Enter fastboot mode

Fastboot is a protocol that can be used to re-flash partitions on your device (update the flash file system in Android devices). It is this small tool that comes with the Android SDK (Software Developer Kit), which is an alternative to the Recovery Mode for doing installations and updates.
There are three way to enter fastboot mode
1. use fastboot key: press fastboot key without release, short press reboot key, count 3 second, release fastboot key. you will enter fastboot mode.
2. When you see anything like "Hit any key to stop autoboot". Hit any key in 3 second, you will enter U-boot console. Use "fastboot 0" command in U-boot console to enter fastboot mode.
3. Use "reboot fastboot" command to enter fastboot in kernel console.
Now you are in the fastboot mode. use lsusb command you should see a USB device with VID 0x2207 and PID 0006 in your computer.
$ lsusb
Bus 002 Device 063: ID 2207:0006

You can also use fastboot command to list devices
$ sudo fastboot -i 0x2207 devices

Usage

Write GPT partition layout to mmc device which fastboot want to use it to
store the image

gpt write mmc 1 $partitions

Invoke fastboot command to prepare

fastboot 1

Start fastboot request on PC

fastboot -i 0x2207 flash loader evb-rk3288/spl/u-boot-spl-dtb.bin

You should see something like:

fastboot 1
WARNING: unknown variable: partition-type:loader
Starting download of 357796 bytes
..
downloading of 357796 bytes finished
Flashing Raw Image
........ wrote 357888 bytes to 'loader'