pacman -S base-devel git
git clone git://git.archlinux.org/svntogit/packages.git --single-branch --branch "packages/linux-firmware"
cd packages/trunk/
makepkg -rsi #check linux-firmware 20200519.8ba6fa6-1 you just built and installed has the issue with linux 5.7.5-arch1-1

cd packages/trunk/src/linux-firmware/
git checkout 20200421
cd ../..
makepkg -ersi #check linux-firmware 20200421.78c0348-1 you just built and installed has the issue with linux 5.7.5-arch1-1

cd packages/trunk/src/linux-firmware/
git bisect start
git bisect good 
git bisect bad 20200519
cd ../..
makepkg -ersi #check if linux-firmware 20200517.619d339-1 you just built and installed has the issue with linux 5.7.5-arch1-1

cd packages/trunk/src/linux-firmware/
git bisect $result #where result is bad if the issue is present and good otherwise
cd ../..
makepkg -ersi #check if linux-firmware 20200517.619d339-1 you just built and installed has the issue with linux 5.7.5-arch1-1

repeat the previous four steps until git reports the bisection is complete