U-Boot

U-Boot Port

We've been working on an experimental U-Boot port for the iPod nano (2nd generation), iPod nano (3rd generation), iPod nano (5th generation) and iPod nano (7th generation). It can be started using wInd3x and will start up a CDC-ACM serial console over USB for debugging purposes. Currently it has no storage driver.

The current port expects to be loaded in place of retailOS, eg. by packaging it into an IMG1 and sending it to WTF. This is what wInd3x does. This way, U-Boot does not have to do any of the 'annoying' early boot stuff like bringing up DRAM.

Getting the source

$ git clone git@github.com:freemyipod/u-boot.git
$ cd u-boot

Configuring

$ make apple_n36_defconfig # iPod nano (2nd generation)
$ make apple_n46_defconfig # iPod nano (3rd generation)
$ make apple_n33_defconfig # iPod nano (5th generation)
$ make apple_n31_defconfig # iPod nano (7th generation)

Building

You'll need GCC for ARM (for bare metal targets, not for Linux targets), OpenSSL and GnuTLS.

On Linux distros that use apt, these can be installed using the following command.

# apt install gcc-arm-none-eabi libssl-dev gnutls-dev

Then, build the U-boot binary.

$ make CROSS_COMPILE=arm-none-eabi- u-boot.bin

On macOS, you need to install openssl using homebrew. If the build system does not find it, append the following to the build command:

HOSTCFLAGS="-I/opt/homebrew/Cellar/openssl@3/3.6.1/include" HOSTLDFLAGS="-L/opt/homebrew/Cellar/openssl@3/3.6.1/lib"

substituting your OpenSSL version/path as needed.

Running

If you have a 30-pin breakout cable with the UART pins connected to your computer (for iPod nano (6th generation) and older), or a Alex/DCSD cable (for iPod nano (7th generation)), you can see and interact with U-boot's console. Launch a serial client like picocom before running U-boot. Here is an example command line.

$ picocom --quiet --baud 115200 --imap lfcrlf /dev/ttyUSB0 # or whatever the correct port is

Make sure your user is in the dialout group or similar, so you can access serial ports.

If you have a regular cable, skip this command. USB DFU interface will still be accessible.

iPod nano (2nd generation)

Install Rockbox. Create an u-boot.ipod file by using the following tool from the Rockbox source code:

$ ./rockbox/tools/scramble -add=nn2g u-boot.bin u-boot.ipod

Upload the file to your iPod. Run Rockbox, go to File Manager, find the u-boot.ipod file and select it.

iPod nano (3rd generation), iPod nano (5th generation) and iPod nano (7th generation)

Connect your iPod in DFU Mode and use wInd3x to start U-Boot:

$ ./wInd3x cfw run u-boot.bin

Make sure your user can access the raw USB device, add udev rules (recommended) or run with root (not recommended).

The command may fail the first time, retry one or two times before asking for help.

Pushing an image to U-boot

When successfully started, U-Boot will then enumerate as yet another DFU device, this time ready to receive an U-Boot compatible image. You can load an image using dfu-util:

$ dfu-util --device 05ac:8007 --download foo
$ dfu-util --device 05ac:8007 --detach

Note the --detach command: this is needed to get U-Boot to exit DFU mode and actually execute the image.

Make sure your user can access the raw USB device, add udev rules (recommended) or run with root (not recommended).

Work in progress

You can run the iPod nano (5th generation) image on a iPod nano (4th generation) and it seems to be working at first glance.

You can run the iPod nano (5th generation) image on a iPod nano (6th generation), but if you don't stop the countdown before auto-booting, it gets stuck since the timer which is used to measure how much a second takes is not initialized properly. You can flood the UART console with key presses just while U-boot is printing the messages to skip the auto-boot prompt entiery and get dropped into a U-boot console.

Your mileage may vary. Patches welcome.