Ubuntu Manpage: bootparam - introduction to boot time parameters of the Linux kernel

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' 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.

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NAME

DESCRIPTION

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