bootparam - introduction to boot time parameters of the Linux kernel - man(7) - [phpMan]

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bootparam(7) Miscellaneous Information 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 mo‐
ment 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 para‐
meters. 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 associ‐
ated 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 en‐
vironment 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 ac‐
cepts.

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 specifica‐
tion 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 par‐
tition 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 for‐
mat 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 filesys‐
tem. No processes can write to files on the filesystem in question until it is 're‐
mounted' 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 ma‐
chine to resume from after hibernation. Usually, it is the same as your swap parti‐
tion 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 de‐
vices (auto-probing) in a specific region. This may be because of hardware that re‐
acts 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 explic‐
itly 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 de‐
fault 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 (Documenta‐
tion/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., in‐
vocation of /sbin/init) is performed.

For a detailed description of the initrd feature, see the kernel source file Documen‐
tation/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 de‐
pendent 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 inter‐
nally. 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 be‐
havior 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 de‐
vice. But most devices, such as hard disks, tape drives, and such are only one de‐
vice, 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 fol‐
lowing:

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 de‐
tected, 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 dri‐
vers/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 specifi‐
cations, 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 spec‐
ified will be the IRQ used for the interface that the drive resides on, and is not re‐
ally 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 in‐
terfaces 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 reg‐
istered 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) usu‐
ally 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 de‐
fault 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/dri‐
ver-specific implementation of the param_n values where used. Interested readers should re‐
fer 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/block‐
dev/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 fol‐
lows:

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 en‐
tirely.

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 lat‐
ter comes in handy if you don't want the printer driver to claim all available paral‐
lel 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.

man > bootparam(7)