Handle partitioned and unpartitioned SD cards

Add utils.MBRPartition to implement basic partitioned device support.

Try to mount partition 1 of SDCard first, if that fails, try to mount
entire device as FAT file system.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

software/src/main.py

@@ -5,7 +5,6 @@ import aiorepl
 import asyncio
 import machine
 import micropython
-import os
 import time
 from machine import Pin
 from math import pi, sin, pow
@@ -17,8 +16,7 @@ from mfrc522 import MFRC522
 from mp3player import MP3Player
 from nfc import Nfc
 from rp2_neopixel import NeoPixel
-from rp2_sd import SDCard
-from utils import Buttons, TimerManager
+from utils import Buttons, SDContext, TimerManager
 
 micropython.alloc_emergency_exception_buf(100)
 
@@ -54,30 +52,6 @@ machine.mem32[0x4001c004 + 8*4] = 0x67
 machine.mem32[0x40030000 + 0x00] = 0x10
 
 
-class SDContext:
-    def __init__(self, mosi, miso, sck, ss, baudrate):
-        self.mosi = mosi
-        self.miso = miso
-        self.sck = sck
-        self.ss = ss
-        self.baudrate = baudrate
-
-    def __enter__(self):
-        self.sdcard = SDCard(self.mosi, self.miso, self.sck, self.ss, self.baudrate)
-        try:
-            os.mount(self.sdcard, '/sd')
-        except Exception:
-            self.sdcard.deinit()
-            raise
-        return self
-
-    def __exit__(self, exc_type, exc_value, traceback):
-        try:
-            os.umount('/sd')
-        finally:
-            self.sdcard.deinit()
-
-
 def run():
     asyncio.new_event_loop()
     # Setup LEDs
@@ -105,4 +79,6 @@ def run():
 
 
 if __name__ == '__main__':
-    run()
+    if machine.Pin(17, machine.Pin.IN, machine.Pin.PULL_UP).value() != 0:
+        time.sleep(5)
+        run()

software/src/utils/__init__.py

@@ -2,6 +2,8 @@
 # Copyright (c) 2025 Matthias Blankertz <matthias@blankertz.org>
 
 from utils.buttons import Buttons
+from utils.mbrpartition import MBRPartition
+from utils.sdcontext import SDContext
 from utils.timer import TimerManager
 
-__all__ = ["Buttons", "TimerManager"]
+__all__ = ["Buttons", "MBRPartition", "SDContext", "TimerManager"]

software/src/utils/mbrpartition.py

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Matthias Blankertz <matthias@blankertz.org>
+
+from array import array
+import struct
+
+
+class MBRPartition:
+    def __init__(self, bdev, partno):
+        assert partno >= 0 and partno < 4
+        self.bdev = bdev
+        bdev_len = bdev.ioctl(4, None)
+        bdev_bs = bdev.ioctl(5, None)
+        assert bdev_bs == 512
+        mbr = array('B', 512*b'0')
+        bdev.readblocks(0, mbr)
+        if mbr[510] != 0x55 or mbr[511] != 0xaa:
+            raise ValueError("Not a valid MBR")
+        partofs = 0x1be + partno*16
+        (boot_ind, _, _, _,
+         parttype, _, _, _,
+         lba_start, lba_len) = struct.unpack_from('<BBBBBBBBLL', mbr, partofs)
+        print(f'Partition {partno} bi {boot_ind} type {parttype} start {lba_start} len {lba_len}')
+        if (boot_ind != 0x00 and boot_ind != 0x80) or parttype == 0x00:
+            raise ValueError("Not a valid partition")
+        self.offset = lba_start
+        self.size = lba_len
+        assert lba_start + lba_len <= bdev_len
+
+    def ioctl(self, op, arg):
+        if op == 4:
+            return self.size
+        elif op == 5:
+            return 512
+        else:
+            return None
+
+    def readblocks(self, block, buf):
+        if block >= self.size:
+            raise ValueError("Block out of range")
+        return self.bdev.readblocks(block+self.offset, buf)
+
+    def writeblocks(self, block, buf):
+        if block >= self.size:
+            raise ValueError("Block out of range")
+        return self.bdev.writeblocks(block+self.offset, buf)

software/src/utils/sdcontext.py

@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Matthias Blankertz <matthias@blankertz.org>
+
+import os
+
+from . import MBRPartition
+from rp2_sd import SDCard
+
+
+class SDContext:
+    def __init__(self, mosi, miso, sck, ss, baudrate):
+        self.mosi = mosi
+        self.miso = miso
+        self.sck = sck
+        self.ss = ss
+        self.baudrate = baudrate
+
+    def __enter__(self):
+        self.sdcard = SDCard(self.mosi, self.miso, self.sck, self.ss, self.baudrate)
+        # Try first partition
+        try:
+            self.part = MBRPartition(self.sdcard, 0)
+            os.mount(self.part, '/sd')
+            return self
+        except Exception:
+            print("Failed to mount SDCard partition, trying whole device...")
+        # Try whole device
+        try:
+            os.mount(self.sdcard, '/sd')
+            return self
+        except Exception as ex:
+            self.sdcard.deinit()
+            raise RuntimeError("Could not mount SD card") from ex
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        try:
+            os.umount('/sd')
+        finally:
+            self.sdcard.deinit()