bootparam(7) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

BOOTPARAM(7)            Linux Programmer's Manual           BOOTPARAM(7)

NAME         top

       bootparam - introduction to boot time parameters of the Linux
       kernel

DESCRIPTION         top

       The Linux kernel accepts certain 'command-line options' or 'boot
       time parameters' at the moment it is started.  In general, this
       is used to supply the kernel with information about hardware
       parameters that the kernel would not be able to determine on its
       own, or to avoid/override the values that the kernel would
       otherwise detect.

       When the kernel is booted directly by the BIOS, you have no
       opportunity to specify any parameters.  So, in order to take
       advantage of this possibility you have to use a boot loader that
       is able to pass parameters, such as GRUB.

   The argument list
       The kernel command line is parsed into a list of strings (boot
       arguments) separated by spaces.  Most of the boot arguments have
       the form:

           name[=value_1][,value_2]...[,value_10]

       where 'name' is a unique keyword that is used to identify what
       part of the kernel the associated values (if any) are to be given
       to.  Note the limit of 10 is real, as the present code handles
       only 10 comma separated parameters per keyword.  (However, you
       can reuse the same keyword with up to an additional 10 parameters
       in unusually complicated situations, assuming the setup function
       supports it.)

       Most of the sorting is coded in the kernel source file
       init/main.c.  First, the kernel checks to see if the argument is
       any of the special arguments 'root=', 'nfsroot=', 'nfsaddrs=',
       'ro', 'rw', 'debug', or 'init'.  The meaning of these special
       arguments is described below.

       Then it walks a list of setup functions to see if the specified
       argument string (such as 'foo') has been associated with a setup
       function ('foo_setup()') for a particular device or part of the
       kernel.  If you passed the kernel the line foo=3,4,5,6 then the
       kernel would search the bootsetups array to see if 'foo' was
       registered.  If it was, then it would call the setup function
       associated with 'foo' (foo_setup()) and hand it the arguments 3,
       4, 5, and 6 as given on the kernel command line.

       Anything of the form 'foo=bar' that is not accepted as a setup
       function as described above is then interpreted as an environment
       variable to be set.  A (useless?) example would be to use
       'TERM=vt100' as a boot argument.

       Any remaining arguments that were not picked up by the kernel and
       were not interpreted as environment variables are then passed
       onto PID 1, which is usually the init(1) program.  The most
       common argument that is passed to the init process is the word
       'single' which instructs it to boot the computer in single user
       mode, and not launch all the usual daemons.  Check the manual
       page for the version of init(1) installed on your system to see
       what arguments it accepts.

   General non-device-specific boot arguments
       'init=...'
              This sets the initial command to be executed by the
              kernel.  If this is not set, or cannot be found, the
              kernel will try /sbin/init, then /etc/init, then
              /bin/init, then /bin/sh and panic if all of this fails.

       'nfsaddrs=...'
              This sets the NFS boot address to the given string.  This
              boot address is used in case of a net boot.

       'nfsroot=...'
              This sets the NFS root name to the given string.  If this
              string does not begin with '/' or ',' or a digit, then it
              is prefixed by '/tftpboot/'.  This root name is used in
              case of a net boot.

       'root=...'
              This argument tells the kernel what device is to be used
              as the root filesystem while booting.  The default of this
              setting is determined at compile time, and usually is the
              value of the root device of the system that the kernel was
              built on.  To override this value, and select the second
              floppy drive as the root device, one would use
              'root=/dev/fd1'.

              The root device can be specified symbolically or
              numerically.  A symbolic specification has the form
              /dev/XXYN, where XX designates the device type (e.g., 'hd'
              for ST-506 compatible hard disk, with Y in 'a'–'d'; 'sd'
              for SCSI compatible disk, with Y in 'a'–'e'), Y the driver
              letter or number, and N the number (in decimal) of the
              partition on this device.

              Note that this has nothing to do with the designation of
              these devices on your filesystem.  The '/dev/' part is
              purely conventional.

              The more awkward and less portable numeric specification
              of the above possible root devices in major/minor format
              is also accepted.  (For example, /dev/sda3 is major 8,
              minor 3, so you could use 'root=0x803' as an alternative.)

       'rootdelay='
              This parameter sets the delay (in seconds) to pause before
              attempting to mount the root filesystem.

       'rootflags=...'
              This parameter sets the mount option string for the root
              filesystem (see also fstab(5)).

       'rootfstype=...'
              The 'rootfstype' option tells the kernel to mount the root
              filesystem as if it where of the type specified.  This can
              be useful (for example) to mount an ext3 filesystem as
              ext2 and then remove the journal in the root filesystem,
              in fact reverting its format from ext3 to ext2 without the
              need to boot the box from alternate media.

       'ro' and 'rw'
              The 'ro' option tells the kernel to mount the root
              filesystem as 'read-only' so that filesystem consistency
              check programs (fsck) can do their work on a quiescent
              filesystem.  No processes can write to files on the
              filesystem in question until it is 'remounted' as
              read/write capable, for example, by 'mount -w -n -o
              remount /'.  (See also mount(8).)

              The 'rw' option tells the kernel to mount the root
              filesystem read/write.  This is the default.

       'resume=...'
              This tells the kernel the location of the suspend-to-disk
              data that you want the machine to resume from after
              hibernation.  Usually, it is the same as your swap
              partition or file.  Example:

                  resume=/dev/hda2

       'reserve=...'
              This is used to protect I/O port regions from probes.  The
              form of the command is:

                  reserve=iobase,extent[,iobase,extent]...

              In some machines it may be necessary to prevent device
              drivers from checking for devices (auto-probing) in a
              specific region.  This may be because of hardware that
              reacts badly to the probing, or hardware that would be
              mistakenly identified, or merely hardware you don't want
              the kernel to initialize.

              The reserve boot-time argument specifies an I/O port
              region that shouldn't be probed.  A device driver will not
              probe a reserved region, unless another boot argument
              explicitly specifies that it do so.

              For example, the boot line

                  reserve=0x300,32  blah=0x300

              keeps all device drivers except the driver for 'blah' from
              probing 0x300-0x31f.

       'panic=N'
              By default, the kernel will not reboot after a panic, but
              this option will cause a kernel reboot after N seconds (if
              N is greater than zero).  This panic timeout can also be
              set by

                  echo N > /proc/sys/kernel/panic

       'reboot=[warm|cold][,[bios|hard]]'
              Since Linux 2.0.22, a reboot is by default a cold reboot.
              One asks for the old default with 'reboot=warm'.  (A cold
              reboot may be required to reset certain hardware, but
              might destroy not yet written data in a disk cache.  A
              warm reboot may be faster.)  By default, a reboot is hard,
              by asking the keyboard controller to pulse the reset line
              low, but there is at least one type of motherboard where
              that doesn't work.  The option 'reboot=bios' will instead
              jump through the BIOS.

       'nosmp' and 'maxcpus=N'
              (Only when __SMP__ is defined.)  A command-line option of
              'nosmp' or 'maxcpus=0' will disable SMP activation
              entirely; an option 'maxcpus=N' limits the maximum number
              of CPUs activated in SMP mode to N.

   Boot arguments for use by kernel developers
       'debug'
              Kernel messages are handed off to a daemon (e.g., klogd(8)
              or similar) so that they may be logged to disk.  Messages
              with a priority above console_loglevel are also printed on
              the console.  (For a discussion of log levels, see
              syslog(2).)  By default, console_loglevel is set to log
              messages at levels higher than KERN_DEBUG.  This boot
              argument will cause the kernel to also print messages
              logged at level KERN_DEBUG.  The console loglevel can also
              be set on a booted system via the /proc/sys/kernel/printk
              file (described in syslog(2)), the syslog(2)
              SYSLOG_ACTION_CONSOLE_LEVEL operation, or dmesg(8).

       'profile=N'
              It is possible to enable a kernel profiling function, if
              one wishes to find out where the kernel is spending its
              CPU cycles.  Profiling is enabled by setting the variable
              prof_shift to a nonzero value.  This is done either by
              specifying CONFIG_PROFILE at compile time, or by giving
              the 'profile=' option.  Now the value that prof_shift gets
              will be N, when given, or CONFIG_PROFILE_SHIFT, when that
              is given, or 2, the default.  The significance of this
              variable is that it gives the granularity of the
              profiling: each clock tick, if the system was executing
              kernel code, a counter is incremented:

                  profile[address >> prof_shift]++;

              The raw profiling information can be read from
              /proc/profile.  Probably you'll want to use a tool such as
              readprofile.c to digest it.  Writing to /proc/profile will
              clear the counters.

   Boot arguments for ramdisk use
       (Only if the kernel was compiled with CONFIG_BLK_DEV_RAM.)  In
       general it is a bad idea to use a ramdisk under Linux—the system
       will use available memory more efficiently itself.  But while
       booting, it is often useful to load the floppy contents into a
       ramdisk.  One might also have a system in which first some
       modules (for filesystem or hardware) must be loaded before the
       main disk can be accessed.

              In Linux 1.3.48, ramdisk handling was changed drastically.
              Earlier, the memory was allocated statically, and there
              was a 'ramdisk=N' parameter to tell its size.  (This could
              also be set in the kernel image at compile time.)  These
              days ram disks use the buffer cache, and grow dynamically.
              For a lot of information on the current ramdisk setup, see
              the kernel source file Documentation/blockdev/ramdisk.txt
              (Documentation/ramdisk.txt in older kernels).

              There are four parameters, two boolean and two integral.

       'load_ramdisk=N'
              If N=1, do load a ramdisk.  If N=0, do not load a ramdisk.
              (This is the default.)

       'prompt_ramdisk=N'
              If N=1, do prompt for insertion of the floppy.  (This is
              the default.)  If N=0, do not prompt.  (Thus, this
              parameter is never needed.)

       'ramdisk_size=N' or (obsolete) 'ramdisk=N'
              Set the maximal size of the ramdisk(s) to N kB.  The
              default is 4096 (4 MB).

       'ramdisk_start=N'
              Sets the starting block number (the offset on the floppy
              where the ramdisk starts) to N.  This is needed in case
              the ramdisk follows a kernel image.

       'noinitrd'
              (Only if the kernel was compiled with CONFIG_BLK_DEV_RAM
              and CONFIG_BLK_DEV_INITRD.)  These days it is possible to
              compile the kernel to use initrd.  When this feature is
              enabled, the boot process will load the kernel and an
              initial ramdisk; then the kernel converts initrd into a
              "normal" ramdisk, which is mounted read-write as root
              device; then /linuxrc is executed; afterward the "real"
              root filesystem is mounted, and the initrd filesystem is
              moved over to /initrd; finally the usual boot sequence
              (e.g., invocation of /sbin/init) is performed.

              For a detailed description of the initrd feature, see the
              kernel source file Documentation/admin-guide/initrd.rst
              (or Documentation/initrd.txt before Linux 4.10).

              The 'noinitrd' option tells the kernel that although it
              was compiled for operation with initrd, it should not go
              through the above steps, but leave the initrd data under
              /dev/initrd.  (This device can be used only once: the data
              is freed as soon as the last process that used it has
              closed /dev/initrd.)

   Boot arguments for SCSI devices
       General notation for this section:

       iobase -- the first I/O port that the SCSI host occupies.  These
       are specified in hexadecimal notation, and usually lie in the
       range from 0x200 to 0x3ff.

       irq -- the hardware interrupt that the card is configured to use.
       Valid values will be dependent on the card in question, but will
       usually be 5, 7, 9, 10, 11, 12, and 15.  The other values are
       usually used for common peripherals like IDE hard disks,
       floppies, serial ports, and so on.

       scsi-id -- the ID that the host adapter uses to identify itself
       on the SCSI bus.  Only some host adapters allow you to change
       this value, as most have it permanently specified internally.
       The usual default value is 7, but the Seagate and Future Domain
       TMC-950 boards use 6.

       parity -- whether the SCSI host adapter expects the attached
       devices to supply a parity value with all information exchanges.
       Specifying a one indicates parity checking is enabled, and a zero
       disables parity checking.  Again, not all adapters will support
       selection of parity behavior as a boot argument.

       'max_scsi_luns=...'
              A SCSI device can have a number of 'subdevices' contained
              within itself.  The most common example is one of the new
              SCSI CD-ROMs that handle more than one disk at a time.
              Each CD is addressed as a 'Logical Unit Number' (LUN) of
              that particular device.  But most devices, such as hard
              disks, tape drives, and such are only one device, and will
              be assigned to LUN zero.

              Some poorly designed SCSI devices cannot handle being
              probed for LUNs not equal to zero.  Therefore, if the
              compile-time flag CONFIG_SCSI_MULTI_LUN is not set, newer
              kernels will by default probe only LUN zero.

              To specify the number of probed LUNs at boot, one enters
              'max_scsi_luns=n' as a boot arg, where n is a number
              between one and eight.  To avoid problems as described
              above, one would use n=1 to avoid upsetting such broken
              devices.

       SCSI tape configuration
              Some boot time configuration of the SCSI tape driver can
              be achieved by using the following:

                  st=buf_size[,write_threshold[,max_bufs]]

              The first two numbers are specified in units of kB.  The
              default buf_size is 32k B, and the maximum size that can
              be specified is a ridiculous 16384 kB.  The
              write_threshold is the value at which the buffer is
              committed to tape, with a default value of 30 kB.  The
              maximum number of buffers varies with the number of drives
              detected, and has a default of two.  An example usage
              would be:

                  st=32,30,2

              Full details can be found in the file
              Documentation/scsi/st.txt (or drivers/scsi/README.st for
              older kernels) in the Linux kernel source.

   Hard disks
       IDE Disk/CD-ROM Driver Parameters
              The IDE driver accepts a number of parameters, which range
              from disk geometry specifications, to support for broken
              controller chips.  Drive-specific options are specified by
              using 'hdX=' with X in 'a'–'h'.

              Non-drive-specific options are specified with the prefix
              'hd='.  Note that using a drive-specific prefix for a non-
              drive-specific option will still work, and the option will
              just be applied as expected.

              Also note that 'hd=' can be used to refer to the next
              unspecified drive in the (a, ..., h) sequence.  For the
              following discussions, the 'hd=' option will be cited for
              brevity.  See the file Documentation/ide/ide.txt (or
              Documentation/ide.txt in older kernels, or
              drivers/block/README.ide in ancient kernels) in the Linux
              kernel source for more details.

       The 'hd=cyls,heads,sects[,wpcom[,irq]]' options
              These options are used to specify the physical geometry of
              the disk.  Only the first three values are required.  The
              cylinder/head/sectors values will be those used by fdisk.
              The write precompensation value is ignored for IDE disks.
              The IRQ value specified will be the IRQ used for the
              interface that the drive resides on, and is not really a
              drive-specific parameter.

       The 'hd=serialize' option
              The dual IDE interface CMD-640 chip is broken as designed
              such that when drives on the secondary interface are used
              at the same time as drives on the primary interface, it
              will corrupt your data.  Using this option tells the
              driver to make sure that both interfaces are never used at
              the same time.

       The 'hd=noprobe' option
              Do not probe for this drive.  For example,

                  hdb=noprobe hdb=1166,7,17

              would disable the probe, but still specify the drive
              geometry so that it would be registered as a valid block
              device, and hence usable.

       The 'hd=nowerr' option
              Some drives apparently have the WRERR_STAT bit stuck on
              permanently.  This enables a work-around for these broken
              devices.

       The 'hd=cdrom' option
              This tells the IDE driver that there is an ATAPI
              compatible CD-ROM attached in place of a normal IDE hard
              disk.  In most cases the CD-ROM is identified
              automatically, but if it isn't then this may help.

       Standard ST-506 Disk Driver Options ('hd=')
              The standard disk driver can accept geometry arguments for
              the disks similar to the IDE driver.  Note however that it
              expects only three values (C/H/S); any more or any less
              and it will silently ignore you.  Also, it accepts only
              'hd=' as an argument, that is, 'hda=' and so on are not
              valid here.  The format is as follows:

                  hd=cyls,heads,sects

              If there are two disks installed, the above is repeated
              with the geometry parameters of the second disk.

   Ethernet devices
       Different drivers make use of different parameters, but they all
       at least share having an IRQ, an I/O port base value, and a name.
       In its most generic form, it looks something like this:

           ether=irq,iobase[,param_1[,...param_8]],name

       The first nonnumeric argument is taken as the name.  The param_n
       values (if applicable) usually have different meanings for each
       different card/driver.  Typical param_n values are used to
       specify things like shared memory address, interface selection,
       DMA channel and the like.

       The most common use of this parameter is to force probing for a
       second ethercard, as the default is to probe only for one.  This
       can be accomplished with a simple:

           ether=0,0,eth1

       Note that the values of zero for the IRQ and I/O base in the
       above example tell the driver(s) to autoprobe.

       The Ethernet-HowTo has extensive documentation on using multiple
       cards and on the card/driver-specific implementation of the
       param_n values where used.  Interested readers should refer to
       the section in that document on their particular card.

   The floppy disk driver
       There are many floppy driver options, and they are all listed in
       Documentation/blockdev/floppy.txt (or Documentation/floppy.txt in
       older kernels, or drivers/block/README.fd for ancient kernels) in
       the Linux kernel source.  See that file for the details.

   The sound driver
       The sound driver can also accept boot arguments to override the
       compiled-in values.  This is not recommended, as it is rather
       complex.  It is described in the Linux kernel source file
       Documentation/sound/oss/README.OSS (drivers/sound/Readme.linux in
       older kernel versions).  It accepts a boot argument of the form:

           sound=device1[,device2[,device3...[,device10]]]

       where each deviceN value is of the following format 0xTaaaId and
       the bytes are used as follows:

       T - device type: 1=FM, 2=SB, 3=PAS, 4=GUS, 5=MPU401, 6=SB16,
       7=SB16-MPU401

       aaa - I/O address in hex.

       I - interrupt line in hex (i.e., 10=a, 11=b, ...)

       d - DMA channel.

       As you can see, it gets pretty messy, and you are better off to
       compile in your own personal values as recommended.  Using a boot
       argument of 'sound=0' will disable the sound driver entirely.

   The line printer driver
       'lp='
              Syntax:

                  lp=0
                  lp=auto
                  lp=reset
                  lp=port[,port...]

              You can tell the printer driver what ports to use and what
              ports not to use.  The latter comes in handy if you don't
              want the printer driver to claim all available parallel
              ports, so that other drivers (e.g., PLIP, PPA) can use
              them instead.

              The format of the argument is multiple port names.  For
              example, lp=none,parport0 would use the first parallel
              port for lp1, and disable lp0.  To disable the printer
              driver entirely, one can use lp=0.

SEE ALSO         top

       klogd(8), mount(8)

       For up-to-date information, see the kernel source file
       Documentation/admin-guide/kernel-parameters.txt.

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                   BOOTPARAM(7)

Pages that refer to this page: reboot(2)boot(7)kernel-command-line(7)