The majority of this chapter is based on the in-kernel documentation for the 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 architecture-specific 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. modules, kernelboot command-line parameters boot command-line parametersmodule-specific boot command-line parameters 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. 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. quiet option loggingloglevel option logginglog_buf_len option klogd program initcall_debugg option error information, console options earlyprintk option debugging, kernelconsole options debug option consoleskernel boot command-line parameters console option boot command-line parametersconsole options blinkenlights parameter These options deal with the console or kernel log, where kernel debugging and error information are displayed. 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. Output console data across the network. netconsole=[src-port]@[src-ip]/[dev],[target-port]@target-ip/[target-mac-address] Send kernel console data across the network using UDP packets to another machine. Options are: 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. 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. 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.) 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. 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. 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. 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. 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. Show timing data on every kernel log message. Cause the kernel to prefix every kernel log message with a timestamp. time option oops dumps, printing words from kernel stack nolapic option noirqbalance option noapic option lapic option kstack option interrupt options boot command-line parametersinterrupt options apic option 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). Advanced Programmable Interrupt Controller (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. Do not use any IOAPICs. Prevent the kernel from using any of the IOAPICs that might be present in the system. Enable the local APIC. Cause the kernel to enable the local APIC even if the BIOS had disabled it. Do not use the local APIC. Tell the kernel not to use the local APIC. Disable kernel IRQ balancing. IRQ balancing Disable all of the built-in kernel IRQ balancing logic. 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. 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. 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. paging, hugepages option noirqdebug option memory options memmap option mem option max_addr option irqpoll option irqfixup option ihash_entries option hugepages option highmem option hash buckets for kernel inode cache debugging, kernelIRQ debugging boot command-line parametersmemory options The kernel handles memory in many different chunks and categories for different purposes. These options allow you to tweak the sizes and settings. memory optionshighmem 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. memory optionshugepages 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. memory optionsihash_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. memory optionsmax_addr Ignore memory. max_addr=n Cause the kernel to ignore all physical memory greater than or equal to the physical address n. memory optionsmem Force memory usage. mem=n[KMG] Set the specific ammount of memory used by the kernel. When used with the memmap= memory optionsmemmap 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). Disable the use of 4 MB pages for kernel memory. mem=nopentium Disable the use of huge (4 MB) pages for kernel memory. 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. 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). memory optionsnoexec 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). memory optionsreserve Reserve some I/O memory. reserve=n[KMG] Force the kernel to ignore some of the I/O memory areas. memory optionsvmalloc 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. memory optionsnorandmaps 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. memory optionsvdso 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. vmalloc option Virtual Dynamic Shared Object (VDSO) vdso option suspend options resume option reserve option randomization, address space of programs power managementsuspend options norandmaps option nonexecutable, mapping memory sections as noexec option boot command-line parameterssuspend options These options change the way the kernel handles suspension for power-saving purposes. suspend optionsresume 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 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. suspend optionsnoresume 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. nosmp option nosep option noresume option non-maskable interrupt (NMI) watchdog nomce option no-hlt option nofxsr option no387 option nmi_watchdog option multiprocessorsnosmp option mce (machine check exception) option math emulation library (387) machine check exception (mce) option lpj (loops per jiffy) option CPUoptionsmce (machine check exception) CPUoptionslpj (loops per jiffy) cachesize option boot command-line parametersCPU options These options CPUoptions control a wide range of behavior regarding timing, processor use in multiprocessor systems, and other processor issues. CPUoptionscachesize 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. 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 SMP systems, 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. CPUoptionsnmi_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. CPUoptionsno387 Always use the 387 emulation library. Always use the 387 math emulation library, 387 math emulation library even if a 387 math coprocessor is present in the system. CPUoptionsnofxsr Disable x86 floating-point save and restore. 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 floating-point registers on a task switch. CPUoptionsno-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. 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. CPUoptionsnomce Disable the machine check exception feature. This option turns the subsystem off. CPUoptionsnosep Disable x86 SYSENTER/SYSEXIT support. SYSENTER/SYSEXIT support Disable x86 SYSENTER/SYSEXIT support in the kernel. This can cause some system calls to take longer. CPUoptionsnosmp Run as a single-processor machine. Tell an SMP kernel to act as a uniprocessor kernel, even on a multiprocessor machine. CPUoptionsnotsc Disable the time stamp counter. time stamp counter Disable the timestamp counter hardware in the system, if present. CPUoptionsmax_cpus Maximum number of CPUs to use. maxcpus=n Specify the maximum number of processors that a SMP kernel should use, even if there are more processors present in the system. scheduler options notsc option migration_cost option max_cpus option isolcpus option CPUisolating from kernel scheduler boot command-line parametersscheduler options These options tweak the parameters used to make scheduling decisions. Most depend on an intimate understanding of how scheduling works in Linux. scheduler optionsisolcpus 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 SMP balancing 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. scheduler optionsmigration_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. scheduler optionsmigration_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. scheduler optionsmigration_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 migration_factor option migration_debug option memoryRamdisk options load_ramdisk option initrd option boot command-line parametersRamdisk 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. Ramdisk optionsinitrd Location of initial ramdisk. initrd=filename Specify where the initial ramdisk for the kernel boot is located. Ramdisk optionsload_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. Ramdisk optionsnoinitrd Do not use any initrd. Do not load any initial ramdisk, even if it is configured in other options passed to the kernel. Ramdisk optionsprompt_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 optionsramdisk_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 optionsramdisk_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 read-write root device read-only root device prompt_ramdisk option noinitrd option boot command-line parametersroot disk options These options control how the kernel finds and handles the filesystem that contains the root root disk optionsroot filesystem. root disk optionsro 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. 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: 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/<diskname> Use the kernel disk name specified by <diskname> as the root disk. /dev/<diskname><decimal> Use the kernel disk name specified by <diskname> and the partition specified by <decimal> as the root disk. /dev/<diskname>p<decimal> Use the kernel disk name specified by <diskname> and the partition specified by <decimal> as the root disk. This is the same as above, but is needed when <diskname> ends with a digit. root disk optionsrootdelay 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. root disk optionsrootflags 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. root disk optionsrootfstype filesystemsrootfstype The root filesystem type. rootfstype=type Try to mount the root filesystem as this type of filesystem. For instance, rootfstype=ext3. root disk optionsrw 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. single-user mode (S) S (single-user) mode kexec options init optionsS (single-user) init options crashkernel option boot command-line parameterskexec options boot command-line parametersinit options The init init optionsinit 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. Program to run at init time. init=filename Run the specified binary as the init process instead of the default /sbin/init program. Ramdisk optionsrdinit init optionsrdinit 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. 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. 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. kexec optionscrashkernel 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). kexec optionselfcorehdr 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 (Read Copy Update) options elfcorehdr option boot command-line parametersRCU (Read Copy Update) 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 (Read Copy Update) optionsrcu.blimit RCU batch limit. rcu.blimit=n Set the maximum number of finished RCU callbacks to process in one batch. RCU (Read Copy Update) optionsrcu.qhimark RCU queue high level. rcu.qhimark=n Batch limiting is disabled when the number of queued RCU callbacks rises above n. RCU (Read Copy Update) optionsrcu.qlowmark RCU queue low level. rcu.qlowmark=n Batch limiting is re-enabled when the number of queued RCU callbacks falls below n. RCU (Read Copy Update) optionsrcu.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. _OSI method, disabling operating system name, faking to ACPI memmap option IRQ balancingACPI options Embedded Controller Description Table (ECDT) ECDT (Embedded Controller Description Table) ACPI (Advanced Configuration and Power Interface)optionsacip_os_name ACPI (Advanced Configuration and Power Interface)optionsaccept_irq_balance These options ACPI (Advanced Configuration and Power Interface)options control parameters that the Advanced Configuration and Power Interface (ACPI) subsystem can use. ACPI (Advanced Configuration and Power Interface)optionsacpi 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 (Advanced Configuration and Power Interface)optionsacpi_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 (Advanced Configuration and Power Interface)optionsacpi_sci ACPI System Control Interrupt trigger mode. acpi_sci=[level|edge|high|low] Set the ACPI System Control Interrupt trigger mode. Enable ACPI IRQ balance. Cause ACPI to balance the active IRQs. This is the default option when operating in APIC mode. ACPI (Advanced Configuration and Power Interface)optionsacpi_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 (Advanced Configuration and Power Interface)optionsacpi_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 (Advanced Configuration and Power Interface)optionsacpi_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. 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 (Advanced Configuration and Power Interface)optionsacpi_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 (Advanced Configuration and Power Interface)optionsacpi_serialize Force serialization of AML methods. Force the serialization of ACPI Machine Language methods. ACPI (Advanced Configuration and Power Interface)optionsacpi_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 XT-PIC timer acting up. ACPI (Advanced Configuration and Power Interface)optionsacpi_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 (Advanced Configuration and Power Interface)optionsacpi_fake_ecdt ECDT workaround. If present, this allows ACPI to workaround BIOS failures when it lacks an Embedded Controller Description Table. ACPI (Advanced Configuration and Power Interface)optionsacpi_generic_hotkey Use generic ACPI hotkey driver. hotkey driver This allows the ACPI consolidated generic hotkey driver to override the platform-specific driver if one is present. ACPI (Advanced Configuration and Power Interface)optionsacpi_pm_good Override pmtimer bug detection. pmtimer Force the kernel to assume that the machine's pmtimer latches its value and always returns good values. ACPI (Advanced Configuration and Power Interface)optionsec_intr ACPI Embedded Controller interrupt mode. ec_intr=n Specify the ACPI embedded controller interrupt mode. embedded controller interrupt mode If n is 0, polling mode will be used, otherwise interrupt mode will be used. Interrupt mode is the default. ACPI (Advanced Configuration and Power Interface)optionsmemmap 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). 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. ACPI (Advanced Configuration and Power Interface)optionspnpacpi Turn Plug and Play ACPI off. pnpacpi=off Disable the Plug and Play ACPI functionality. ACPI (Advanced Configuration and Power Interface)optionsprocessor.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. ACPI (Advanced Configuration and Power Interface)optionsprocessor.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 optionskernel boot processor.max_cstate option pnpacpi option max_report_luns option max_luns option LUNs (Logical Unit Numbers) Logical Unit Numbers (LUNs) C-state, processor boot command-line parametersSCSI 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. SCSI optionskernel bootmax_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. SCSI optionsmax_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. SCSI optionsscsi_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 devicesboot command-line options boot command-line parametersPCI options These options specify different parameters the PCI subsystem can use: 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. 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 HP Pavilion 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. PnP (Plug and Play)BIOS options noisapnp option memorypnp_reserve_mem option I/Oports not used by PnP BIOS DMAs not used by PnP BIOS boot command-line parametersPnP BIOS options PnP (Plug and Play)BIOS optionsnoisapnp Disable the ISA Plug and Play (PnP) subsystem. Disable the ISA PnP subsystem, if it has been enabled in the kernel configuration. PnP (Plug and Play)BIOS optionspnpbios 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 (Plug and Play)BIOS optionspnp_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 (Plug and Play)BIOS optionspnp_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 (Plug and Play)BIOS optionspnp_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 (Plug and Play)BIOS optionspnp_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. boot command-line parametersSELinux SELinux (Security-Enhanced Linux)boot command-line options enforcing option checkreqprot option These options change some fundamental aspects of SELinux startup. SELinux (Security-Enhanced Linux)boot command-line optionscheckreqprot 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 SELinux (Security-Enhanced Linux)boot command-line optionsselinux file. SELinux (Security-Enhanced Linux)boot command-line optionsenforcing 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. 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 (Security-Enhanced Linux)boot command-line optionsselinux_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. thash_entries option shapers option lockd.nlm_grace_period option filesystemsNFS options dhash_entries option boot command-line parametersNFS options boot command-line parametersnetwork options These options networkingoptions control low-level aspects of the networking subsystem. networkingoptionsnetdev 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. Set the number of route cache hash buckets. dhash_entries=n networkingoptionsdhash_entries This option lets you override the default number of hash buckets for the kernel's route cache. Recommended only for kernel network experts. networkingoptionsshapers 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. networkingoptionsthash_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. lockd.nlm_udpport option lockd.nlm_timeout option lockd.nlm_tcpport option These options control NFS startup. NFS optionslockd.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. NFS optionslockd.nlm_tcpport Assign a TCP port to the lock manager. lockd.nlm_tcpport=port Set the TCP port that the NFS lock manager should use. port must be a valid TCP port value. NFS optionslockd.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. NFS optionslockd.nlm_udpport Assign a UDP port to the lock manager. lockd.nlm_udpport=port Set the UDP port that the NFS lock manager should use. port must be a valid UDP port value. 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 optionsnfs.callback_tcpport Set the NFSv4 TCP port for the callback channel. nfs.callback_tcpport=port Specify the TCP port that the NFSv4 callback channel should listen on. port must be a valid TCP port value. NFS optionsnfs.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. USB (Universal Serial Bus)nousb option parport option parallel port options nousb option lp option hardware options boot command-line parametershardware-specific options These options specify different parameters, depending on the hardware present in the system. hardware optionsnousb Disable the USB subsystem. If this option is present, the USB subsystem will not be initialized. hardware optionslp 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. hardware optionsparport 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 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. hardware optionsparport_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. hardware optionsnr_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 options parport_init_mode option nr_uarts option enable_timer_pin_1 option enable_8254_timer option disable_timer_pin_1 option boot command-line parameterstimer options These options override default kernel behavior to fix problems with certain chips. timer optionsenable_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. timer optionsdisable_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. timer optionsenable_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. timer optionsdisable_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. timer optionshpet Disable HPET and use PIT instead. hpet=disable Disable the HPET timer source and tell the kernel to use the PIT timer source instead. timer optionsclocksource 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. profile option pause_on_oops option panic option NUMA nodes, distributing large hashes across max_loop option hpet option hashdist option elevator option disable_8254_timer option dhash_entries option clocksource option boot command-line parametersmiscellaneous 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. 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. 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. 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. 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). 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. 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. 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. 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.