kernel-options.txt

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				  Command Line Options for Linux/m68k
				  ===================================

Last Update: 2 May 1999
Linux/m68k version: 2.2.6
Author: [e-mail] (Roman Hodek)
Update: [e-mail] (Jes Sorensen) and [e-mail] (Chris Lawrence)

0) Introduction
===============

  Often I've been asked which command line options the Linux/m68k
kernel understands, or how the exact syntax for the ... option is, or
... about the option ... . I hope, this document supplies all the
answers...

  Note that some options might be outdated, their descriptions being
incomplete or missing. Please update the information and send in the
patches.


1) Overview of the Kernel's Option Processing
=============================================

The kernel knows three kinds of options on its command line:

  1) kernel options
  2) environment settings
  3) arguments for init

To which of these classes an argument belongs is determined as
follows: If the option is known to the kernel itself, i.e. if the name
(the part before the '=') or, in some cases, the whole argument string
is known to the kernel, it belongs to class 1. Otherwise, if the
argument contains an '=', it is of class 2, and the definition is put
into init's environment. All other arguments are passed to init as
command line options.

  This document describes the valid kernel options for Linux/m68k in
the version mentioned at the start of this file. Later revisions may
add new such options, and some may be missing in older versions.

  In general, the value (the part after the '=') of an option is a
list of values separated by commas. The interpretation of these values
is up to the driver that "owns" the option. This association of
options with drivers is also the reason that some are further
subdivided.


2) General Kernel Options
=========================

2.1) root=
----------

Syntax: root=/dev/<device>
    or: root=<hex_number>

This tells the kernel which device it should mount as the root
filesystem. The device must be a block device with a valid filesystem
on it.

  The first syntax gives the device by name. These names are converted
into a major/minor number internally in the kernel in an unusual way.
Normally, this "conversion" is done by the device files in /dev, but
this isn't possible here, because the root filesystem (with /dev)
isn't mounted yet... So the kernel parses the name itself, with some
hardcoded name to number mappings. The name must always be a
combination of two or three letters, followed by a decimal number.
Valid names are:

  /dev/ram: -> 0x0100 (initial ramdisk)
  /dev/hda: -> 0x0300 (first IDE disk)
  /dev/hdb: -> 0x0340 (second IDE disk)
  /dev/sda: -> 0x0800 (first SCSI disk)
  /dev/sdb: -> 0x0810 (second SCSI disk)
  /dev/sdc: -> 0x0820 (third SCSI disk)
  /dev/sdd: -> 0x0830 (forth SCSI disk)
  /dev/sde: -> 0x0840 (fifth SCSI disk)
  /dev/fd : -> 0x0200 (floppy disk)
  /dev/xda: -> 0x0c00 (first XT disk, unused in Linux/m68k)
  /dev/xdb: -> 0x0c40 (second XT disk, unused in Linux/m68k)
  /dev/ada: -> 0x1c00 (first ACSI device)
  /dev/adb: -> 0x1c10 (second ACSI device)
  /dev/adc: -> 0x1c20 (third ACSI device)
  /dev/add: -> 0x1c30 (forth ACSI device)

The last four names are available only if the kernel has been compiled
with Atari and ACSI support.

  The name must be followed by a decimal number, that stands for the
partition number. Internally, the value of the number is just
added to the device number mentioned in the table above. The
exceptions are /dev/ram and /dev/fd, where /dev/ram refers to an
initial ramdisk loaded by your bootstrap program (please consult the
instructions for your bootstrap program to find out how to load an
initial ramdisk). As of kernel version 2.0.18 you must specify
/dev/ram as the root device if you want to boot from an initial
ramdisk. For the floppy devices, /dev/fd, the number stands for the
floppy drive number (there are no partitions on floppy disks). I.e.,
/dev/fd0 stands for the first drive, /dev/fd1 for the second, and so
on. Since the number is just added, you can also force the disk format
by adding a number greater than 3. If you look into your /dev
directory, use can see the /dev/fd0D720 has major 2 and minor 16. You
can specify this device for the root FS by writing "root=/dev/fd16" on
the kernel command line.

[Strange and maybe uninteresting stuff ON]

  This unusual translation of device names has some strange
consequences: If, for example, you have a symbolic link from /dev/fd
to /dev/fd0D720 as an abbreviation for floppy driver #0 in DD format,
you cannot use this name for specifying the root device, because the
kernel cannot see this symlink before mounting the root FS and it
isn't in the table above. If you use it, the root device will not be 
set at all, without an error message. Another example: You cannot use a
partition on e.g. the sixth SCSI disk as the root filesystem, if you
want to specify it by name. This is, because only the devices up to
/dev/sde are in the table above, but not /dev/sdf. Although, you can
use the sixth SCSI disk for the root FS, but you have to specify the
device by number... (see below). Or, even more strange, you can use the
fact that there is no range checking of the partition number, and your
knowledge that each disk uses 16 minors, and write "root=/dev/sde17"
(for /dev/sdf1).

[Strange and maybe uninteresting stuff OFF]

  If the device containing your root partition isn't in the table
above, you can also specify it by major and minor numbers. These are
written in hex, with no prefix and no separator between. E.g., if you
have a CD with contents appropriate as a root filesystem in the first
SCSI CD-ROM drive, you boot from it by "root=0b00". Here, hex "0b" =
decimal 11 is the major of SCSI CD-ROMs, and the minor 0 stands for
the first of these. You can find out all valid major numbers by
looking into include/linux/major.h.


2.2) ro, rw
-----------

Syntax: ro
    or: rw

These two options tell the kernel whether it should mount the root
filesystem read-only or read-write. The default is read-only, except
for ramdisks, which default to read-write.


2.3) debug
----------

Syntax: debug

This raises the kernel log level to 10 (the default is 7). This is the
same level as set by the "dmesg" command, just that the maximum level
selectable by dmesg is 8.


2.4) debug=
-----------

Syntax: debug=<device>

This option causes certain kernel messages be printed to the selected
debugging device. This can aid debugging the kernel, since the
messages can be captured and analyzed on some other machine. Which
devices are possible depends on the machine type. There are no checks
for the validity of the device name. If the device isn't implemented,
nothing happens.

  Messages logged this way are in general stack dumps after kernel
memory faults or bad kernel traps, and kernel panics. To be exact: all
messages of level 0 (panic messages) and all messages printed while
the log level is 8 or more (their level doesn't matter). Before stack
dumps, the kernel sets the log level to 10 automatically. A level of
at least 8 can also be set by the "debug" command line option (see
2.3) and at run time with "dmesg -n 8".

Devices possible for Amiga:

 - "ser": built-in serial port; parameters: 9600bps, 8N1
 - "mem": Save the messages to a reserved area in chip mem. After
          rebooting, they can be read under AmigaOS with the tool
          'dmesg'.

Devices possible for Atari:

 - "ser1": ST-MFP serial port ("Modem1"); parameters: 9600bps, 8N1
 - "ser2": SCC channel B serial port ("Modem2"); parameters: 9600bps, 8N1
 - "ser" : default serial port
           This is "ser2" for a Falcon, and "ser1" for any other machine
 - "midi": The MIDI port; parameters: 31250bps, 8N1
 - "par" : parallel port
           The printing routine for this implements a timeout for the
           case there's no printer connected (else the kernel would
           lock up). The timeout is not exact, but usually a few
           seconds.


2.6) ramdisk=
-------------

Syntax: ramdisk=<size>

  This option instructs the kernel to set up a ramdisk of the given
size in KBytes. Do not use this option if the ramdisk contents are
passed by bootstrap! In this case, the size is selected automatically
and should not be overwritten.

  The only application is for root filesystems on floppy disks, that
should be loaded into memory. To do that, select the corresponding
size of the disk as ramdisk size, and set the root device to the disk
drive (with "root=").


2.7) swap=
2.8) buff=
-----------

  I can't find any sign of these options in 2.2.6.


3) General Device Options (Amiga and Atari)
===========================================

3.1) ether=
-----------

Syntax: ether=[<irq>[,<base_addr>[,<mem_start>[,<mem_end>]]]],<dev-name>

  <dev-name> is the name of a net driver, as specified in
drivers/net/Space.c in the Linux source. Most prominent are eth0, ...
eth3, sl0, ... sl3, ppp0, ..., ppp3, dummy, and lo.

  The non-ethernet drivers (sl, ppp, dummy, lo) obviously ignore the
settings by this options. Also, the existing ethernet drivers for
Linux/m68k (ariadne, a2065, hydra) don't use them because Zorro boards
are really Plug-'n-Play, so the "ether=" option is useless altogether
for Linux/m68k.


3.2) hd=
--------

Syntax: hd=<cylinders>,<heads>,<sectors>

  This option sets the disk geometry of an IDE disk. The first hd=
option is for the first IDE disk, the second for the second one.
(I.e., you can give this option twice.) In most cases, you won't have
to use this option, since the kernel can obtain the geometry data
itself. It exists just for the case that this fails for one of your
disks.


3.3) max_scsi_luns=
-------------------

Syntax: max_scsi_luns=<n>

  Sets the maximum number of LUNs (logical units) of SCSI devices to
be scanned. Valid values for <n> are between 1 and 8. Default is 8 if
"Probe all LUNs on each SCSI device" was selected during the kernel
configuration, else 1.


3.4) st=
--------

Syntax: st=<buffer_size>,[<write_thres>,[<max_buffers>]]

  Sets several parameters of the SCSI tape driver. <buffer_size> is
the number of 512-byte buffers reserved for tape operations for each
device. <write_thres> sets the number of blocks which must be filled
to start an actual write operation to the tape. Maximum value is the
total number of buffers. <max_buffer> limits the total number of
buffers allocated for all tape devices.


3.5) dmasound=
--------------

Syntax: dmasound=[<buffers>,<buffer-size>[,<catch-radius>]]

  This option controls some configurations of the Linux/m68k DMA sound
driver (Amiga and Atari): <buffers> is the number of buffers you want
to use (minimum 4, default 4), <buffer-size> is the size of each
buffer in kilobytes (minimum 4, default 32) and <catch-radius> says
how much percent of error will be tolerated when setting a frequency
(maximum 10, default 0). For example with 3% you can play 8000Hz
AU-Files on the Falcon with its hardware frequency of 8195Hz and thus
don't need to expand the sound.



4) Options for Atari Only
=========================

4.1) video=
-----------

Syntax: video=<fbname>:<sub-options...>

The <fbname> parameter specifies the name of the frame buffer,
eg. most atari users will want to specify `atafb' here. The
<sub-options> is a comma-separated list of the sub-options listed
below.

NB: Please notice that this option was renamed from `atavideo' to
    `video' during the development of the 1.3.x kernels, thus you
    might need to update your boot-scripts if upgrading to 2.x from
    an 1.2.x kernel.

NBB: The behavior of video= was changed in 2.1.57 so the recommended
option is to specify the name of the frame buffer.

4.1.1) Video Mode
-----------------

This sub-option may be any of the predefined video modes, as listed
in atari/atafb.c in the Linux/m68k source tree. The kernel will