklist limit that might be present for this
processor.
processor.nocst Ignore the _CST method for C-states.
Causes the ACPI core to ignore the _CST method of determining
the processor C-states and use the legacy FADT method instead.
SCSI Options
These options specify different parameters the SCSI subsystem can use. A number
of SCSI driver-specific options are also available; please see the different driver
documentation files in the kernel directory Documentation/scsi/ for details.
max_luns Maximum number of SCSI LUNS to probe.
max_luns=n
Specify the maximum number of SCSI LUNS that the system
should probe. n is an integer from 1 to 4,294,967,295.
max_report_
luns
Maximum number of SCSI LUNS received.
max_report_luns=n
Specify the maximum number of SCSI LUNs that the system can
receive. n is an integer from 1 to 16,384.
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scsi_dev_flags SCSI black/white list.
scsi_dev_flags=vendor:model:flags
This option lets the user add entries to the SCSI black/white list for
a specific vendor and model of device.
PCI Options
These options specify different parameters the PCI subsystem can use:
PCI pci=option[,option...]
Each option can be one of the following:
off
Do not probe for the PCI bus.
bios
Force the use of the PCI BIOS by not accessing the hardware
directly. This means that the kernel should trust the BIOS,
which is not the standard thing to do (as BIOSes are known to
lie more often than they are known to be valid). Use this only
if your machine has a nonstandard PCI host bridge and the
normal boot method is not working properly.
nobios
Do not use the PCI BIOS, but access the hardware directly
instead. This is the default method of probing for PCI devices
in all kernels after 2.6.13.
conf1
Force use of PCI Configuration Mechanism 1 (a way to access
PCI memory on i386 machines).
conf2
Force use of PCI Configuration Mechanism 2 (a way to access
PCI memory on i386 machines).
nommconf
Disable use of the ACPI MMCONFIG table for PCI
configuration.
nomsi
If the PCI_MSI kernel config parameter is enabled, this kernel
boot option can be used to disable the use of MSI interrupts
system-wide.
nosort
Do not sort PCI devices according to order given by the PCI
BIOS. This sorting is done to get a device order compatible
with much older kernel versions.
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biosirq
Use PCI BIOS calls to get the interrupt routing table. These
calls are known to be buggy on several machines and hang
these machine when used, but on other machines they are the
only way to get the interrupt routing table. Try this option if
the kernel is unable to allocate IRQs or discover secondary
PCI buses on your motherboard.
rom
Assign address space to expansion ROMs. Use this with
caution as certain devices share address decoders between
ROMs and other resources.
irqmask=0xnnnn
Set a bit mask of IRQs allowed to be assigned automatically to
PCI devices. You can make the kernel exclude IRQs of your
ISA cards this way.
pirqaddr=0xn
Specify the physical address of the PIRQ table (normally
generated by the BIOS) if it is outside the F0000–100000
(hexadecimal) range.
lastbus=n
Scan all buses through bus n. Can be useful if the kernel is
unable to find your secondary buses and you want to tell it
explicitly which ones they are.
assign-busses
Always use your own PCI bus numbers, overriding whatever
the firmware may have done.
usepirqmask
Honor the possible IRQ mask stored in the BIOS $PIR table.
This is needed on some systems with broken BIOSes, notably
some HPP avilion N5400 and Omnibook XE3 notebooks.
This will have no effect if ACPI IRQ routing is enabled.
noacpi
Do not use ACPI for IRQ routing or for PCI scanning.
routeirq
Do IRQ routing for all PCI devices. This is normally done in
pci_enable_device( ), so this option is a temporary
workaround for broken drivers that don’t call it.
firmware
Do not re-enumerate the bus, but instead just use the configuration
from the bootloader. This is currently used on IXP2000
systems where the bus has to be configured a certain way for
adjunct CPUs.
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Plug and Play BIOS Options
noisapnp Disable the ISA Plug and Play (PnP) subsystem.
Disable the ISA PnP subsystem, if it has been enabled in the kernel
configuration.
pnpbios PnP BIOS settings.
pnpbios=[on|off|curr|no-curr]
Set the main PnP BIOS settings. on enables the PnP BIOS
subsystem. off disables the PnP BIOS subsystem. curr tells the PnP
BIOS subsystem to use the current static settings and no-curr tells
the subsystem to probe for dynamic settings if possible.
pnp_reserve_
irq
PnP BIOS reserved IRQs.
pnp_reserve_irq=irq1[,irq2...]
List of the IRQs that the PnP BIOS subsystem should not use for
autoconfiguration.
pnp_reserve_
dma
PnP BIOS reserved DMAs.
pnp_reserve_dma=dma1[,dma2...]
List of the DMAs that the PnP BIOS subsystem should not use for
autoconfiguration.
pnp_reserve_io PnP BIOS reserved I/O ports.
pnp_reserve_io=io1,size1[,io2,size2...]
I/O ports that the PnP BIOS subsystem should not use for autoconfiguration.
Each port is listed by its starting location and size.
pnp_reserve_
mem
PnP BIOS reserved memory regions.
pnp_reserve_mem=mem1,size1[,mem2,size2...]
Memory regions that the PnP BIOS subsystem should not use for
autoconfiguration. Each region is listed by its starting location and
size.
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SELinux Options
These options change some fundamental aspects of SELinux startup.
checkreqprot Set the initial checkreqprot flag value.
checkreqprot=[0|1]
Set the initial checkreqprot flag value. 0 means that the check
protection will be applied by the kernel and will include any
implied execute protection. 1 means that the check protection is
requested by the application. The default value is set by a kernel
configuration option.
The value can be changed at runtime via the /selinux/checkreqprot
file.
enforcing Set the initial enforcing status.
enforcing=[0|1]
Specify whether SELinux enforces its rules upon boot. 0 means that
SELinux will just log policy violations but will not deny access to
anything. 1 means that the enforcement will be fully enabled with
denials as well as logging. The default value is 0.
The value can be changed at runtime via the /selinux/enforce file.
selinux Enable or disable SELinux at boot time.
selinux=[0|1]
This option allows SELinux to be enabled (1) or disabled (0) to
boot time. The default value is set by a kernel configuration option.
If SELinux is enabled at boot time, the /selinux/disable file can be
used later to disable it prior to the initial policy load.
selinux_
compat_net
Set the network control model.
selinux_compat_net=[0|1]
Set the initial value for the SELinux network control model. 0 uses
the new secmark-based packet controls, and 1 uses the legacy
packet controls. 0 is the default and preferred value.
This value can be changed at runtime via the /selinux/compat_net
file.
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Network Options
These options control low-level aspects of the networking subsystem.
netdev Set various network device parameters.
netdev=[irq],[io],[mem_start],[mem_end],[name]
Specify network device parameters, which are specific to the driver
used by the network device. Some drivers’ source files document
the applicable options. This option does not usually apply to PCI,
USB, or other plug-and-play network devices. It is intended for use
only on devices that can not discover their own resource
assignments.
rhash_entries Set the number of route cache hash buckets.
dhash_entries=n
This option lets you override the default number of hash buckets
for the kernel’s route cache. Recommended only for kernel
network experts.
shapers Set the maximum number of network shapers.
shapers=n
This option lets you set the maximum number of network shapers
that the kernel can use.
thash_entries Set the number of TCP connection hash buckets.
thash_entries=n
This option lets you override the default number of hash buckets
for the kernel’s TCP connection cache.
Network File System Options
These options control NFS startup.
lockd.nlm_
grace_period
Assign a grace period to the lock manager.
lockd.nlm_grace_period=n
Set the NFS lock manager grace period. n is measured in seconds.
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lockd.nlm_
tcpport
Assign a TCP port to the lock manager.
lockd.nlm_tcpport=port
Set the TCPport that the NFS lock manager should use. port must
be a valid TCP port value.
lockd.nlm_
timeout
Assign a new timeout value to the lock manager.
lockd.nlm_timeout=n
Override the default time value for the NFS lock manager. n is
measured in seconds. If this option is not specified, the default of
10 seconds will be used.
lockd.nlm_
udpport
Assign a UDP port to the lock manager.
lockd.nlm_udpport=port
Set the UDPport that the NFS lock manager should use. port must
be a valid UDP port value.
nfsroot Specifies the NFS root filesystem.
nfsroot=[server-ip:]root-dir[,nfs-options]
Set the NFS root filesystem for diskless boxes, to enable them to
boot properly over NFS. If this parameter is not set, the value /tftpboot/
client_ip_address will be used as the root filesystem with the
default NFS options.
server-ip
IP address of the NFS server to connect to.
root-dir
Directory on the NFS server to mount as root. If there is a %s
token in this string, it will be replaced with the ASCII representation
of the client’s IP address.
nfs-options
The standard NFS options, such as ro, separated by commas.
nfs.callback_
tcpport
Set the NFSv4 TCP port for the callback channel.
nfs.callback_tcpport=port
Specify the TCPport that the NFSv4 callback channel should listen
on. port must be a valid TCP port value.
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nfs.idmap_
cache_timeout
Set the maximum lifetime for idmapper cache entries.
nfs.idmap_cache_timeout=n
Specify the maximum lifetime for idmapper cache entries. n is
measured in seconds.
Hardware-Specific Options
These options specify different parameters, depending on the hardware present in
the system.
nousb Disable the USB subsystem.
If this option is present, the USB subsystem will not be initialized.
lp Parallel port and its mode.
lp=[0|port[,port...]|reset|auto]
Specify the parallel port to use. The lp=port1,port2... format associates
a sequence of parallel ports to devices, starting with lp0. An
example is lp=none,parport0, which would suppress configuration
of the lp0 device and cause the lp1 device to use the first parallel
port.
lp=0
Disables the printer driver.
lp=reset
Causes the attached printers to be reset. This option can be
combined with the port specifications.
lp=auto
Causes the kernel to examine the device ID from each port to
determine whether a IEEE 1284-compatible printer is
attached. If so, the kernel will manage that printer.
parport Specify the parallel port parameters.
parport=[setting[,setting...]
Specify settings for parallel port drivers. Parallel ports are assigned
in the order they are specified on the command line, starting with
parport0.
auto forces the driver to use any IRQ/DMA settings detected (the
default is to ignore detected IRQ/DMA settings because of possible
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conflicts). You can also specify the base address, IRQ, and DMA
settings in the format 0xnnnn[,irq[,dma]]. irq and dma can be
numbers, auto to use detected settings on that particular port, or
nofifo to avoid using a FIFO even if it is detected.
parport_init_
mode
Parallel port initialization mode.
parport_init_mode=[spp|ps2|epp|ecp|ecpepp]
Specifies the mode for operating the parallel port. This is necessary
on the Pegasos computer where the firmware has no options for
setting up the parallel port mode. This option works for parallel
port chips of type 686a and 8231.
nr_uarts Maximum number of UARTs to be registered.
nr_uarts=n
Specifies the maximum number of different UARTs that can be
registered in the kernel.
Timer-Specific Options
These options override default kernel behavior to fix problems with certain chips.
enable_timer_
pin_1
Enable pin 1 of the APIC timer.
Enable pin 1 of the APIC timer. This option can be useful to work
around chipset bugs (on some ATI chipsets in particular). The
kernel tries to set a reasonable default, but sometimes this option is
necessary to override it.
disable_timer_
pin_1
Disable pin 1 of the APIC timer.
Disable pin 1 of the APIC timer. Useful for the same reasons as
enable_timer_pin_1.
enable_8254_
timer
Enable interrupt 0 timer routing over the 8254 chip.
Enable interrupt 0 timer routing over the 8254 chip in addition to
routing over the IO-APIC. The kernel tries to set a reasonable
default, but sometimes this option is necessary to override it.
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disable_8254_
timer
Disable interrupt 0 timer routing over the 8254 chip.
Disable interrupt 0 timer routing over the 8254 chip in addition to
routing over the IO-APIC. The kernel tries to set a reasonable
default, but sometimes this option is necessary to override it.
hpet Disable HPET and use PIT instead.
hpet=disable
Disable the HPET timer source and tell the kernel to use the PIT
timer source instead.
clocksource Set the specific clocksource.
clocksource=[hpet|pit|tsc|acpi_pm|cyclone|scx200_hrt]
Override the default kernel clocksource and use the clocksource
with the specified name instead.
Miscellaneous Options
These options should always be available and don’t depend on any specific
subsystem or hardware being present in the system in order to work properly.
dhash_entries Set the number of dentry hash buckets.
dhash_entries=n
This option lets you override the default number of hash buckets
for the kernel’s dentry cache. Recommended only for kernel
experts.
elevator Set the default I/O scheduler elevator.
elevator=[anticipatory|cfq|deadline|noop]
Specify the I/O scheduler. See Chapter 11 for a list of the different
I/O schedulers available, and what they do.
hashdist Distribute large hashes across NUMA nodes.
hashdist=[0|1]
Large hashes that are allocated during the boot process on the IA-
64 platform are, by default, distributed across the different NUMA
nodes. This option lets the user turn this option on or off.
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combined_
mode
Specify IDE driver usage.
combined_mode=[combined|ide|libata]
Control which driver uses the IDE ports in combined mode: the
legacy IDE driver, libata, or both. Note that using the ide or libata
options may affect your device naming (e.g., by changing hdc to
sdb).
max_loop Maximum number of loopback devices.
max_loop=n
Specify the maximum number of loopback filesystem devices that
can be mounted at the same time. n is an integer from 1 to 256.
panic Time to wait after panic before rebooting.
panic=n
Specify the amount of time in seconds that the kernel should wait
after a panic happens before it reboots. If this is set to 0 (the default
value), the kernel will not reboot after panicking; it will simply
halt.
pause_on_oops Delay between kernel oopses.
pause_on_oops=n
Tell the kernel to halt all CPUs after the first oops for n seconds
before continuing. This is useful if oopses keep scrolling off of the
screen before you can write them down or take a picture of them.
profile Control the kernel profiling.
profile=[schedule,][number]
This option affects how the kernel profiler is calculated. If schedule
is specified, the schedule points are affected by the value set in
number. If schedule is not specified, number is the step size as a
power of two for statistical time-based profiling in the kernel.
The most common use of this option is profile=2.
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