Fastboot

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Using fastboot on sunxi devices

Fastboot is a diagnostic protocol, primarily used to modify the filesystems on the flash device via USB or UDP. It requires that the device is started in a boot loader or Seconday Boot Loader mode.

This document currently walks through an installation with a device with eMMC storage.

Prerequisites

  • fastboot binaries installed on client machine
  • sunxi-tools installed on client machine (optional)
  • u-boot tools (mkimage)
  • u-boot binaries for the target sunxi device
  • filesystem images
    • root file system containing operating system
    • vfat file system
  • Sunxi device connected to client via OTG port

Getting your device into Fastboot mode

To enter into fastboot mode, execute the `fastboot` command in U-Boot:

 fastboot usb 0

On the client machine, you can check whether the device is visible using the `fastboot devices` command. And for fun, you can also fetch the bootloader version using the fastboot protocol:

 fastboot devices 
 1234567890abcdef	fastboot
 fastboot getvar bootloader-version
 bootloader-version: U-Boot 2018.11-rc2-00033-ge3beca3a2f
 finished. total time: 0.000s

Preparing the device for flashing

Now that the device is in the fastboot mode, we can continue with creating the partitions on the device. By default, u-boot for sunxi defines following partitions:

  • loader1 - partition for secondary program loader
  • loader2 - partition for u-boot
  • esp - EFI system partition, also used for u-boot to look up boot.scr
  • system - Root partition for system

These partitions have also assigned GUID's according to Discoverable Partitions Specification, to enable automatic discovery of partitions and their mountpoints.

You can start by formatting the internal storage by executing the `fastboot oem format` command from client:

 fastboot oem format

This equivalent to running the `gpt write mmc 1 $partitions` from u-boot.

Flashing the device

Now that we have the partitions created, all that is left for us is to flash the data.

`loader1` is used for storing the Seconday Program Loader, in our case, it is the `spl/sunxi-spl.bin` in the u-boot directory:

 fastboot flash loader1 spl/sunxi-spl.bin

`loader2` is for storing the u-boot binary. `u-boot.img` in the u-boot source directory:

 fastboot flash loader2 u-boot.img

`esp` partition (EFI System Partition) can be kept empty, although if it is VFAT partition, u-boot automatically looks up the `boot.scr` file for device-specific configuration. (You can create empty vfat partition by `fallocate -l 32M esp.img && mkfs.vfat esp.img`)

 fastboot flash esp esp.img

`system` partition is where the operating system resides. Creating that is left as an exercise to the reader.

 fastboot flash system system.img

Now, if everything has been properly set up (aka proper kernel with machine-specific dtb installed on system.img, and boot.scr on esp partition), you can reboot the machine:

 fastboot reboot
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