Operation Manual

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 Debian 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-The-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.
The 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.
The kgdboc kernel option enables debugging of the Linux kernel over the serial console created using the console parameter.
For most users, this is unnecessary. For developers, having access to kernel debugging over a serial connection is most useful.
Many distributions leave this enabled just in case.
The 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.
The 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 Debian distribution, 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 systems, the rootfstype option specifically tells the Debian distribution uses
an EXT4 file system.
Finally, the rootwait parameter tells the kernel that it should not try to boot the system any further until the device containing the