# bootparam

## 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 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'**
           **'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'**
           **'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.
    


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