bootparam

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

NAME
       bootparam - introduction to boot time parameters of the Linux kernel

DESCRIPTION
       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 pos-
       sibility 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  argu-
       ments) 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 sepa-
       rated parameters per keyword.  (However, you can reuse the same keyword
       with  up to an additional 10 parameters in unusually complicated situa-
       tions, 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 argu-
       ment 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 argu-
       ment.

       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 desig-
              nates 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  conven-
              tional.

              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 revert-
              ing 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.  Exam-
              ple:

                  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 prob-
              ing, 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 prob-
              ing 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/ker-
              nel/printk  file  (described  in  syslog(2)), the syslog(2) SYS-
              LOG_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_PRO-
              FILE_SHIFT, when that is given, or 2, the default.  The signifi-
              cance  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 hard-
       ware) must be loaded before the main disk can be accessed.

              In Linux 1.3.48, ramdisk handling was changed drastically.  Ear-
              lier,  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  informa-
              tion  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  Documenta-
              tion/initrd.txt before Linux 4.10).

              The 'noinitrd' option tells the kernel that although it was com-
              piled 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  usu-
       ally 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 check-
       ing.  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 ker-
              nel 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  unspeci-
              fied  drive in the (a, ..., h) sequence.  For the following dis-
              cussions, 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  cylin-
              der/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  perma-
              nently.  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 argu-
              ment, 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 accom-
       plished 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 Docu-
       mentation/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  com-
       piled-in values.  This is not recommended, as it is rather complex.  It
       is   described   in   the   Linux   kernel   source   file   Documenta-
       tion/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
       klogd(8), mount(8)

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

COLOPHON
       This  page  is  part of release 4.15 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                             2017-09-15                      BOOTPARAM(7)
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