[!NOTE]
ccache is default disabled because it creats issues in particular situation.
If you are aware its pros and cons and you insist enable it, following this guide: https://github.com/portapack-mayhem/mayhem-firmware/wiki/Notes-About-ccache
If you don't know what's this, just use default.

This guide not works on Asahi, please make sure you are on x86_64 platform

1. Install dependence

sudo pacman -S git tar wget dfu-util cmake make python3 bzip2 lz4 curl hackrf python-distutils-extra python-setuptools python-pip python-yaml

Check the output and make sure the packages listed above were installed correctly.

2. Install ARM gcc-arm-none-eabi package from AUR

you can follow the instructions in Debian based distro page in this wiki as well. This doesn’t work before and it turns out that it’s nushell caused the bug. If you don’t use nushell, you are all good with that set up.

This will automatically add the binaries of arm toolchain to the /usr/bin of your system. Note that you are adding an old toolchain into your system.

1. Go to the page of gcc-arm-none-eabi package in AUR.
2. Click View Changes to check the commit history of this AUR package.
3. Click version 9-2020-q2 to check the specific version of gcc-arm-none-eabi, then click Download to download the package for makepkg.
4. Assuming you download the package to ~.
5. Create a directory to satisfying and checking the package. mkdir AUR
6. mv aur-11b618acbed084c37cdf1568a1bc2b05152af7e1.tar.gz ./AUR
7. cd AUR
8. tar -xvf aur-11b618acbed084c37cdf1568a1bc2b05152af7e1.tar.gz
9. cd aur-11b618acbed084c37cdf1568a1bc2b05152af7e1
10. makepkg

(ARM already fixed this but in case of it happend again we'll leave it here.)
Note that since the SSL certificate of the file this makepkg pointed to already expired, the curl wouldn't download it correctly, thus, you have to add -k argument to your makepkg.conf:
sudo vim /etc/makepkg.conf
Edit the line
'https::/usr/bin/curl -qgb "" -fLC - --retry 3 --retry-delay 3 -o %o %u'
to
'https::/usr/bin/curl -qgb "" -fLC - --retry 3 --retry-delay 3 -k -o %o %u'
(You may change it back after installing, if you needed)
Then makepkg, waiting it finished.

11. Install the package with pacman:
    sudo pacman -U gcc-arm-none-eabi-bin-9_2020_q2_update-1-x86_64.pkg.tar.zst

3. Clone your repo to local and satisfying the sub-module

cd ~
git clone https://github.com/portapack-mayhem/mayhem-firmware.git
cd portapack-mayhem  
git submodule update --init --recursive

4. Give permission to the repo directory in your local and compile it

No need to do this step if all the things you do are on one user.

sudo chown -R my_user:my_usergroup ~/mayhem-firmware
cd ~/mayhem-firmware
mkdir build
cd build
cmake ..
make firmware

If you want, use -j argument to increase the compile speed, for example make -j firmware to auto decide the number of threads to compile, or manually set the thread numbers, for example make -j4 firmware

Notes

1. You cannot directly install gcc-arm-none-eabi from AUR using yay or others tool, otherwise the version would be not match.

2. (ARM already fixed this but in case of it happend again we'll leave it here.)
   After installing gcc-arm-none-eabi you may change the makepkg.conf back:

sudo vim /etc/makepkg.conf
Edit the line
'https::/usr/bin/curl -qgb "" -fLC - --retry 3 --retry-delay 3 -k -o %o %u'
to
'https::/usr/bin/curl -qgb "" -fLC - --retry 3 --retry-delay 3 -o %o %u'