Advanced Raspberry Pi Configuration - Raspberry Pi User Guide - page 111

In a Linux-based desktop or laptop, these options are normally passed to the kernel by a tool
known as a bootloader , which has its own configuration file. On the Pi, the options are simply
entered directly into cmdline.txt to be read by the Pi at startup.
Almost any kernel option supported by Linux can be entered into the cmdline.txt file, to
alter things like the appearance of the console or which kernel is loaded. As an example, here
is the cmdline.txt file from the Raspbian distribution, which should be written in the file
as one continuous line:
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 ↵
kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 ↵
rootfstype=ext4 rootwait
The first option, dwg_otc.lpm_enable , tells the Pi to disable the On-he-Go (OTG) mode
of its USB controller, to prevent problems that can occur when the functionality is enabled
without proper support in the operating system. The majority of Linux distributions for the
Pi disable this mode.
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 bps (bits 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-filled screen you see
when you first 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 first console option.
he root option tells the Linux kernel where it can find its root file system , containing all the
files and directories required for the system to operate. In the case of the default Raspbian dis-
tribution, this is on the second partition of the SD card—device mmcblk0p2 . This 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 file system stored on the SD card.
In addition to telling the kernel where to find its root file system, it also needs to know what
format the partition was created in. Because Linux supports a variety of different file sys-
tems, the rootfstype option specifically tells the Raspbian distribution to use an EXT4 file
system.