Here is a very quick howto that walks you through the few steps needed to compile the Rockbox firmware from source.

Get yourself ready

Just make sure that you have all the basic tools installed.

apt-get install build-essential libtool-bin flex bison texinfo

Get the source code

Rockbox is versioned on github:

git clone git://git.rockbox.org/rockbox

Build the toolchain

This is documented on the official wiki: http://www.rockbox.org/wiki/CrossCompiler.

cd rockbox
export RBDEV_PREFIX="/opt/rockbox"
sudo mkdir "$RBDEV_PREFIX"
# Set yourself as the owner of the build directory to avoid compiling as root
sudo chown $USER:$USER "$RBDEV_PREFIX"
cd tools
./rockboxdev.sh
# Set ownership back to root
sudo chown -R root:root "$RBDEV_PREFIX"

It may happen that the build fails with such error:

../../gcc-4.4.4/gcc/toplev.c: At top level:
../../gcc-4.4.4/gcc/toplev.c:536:1: error: redefinition of ‘floor_log2’
floor_log2 (unsigned HOST_WIDE_INT x)

In such case, you need to modify the CFLAGS used to build the toolchain, as suggested on this page: http://www.rockbox.org/irc/logs/rockbox-20150803.txt. At the moment it’s at the line 214 of the file rockboxdev.sh. Replace CFLAGS=-U_FORTIFY_SOURCE by CFLAGS="-U_FORTIFY_SOURCE -fgnu89-inline".

Build the firmware

This is documented on the official wiki: http://www.rockbox.org/wiki/HowToCompile

mkdir build && cd build
PATH=/opt/rockbox/bin:$PATH
../tools/configure
make -j

Then, you need to pack all the firmware files in an archive. For the small archive, use the target zip. For a bigger archive with additional stuff (like plenty of fonts), use the target fullzip.

make fullzip

Install the firmware on your device

I assume you already installed Rockbox before on your favorite audio player, and you’re just updating the firmware right now. It means that I won’t explain how to install the bootloader.

So just plug your device, mount it. Here we assume it’s mounted at the location /media/$USER/iPodClassic (yep I’ve got an iPod, now you know).

Installing the firmware is a simple as unpacking the archive at the root your device filesystem. The archive only contains a directory named .rockbox that holds the firmware files. If you really want to restart from scratch, you can delete this directory beforehand.

rm -fr /media/$USER/iPodClassic/.rockbox

Otherwise, just unpack the archive and overwrite everything.

unzip rockbox-full.zip -d /media/$USER/iPodClassic/
  Archive:  rockbox-full.zip
  replace /media/user/iPodClassic/.rockbox/rockbox-info.txt? [y]es, [n]o, [A]ll, [N]one, [r]ename: A
  inflating ...
  ...
sync

If unsure, backup before doing anything foolish !

Reboot your audio player

That’s it. Just reboot and unplug your audio player.

If you’re the owner of an iPod Classic/6g like me, and tend to forget how to restart it, here is the procedure: hold the “Menu” and “Select” buttons for about 5 seconds.