Hardware Reference
In-Depth Information
he
console
option tells Linux that it should create a serial console—device
ttyAMA0
—
and at what speed it should operate. In most cases, the speed should be left at the default of
115,200 Kb/s (kilobytes per second). If the Pi is being used to communicate with older
devices, this can be reduced accordingly.
he
kgdboc
kernel option enables debugging of the Linux kernel over the serial console cre-
ated using the
console
parameter. For most users, this is unnecessary. For developers, hav-
ing access to kernel debugging over a serial connection is most useful. Many distributions
leave this enabled just in case.
he second
console
entry creates the device
tty1
, which is the text-illed screen you see
when you irst boot the Pi. Without this entry, you wouldn't be able to use the Pi without
connecting something to the serial console created by the irst
console
option.
he
root
option tells the Linux kernel where it can ind its
root ile system
, containing all the
iles and directories required for the system to operate. In the case of the default Debian
distribution, this is on the second partition of the SD card—device
mmcblk0p2
. his option
can be altered to address an external storage device connected over USB, which can speed up
the operation of the Pi considerably compared to having the root ile system stored on the
SD card.
In addition to telling the kernel where to ind its root ile system, it also needs to know what
format the partition was created in. Because Linux supports a variety of diferent ile sys-
tems, the
rootfstype
option speciically tells the Debian distribution uses an EXT4 ile
system.
Finally, the
rootwait
parameter tells the kernel that it should not try to boot the system
any further until the device containing the root ile system is available. Without this option,
the Pi can get stuck as it begins to boot before the relatively slow SD card is fully ready for
access.
With the exception of the
dwc_otg
setting, none of these kernel parameters are unique to
the Pi. he bootloader coniguration of any Linux distribution will include a list of options
very similar to those of
cmdline.txt
.
Typically, you should leave the
cmdline.txt
alone. It's created by the distribution main-
tainers speciically for that version of Linux, and may difer from one distribution to the
