Build an image for booting in USB Mass-Storage mode

This manual describes how to build bootstream to boot Linux from USB Mass Storage Gadget. In this case, the contents of the SD-card becomes available via USB as a normal USB flash drive.

The structure of the bootstream:

• U-Boot SPL - primary boot loader performs initial initialization
• U-Boot - the boot loader itself, runs Linux
• The Linux kernel image consists of several parts
  • zImage - kernel
  • initramfs - file system with modules to support USB gadget, is included in zImage
  • DTB - nformation about the periphery required for the kernel to work. Is appended to the end of the zImage, CONFIG_ARM_APPENDED_DTB option should be enabled in the config

Download bootstream

Building Linux

• To get the kernel sources from git,

$ git clone https://github.com/contactless/linux.git
$ cd linux
$ git checkout dev/v4.1.15 

• Download initramfs from repository and unzip

$ unzip initramfs_usbgadget.zip
$ sudo tar xvf initramfs_usbgadget.tar

• Initialize the environment to build the kernel. If necessary, change the DT and CROSS_COMPILE variables in this file

$ . ./scripts/kbuild-env.sh
$ set +e

• Set the config of the kernel

$ make mxs_usbgadget_defconfig

• Build the kernel and modules by installing them in unpacked initramfs

$ INSTALL_MOD_PATH=./initramfs_usbgadget/ ./scripts/kbuild.sh

• Build kernel with appended DTB and initramfs

$ ./scripts/kbuild-dtb.sh

Building U-Boot

• Get u-Boot sources from git. You need a wb_v2015.07 branch

$ cd ..
$ git clone https://github.com/contactless/u-boot.git
$ cd u-boot
$ git checkout wb_v2015.07

• Set u-Boot config

$ make mx28_wirenboard5_usbgadget_defconfig

• Copy a previously collected image of [[kernel+initramfs]+DTB] to the U-Boot folder

$ cp ../linux/zImage-imx28-evk ./zImage

• Build a combined image [U-Boot+[[kernel+initramfs]+DTB|kernel+initramfs]+DTB] into U-Boot folder

$ cp ../linux/zImage-imx28-evk ./zImage

• Build a combined [U-Boot+[[kernel+initramfs]+DTB]] image suitable for booting to USB (bootstream)

$ CROSS_COMPILE='arm-linux-gnueabi-' make -j4 u-boot.sb

Assembling mxsldr

This utility is needed to download the resulting image via USB. Requires libusb-1.0 library installed (apt-get install libusb-1.0-0-dev)

$ cd ..
$ git clone http://git.denx.de/mxsldr.git
$ cd mxsldr
$ make

Loading a built image using mxsldr

$ ./mxsldr ../u-boot/u-boot.sb