wip(rpi4): park the Pi 4 sketch — superseded by the Pi 5 machinery in #87

Never-committed working tree state found in the dev worktree: a Pi 4 hardware
module, a setup walkthrough, an upboard-serial refactor, and flake wiring that
does not evaluate (aarch64 packages block nested inside packages.x86_64-linux;
pkgs-aarch64 built with `inherit aarch64` instead of `system`, so it is really
x86; references a nixosConfigurations.rpi4-installed that is never defined).

Parked verbatim for reference. The real Pi 4 target is rebuilt on top of #87's
mkPiInstalled/mkPiImage shape in feat/rpi4-target.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013A6683cCHnQxFUosx1krY4

deploy/nixos/hardware/raspberry-pi4.nix

@@ -0,0 +1,124 @@
+# Raspberry Pi 4 Hardware Configuration
+# For bitSpire deployments on Raspberry Pi 4
+# Requires: 4GB+ RAM (8GB recommended for Electron + kiosk)
+# Storage: SD card or external SSD/eMMC (minimum 32GB)
+#
+# Note: Import shared serial peripheral configuration for consistency
+# with other bitSpire hardware (useful for future extensions).
+
+{ config, lib, pkgs, ... }:
+
+{
+  # Import shared serial peripheral configuration (useful for future hardware extensions)
+  imports = [ ./upboard-serial.nix ];
+
+  # Raspberry Pi 4 uses aarch64 architecture
+  boot.kernelPackages = pkgs.linuxPackages_rpi4;
+
+  boot = {
+    loader = {
+      # Raspberry Pi 4 uses the Raspberry Pi firmware bootloader (rpi-bootloader)
+      grub.enable = lib.mkForce false;
+      # raspberryPi.enable has been removed in recent nixpkgs
+      # rpi-bootloader is now built into rpi4 kernel packages
+    };
+
+    initrd.availableKernelModules = [
+      "xhci_pci"
+      "usb_storage"
+      "sdhci_pci"
+      "sdhci"
+      "sd_mod"
+      "brcmmac"
+      "brcmfmac"
+      "mmc_block"
+      "cma"
+    ];
+
+    initrd.kernelModules = [
+      "sdhci_pci"
+      "sdhci"
+      "mmc_block"
+      "brcmmac"
+      "brcmfmac"
+      "cma"
+    ];
+
+    kernelModules = [
+      "kvm-arm"
+      "uas"
+    ];
+
+    kernelParams = [
+      "console=tty1"
+      "console=ttyAMA0,115200n8"
+      "console=ttyS0,115200n8"
+      "root=/dev/mmcblk0p2"
+      "rootfstype=ext4"
+      "rootwait"
+      "rw"
+      # Disable PCIe to save power (optional - uncomment if needed)
+      # "pcie_aspm=off"
+    ];
+  };
+
+  # Filesystem layout: SD card with GPT partition table
+  # Partition 1: EFI System Partition (FAT32)
+  # Partition 2: Root filesystem (ext4)
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/nixos";
+    fsType = "ext4";
+    options = [ "noatime" ];
+  };
+
+  fileSystems."/boot" = {
+    device = "/dev/disk/by-label/ESP";
+    fsType = "vfat";
+    options = [ "defaults" "nofail" ];
+  };
+
+  # Swap configuration for better reliability on SD cards
+  swapDevices = [{
+    device = "/swapfile";
+    size = 2048; # 2GB swap
+  }];
+
+  # Power management for Raspberry Pi
+  powerManagement = {
+    enable = true;
+    cpuFreqGovernor = "ondemand";
+  };
+
+  # Disable suspend/hibernate for kiosk stability
+  systemd.targets = {
+    sleep.enable = false;
+    suspend.enable = false;
+    hibernate.enable = false;
+    hybrid-sleep.enable = false;
+  };
+
+  # Hardware-specific settings
+  hardware = {
+    # Raspberry Pi 4 has no discrete GPU (uses VideoCore)
+    graphics = {
+      enable = true;
+    };
+
+    # USB mass storage drivers (not applicable for ARM - handled by kernel modules)
+    # usb.enable = true;
+    # usb.enableUSBHub = true;
+
+    # RPi4-specific tweaks
+    enableRedistributableFirmware = true;
+    # CPU firmware (not applicable for ARM - no x86 microcode)
+    # cpu.amd64.updateMicrocode = false; # Not applicable for ARM
+  };
+
+  # USB configuration (not applicable for ARM)
+  # hardware.usb.enable = true;
+  # hardware.usb.enableUSBHub = true;
+
+  # Network configuration
+  # useDHCP is set in configuration.nix (base config)
+  networking.interfaces.eth0.useDHCP = true;
+}