add SD card mode instructions

Signed-off-by: Florian Agbuya <fa@m-labs.ph>

README.md

@@ -56,5 +56,12 @@ ssh access is not required for linien client if linien-server is started. If not
 | Set the linien-server service to start at bootup     | `linien-server enable`                  |
 | Set the linien-server service not to start at bootup | `linien-server disable`                 |
 
-4. In the dev shell, run `linien` to launch the GUI. Add new device. Username is `root`.
-5. Select the newly added device and click connect in the GUI to connect and start the GUI.
+5. In the dev shell, run `linien` to launch the GUI. Add new device. Username is `root`.
+6. Select the newly added device and click connect in the GUI to connect and start the GUI.
+
+## SD Card Mode
+By default, the SD card is mounted in read-only mode with RAM overlay to prevent data corruption when power is unexpectedly removed.
+
+1. To toggle between read-only and read-write modes, run the command: `toggle-sd-mode`.
+2. The system will automatically toggle between modes and reboot to apply changes.
+3. When in read-write mode, always perform proper shutdown before removing power.

flake.nix

@@ -411,6 +411,22 @@
         hardeningDisable = [ "fortify" ];
       };
 
+      toggle-sd-mode = pkgs-armv7l.writeScriptBin "toggle-sd-mode" ''
+        #!/bin/sh
+        mkdir -p /boot
+        mount /dev/mmcblk0p1 /boot
+        if [ -f /boot/rw_mode ]; then
+          rm -f /boot/rw_mode
+          echo "System will boot in read-only mode after reboot."
+        else
+          touch /boot/rw_mode
+          echo "System will boot in read-write mode after reboot."
+        fi
+        umount /boot
+        echo "Rebooting..."
+        reboot
+      '';
+
       board-package-set = { board }: let
         not-os-configured = (import patched-not-os {
           inherit nixpkgs;
@@ -420,6 +436,7 @@
             ({ config, pkgs, lib, ... }: {
               environment.systemPackages = [
                 (pkgs.python3.withPackages(ps: with ps; [ pyfastservo linien-server]))
+                toggle-sd-mode
               ];
               boot.postBootCommands = lib.mkAfter ''
 

not-os-patches/pr-28.patch

@@ -59,7 +59,7 @@ index d7b0bf3..70353a1 100644
        '';
      }
 diff --git a/stage-1.nix b/stage-1.nix
-index 331fecd..aa5148e 100644
+index 331fecd..3b6b98b 100644
 --- a/stage-1.nix
 +++ b/stage-1.nix
 @@ -117,11 +117,6 @@ let
@@ -74,18 +74,63 @@ index 331fecd..aa5148e 100644
      root=/dev/vda
      realroot=tmpfs
      for o in $(cat /proc/cmdline); do
-@@ -164,7 +159,9 @@ let
+@@ -163,20 +158,41 @@ let
+     chmod 755 /mnt/
      mkdir -p /mnt/nix/store/
  
- 
+-
 -    ${if config.not-os.nix then ''
+-    # make the store writeable
+-    mkdir -p /mnt/nix/.ro-store /mnt/nix/.overlay-store /mnt/nix/store
+-    mount $root /mnt/nix/.ro-store -t squashfs
+-    if [ $realroot = $1 ]; then
+-      mount tmpfs -t tmpfs /mnt/nix/.overlay-store -o size=1G
+-    fi
+-    mkdir -pv /mnt/nix/.overlay-store/work /mnt/nix/.overlay-store/rw
+-    modprobe overlay
+-    mount -t overlay overlay -o lowerdir=/mnt/nix/.ro-store,upperdir=/mnt/nix/.overlay-store/rw,workdir=/mnt/nix/.overlay-store/work /mnt/nix/store
+-    '' else ''
+-    # readonly store
+-    mount $root /mnt/nix/store/ -t squashfs
 +    ${if config.not-os.sd && config.not-os.nix then ''
-+    mount $root /mnt
-+    '' else if config.not-os.nix then ''
-     # make the store writeable
-     mkdir -p /mnt/nix/.ro-store /mnt/nix/.overlay-store /mnt/nix/store
-     mount $root /mnt/nix/.ro-store -t squashfs
-@@ -190,6 +187,11 @@ let
++      USE_READONLY=1
++      if [ -e /dev/mmcblk0p1 ]; then
++        mkdir -p /boot
++        mount -t vfat -o ro /dev/mmcblk0p1 /boot
++        if [ -e /boot/rw_mode ]; then
++          echo "Found rw_mode flag, enabling read-write mode"
++          USE_READONLY=0
++        fi
++        umount /boot
++      fi
++      if [ "$USE_READONLY" = "1" ]; then
++        mkdir -p /mnt.ro /mnt.overlay
++        mount -o ro $root /mnt.ro
++        mount -t tmpfs -o size=1G tmpfs /mnt.overlay
++        mkdir -p /mnt.overlay/upper /mnt.overlay/work
++        mount -t overlay overlay -o lowerdir=/mnt.ro,upperdir=/mnt.overlay/upper,workdir=/mnt.overlay/work /mnt
++      else
++        mount $root /mnt
++      fi
++    ''
++    else if config.not-os.nix then ''
++      # make the store writeable
++      mkdir -p /mnt/nix/.ro-store /mnt/nix/.overlay-store /mnt/nix/store
++      mount $root /mnt/nix/.ro-store -t squashfs
++      if [ $realroot = $1 ]; then
++        mount tmpfs -t tmpfs /mnt/nix/.overlay-store -o size=1G
++      fi
++      mkdir -pv /mnt/nix/.overlay-store/work /mnt/nix/.overlay-store/rw
++      modprobe overlay
++      mount -t overlay overlay -o lowerdir=/mnt/nix/.ro-store,upperdir=/mnt/nix/.overlay-store/rw,workdir=/mnt/nix/.overlay-store/work /mnt/nix/store
++    ''
++    else ''
++      # readonly store
++      mount $root /mnt/nix/store/ -t squashfs
+     ''}
+ 
+     ${lib.optionalString enablePlymouth ''
+@@ -190,6 +206,11 @@ let
    initialRamdisk = pkgs.makeInitrd {
      contents = [ { object = bootStage1; symlink = "/init"; } ];
    };
@@ -97,7 +142,7 @@ index 331fecd..aa5148e 100644
  in
  {
    options = {
-@@ -205,6 +207,7 @@ in
+@@ -205,6 +226,7 @@ in
    config = {
      system.build.bootStage1 = bootStage1;
      system.build.initialRamdisk = initialRamdisk;
@@ -152,10 +197,10 @@ index c61f9d6..fbdf0fd 100644
    };
  }
 diff --git a/zynq_image.nix b/zynq_image.nix
-index 3fa23ab..069fe89 100644
+index 3fa23ab..1790801 100644
 --- a/zynq_image.nix
 +++ b/zynq_image.nix
-@@ -1,66 +1,89 @@
+@@ -1,66 +1,91 @@
 -{ config, pkgs, ... }:
 +{ lib, config, pkgs, ... }:
  
@@ -167,6 +212,8 @@ index 3fa23ab..069fe89 100644
 +  customKernel = (pkgs.linux_6_6.override {
      extraConfig = ''
        OVERLAY_FS y
++      NLS_CODEPAGE_437 y
++      NLS_ISO8859_1 y
 +      MEDIA_SUPPORT n
 +      FB n
 +      DRM n