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Gentoo Linux on Macbook Air

This page covers various features, nuances, notes, etc. of my Gentoo system running on a 2012 Macbook Air. This laptop was initially bought as a tinkering project for 150USD. While this is not the most powerful device, it is suitable for just about anything I encounter on a daily basis and is used as my primary system. My intentions for this system are reliability and speed most importantly. Follows is a tidy OS with minimal kernel parameters set, fine-tuned USE flags, and a system I'm very familiar with. Much of my configuration can be found on my Github.

laptop image laptop open image

hardware features
kernel
bootloader
portage
applications
miscellaneous
screenshots

Hardware Features

This is a Macbook Air 5,2 (EMC 2559) fit with the following (see lspci, lsusb)

Modifications

Since 2010, Apple Macbooks have used their proprietary SSDs with non-standard connectors. When I bought this machine, it came with a 128 GB Samgsung "blade" SSD which I was not very keen on keeping. Luckily, there exists adapters which simply act as a bridge between a standard M.2 SSD and Apple's connector. Installation was quite easy and the adapter has been reliable since. This laptop also came with what I presume was the original battery, as it lasted about 15 minutes after being charged and would lose power randomly. A suitable replacement can be found roughly 50USD. With this, I get about 5 hours on a full charge, mostly thanks to TLP and thermald. Now, if only the memory wasn't soldered directly to the motherboard, or I could find the courage to replace it.

Kernel

After spending quite some time narrowing down kernel enabled, I have a 7.8MB kernel image with ~4000 parameters set. Note, this configuration is very hardware and user specific. I opted for zero modules since I don't need any out-of-tree hardware support. I also did not configure anything very drastically in either direction, like extreme security, sacraficing usability, etc. I chose to use gentoo-sources to provide kernel source as it contains some handy patches and is easy to obtain via emerge. A base 6.8 kernel builds locally (with 4 cores!) in approximately 17 minutes, which is not too much of a pain when troubleshooting on bare metal. I also chose to omit an initramfs, which means CPU microcode can/should be built directly into the kernel. This can be achieved by first installing sys-firmware/intel-microcode. This is also a good time to add microcode signature to /etc/portage/make.conf 
$ iucode_tool -S -l /lib/firmware/intel-ucode/*
        iucode_tool: system has processor(s) with signature 0x000306a9
microcode bundle 1: /lib/firmware/intel-ucode/06-3a-09
selected microcodes:
  001/001: sig 0x000306a9, pf_mask 0x12, 2019-02-13, rev 0x0021, size 14336
# echo 'MICROCODE_SIGNATURES="-s 0x000306a9"' >> /etc/portage/make.conf
CPU's signature is found in bundle 1, so the filename to use is /lib/firmware/intel-ucode/06-3a-09. Ensure the following kernel options are set approximately, where the argument passed to CONFIG_EXTRA_FIRMWARE is the filename of our microcode 
CONFIG_MICROCODE=y
CONFIG_FW_LOADER=y
CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09"
CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
Check dmesg for microcode output to ensure all was successful. Specific configuation for this machine can be found at 6.8.9 .config. For some more kernel related stuff, see here.

Bootloader

Spoiler: there is none. For the duration of my time using Linux, I have used GRUB across various systems. In my experience, it's been ok, it does what it needs to, allows for extensive configuration like theming and requiring a password. However, some time ago it went kaput on this machine and despite manually setting the root partition in /boot/grub/grub.cfg, it seems the incorrect value was passed to the kernel at boot and the partition could not be found. Every time I would reboot I would have to manually set it from GRUB command line (which is a handy trick to know nonetheless): 
grub> ls
(hd0) (hd0,gpt0) (hd0,gpt1) (hd0,gpt2)
grub> ls (hd0,gpt1)/
vmlinuz vmlinuz.old
grub> root=(hd0,gpt1)
grub> linux /boot/vmlinuz root=/dev/sda3
grub> boot
Since this isn't a multiboot machine, GRUB can simply be omitted and expose the kernel image as an EFI stub. First, the following kernel options must be set 
CONFIG_EFI=y
CONFIG_EFI_STUB=y
CONFIG_CMDLINE_BOOL=y
CONFIG_CMDLINE="root=PARTUUID=<uuid> ro"
Where root is set to the PARTUUID value of the root partition. Use blkid to obtain this Once kernel has been recompiled, copy the image to the EFI partition and create a new boot entry. It is also wise to remove unneeded entries to conserve limited space. (below command selects the first entry with -b and deletes it with -B) 
# efibootmgr -b 1 -B
# cp arch/x86/boot/bzImage /boot/efi/gentoo/bzImage-x.y.z.efi
# efibootmgr -c -d /dev/sda -L "gentoo efi stub" -l "\efi\gentoo\bzImage-x.y.z.efi"
Reboot and the bootable system should be shown in Mac's EFI program.

Portage

Profile

When initially setting this up, I opted for a multilib system as I was still using some 32-bit programs at the time. Since, I have switched to a no-multilib profile. Unlike adding multilib support to a 64-bit only system, going from multilib to no-multilib is quite easy. 
# eselect profile list
  ...
  [12]  default/linux/amd64/17.1/no-multilib (stable)
  ...
# eselect profile set 12
# emerge -auDN @world
# emerge --depclean
It was pleasant to see 80+ packages removed after the switch. And have much reduced overhead in my system overall. I did have to manually set a few USE flags the were originally set by default profile.

Compiler

As Clang/LLVM has become more and more stable and is able to compile (almost) all packages installed, I chose to switch the default compiler, linker and other bits over from GCC. This is easy enough in /etc/portage/make.conf. 
CC="clang"
CXX="clang++"
AR="llvm-ar"
NM="llvm-nm"
RANLIB="llvm-ranlib"
For packages that Clang fails to compile, a GCC fallback environment can be made containing the previous variables, then applied to certain packages (e.g. gui-apps/waybar) by adding an entry in /etc/portage/package.env. Some advantages of using Clang over GCC are slightly faster compilation times and reduced memory usage, and potentially safer binaries overall.

USE Flags

My USE flags and related options are quite minimal. Global flags include 
USE="alsa bluetooth clang dbus elogind gles2 jpeg png \
     pulseaudio sdl udev wayland X -systemd -vulkan -nls"
ACCEPT_LICENSE="*"
ACCEPT_KEYWORDS="~amd64"
For reference, some primary features used on this system are Wayland, audio, elogind, and OpenRC. Since no-multilib already has very few things enabled, not many flags needed to be disabled, although I have quite a few package specific USE flags disabled to help reduce compile times and remove dependencies that may only be required for one or two packages.

Applications

Packages in my world file can be found in package list, and list of all system packages in full package list. Some notable utilities I use are below.

Miscellaneous

Another QOL modification for me was disabling the rather obnoxious startup sound present on all Macbooks. To do this, kernel must be built with efivarfs support (CONFIG_EFIVAR_FS) and the partition must be mounted at /sys/firmware/efi/efivars/. Next, the following commands must be run as proper root, not just with elevated permissions. Begin by removing the immutable flag from the appropriate file. Mac's EFI terminal uses the bell code (0x07) to audibly play system messages/warnings. It can be set to null character to silence it. Lastly, replace the immutable flag. 
chattr -i /sys/firmware/efi/efivars/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82
printf "\x07\x00\x00\x00\x00" > /sys/firmware/efi/efivars/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82
chattr +i /sys/firmware/efi/efivars/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82

Screenshots

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