All Arduino and AdaFruit SAMD boards with a flash-based bootloader have

a bug where reading 64 bytes or more over USB returns corrupted data.
This is because the bootloaders try to send 64+ byte USB packets with a
pointer into flash instead of SRAM which is not allowed on the SAMD.
Instead we detect the Arduino bootloader and break read transactions
into multiple requests no larger than 63 bytes.

Author: shumatech

src/Samba.cpp

@@ -56,6 +56,7 @@ Samba::Samba() :
     _canChipErase(false),
     _canWriteBuffer(false),
     _canChecksumBuffer(false),
+    _readBufferSize(0),
     _debug(false),
     _isUsb(false)
 {
@@ -122,6 +123,12 @@ Samba::init()
             }
             extIndex++;
         }
+
+        // All SAMD-based Arduino/AdaFruit boards have a bug in their bootloader
+        // that trying to read 64 bytes or more over USB corrupts the data.
+        // We must limit these boards to read chunks of 63 bytes.
+        if (_isUsb)
+            _readBufferSize = 63;
     }
 
     _port->timeout(TIMEOUT_NORMAL);
@@ -452,29 +459,43 @@ void
 Samba::read(uint32_t addr, uint8_t* buffer, int size)
 {
     uint8_t cmd[20];
+    int chunk;
 
     if (_debug)
         printf("%s(addr=%#x,size=%#x)\n", __FUNCTION__, addr, size);
 
     // The SAM firmware has a bug reading powers of 2 over 32 bytes
     // via USB.  If that is the case here, then read the first byte
     // with a readByte and then read one less than the requested size.
-    if (_isUsb && size > 32 && !(size & (size - 1)))
+    if (_isUsb && _readBufferSize == 0 && size > 32 && !(size & (size - 1)))
     {
         *buffer = readByte(addr);
         addr++;
         buffer++;
         size--;
     }
 
-    snprintf((char*) cmd, sizeof(cmd), "R%08X,%08X#", addr, size);
-    if (_port->write(cmd, sizeof(cmd) - 1) != sizeof(cmd) - 1)
-        throw SambaError();
+    while (size > 0)
+    {
+        // Handle any limitations on the size of the read
+        if (_readBufferSize > 0 && size > _readBufferSize)
+            chunk = _readBufferSize;
+        else
+            chunk = size;
 
-    if (_isUsb)
-        readBinary(buffer, size);
-    else
-        readXmodem(buffer, size);
+        snprintf((char*) cmd, sizeof(cmd), "R%08X,%08X#", addr, chunk);
+        if (_port->write(cmd, sizeof(cmd) - 1) != sizeof(cmd) - 1)
+            throw SambaError();
+
+        if (_isUsb)
+            readBinary(buffer, chunk);
+        else
+            readXmodem(buffer, chunk);
+
+        size -= chunk;
+        addr += chunk;
+        buffer += chunk;
+    }
 }
 
 void

src/Samba.h

@@ -93,6 +93,7 @@ class Samba
     bool _canChipErase;
     bool _canWriteBuffer;
     bool _canChecksumBuffer;
+    int _readBufferSize;
     bool _debug;
     bool _isUsb;
     SerialPort::Ptr _port;