From 9d92c9d0a797b19662968b250919d075317d7379 Mon Sep 17 00:00:00 2001
From: Tom Fitzhenry <tom@tom-fitzhenry.me.uk>
Date: Fri, 19 Jun 2020 17:57:04 +1000
Subject: [PATCH] iso-image: add 'serial console' boot entry

Prior to this commit, installation over serial console would requiring
manually having to modify the kernel modeline, as described in
https://github.com/NixOS/nixpkgs/issues/58198 .

This is unnecessarily fiddly, so this commit adds a syslinux boot
entry that has serial enabled.

GRUB already has a serial console entry:
https://github.com/NixOS/nixpkgs/blob/2c07a0800a76be01a3c255f39e21877a9a389f84/nixos/modules/installer/cd-dvd/iso-image.nix#L311-L317

Why 115200 bps? This is already used in other places, e.g. https://github.com/NixOS/nixpkgs/pull/58196

I tested this change by building the image, booting the image, and
observing the boot process over serial:

    $ cd nixos/
    $ nix-build -A config.system.build.isoImage -I nixos-config=modules/installer/cd-dvd/installation-cd-minimal.nix default.nix
    $ sudo cp /nix/store/arcl702c3z8xlndlvnfplq9yhixjvs9k-nixos-20.09pre-git-x86_64-linux.iso/iso/nixos-20.09pre-git-x86_64-linux.iso /dev/sdb

    $ picocom -b 115200 /dev/ttyUSB0
---
 nixos/modules/installer/cd-dvd/iso-image.nix | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index 405fbfa10db..43d20a556f8 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -143,6 +143,13 @@ let
     LINUX /boot/${config.system.boot.loader.kernelFile}
     APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} loglevel=7
     INITRD /boot/${config.system.boot.loader.initrdFile}
+
+    # A variant to boot with a serial console enabled
+    LABEL boot-serial
+    MENU LABEL NixOS ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (serial console=ttyS0,115200n8)
+    LINUX /boot/${config.system.boot.loader.kernelFile}
+    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} console=ttyS0,115200n8
+    INITRD /boot/${config.system.boot.loader.initrdFile}
   '';
 
   isolinuxMemtest86Entry = ''