Hardware Reference
In-Depth Information
kgdboc=
The
kgdboc
parameter was named after the idea “
kgdb
over console.” This allows you to
use a serial console as your primary console as well as use it for kernel debugging. The
primary console, however, need not be a serial console for
kgdboc
to be used.
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The Raspbian image supplies this:
kgdboc=ttyAMA0,115200
This allows kernel debugging to proceed through serial device
/dev/ttyAMA0
, which
is the only serial device supported on the Raspberry Pi.
root=
The Linux kernel needs to know what device holds the root file system. The standard
Raspbian image supplies the following:
root=/dev/mmcblk0p2
This points the kernel to the SD card (
mmcblk0
), partition 2 (non-NOOBS
distribution). See also the
rootfstype
parameter.
The general form of the
root=
parameter supports three forms:
root=MMmm
: Boot from major device
MM
, minor
mm
(hexadecimal).
•
root=/dev/nfs
: Boot a NFS disk specified by
nfsroot
(see also
nfs-root=
and
ip=
).
•
root=/dev/name
: Boot from a device named
/dev/name
.
•
rootfstype=
In addition to specifying the device holding the root file system, the Linux kernel
sometimes needs to know the file system type. This is configured through the
rootfstype
parameter. The standard Raspbian image supplies the following:
rootfstype=ext4
This example indicates that the root file system is the
ext4
type.
The Linux kernel can examine the device given in the root parameter to determine the
file system type. But there are scenarios where the kernel cannot resolve the type or gets
confused. Otherwise, you may want to force a certain file system type. Another situation is
when MTD is used for the root file system. For example, when using JFFS2, it must specified.
elevator=
This option selects the I/O scheduler scheme to be used within the kernel. The standard
Raspbian image specifies the following:
elevator=deadline