applet.memory.24x: use chunked reads.

Currently the I2C applet cannot handle reads of 64K and more,
and 64K I2C EEPROMs exist; before this commit, trying to read exactly
64K from one would result in a hang.

software/glasgow/applet/interface/i2c_master/__init__.py

@@ -141,6 +141,7 @@ async def _cmd_stop(self):
         await self.lower.write([CMD_STOP])
 
     async def _cmd_count(self, count):
+        assert count < 0xffff
         msb = (count >> 8) & 0xff
         lsb = (count >> 0) & 0xff
         await self.lower.write([CMD_COUNT, msb, lsb])

software/glasgow/applet/memory/_24x/__init__.py

@@ -27,24 +27,35 @@ def _carry_addr(self, addr):
             return (i2c_addr, [addr & 0xff])
 
     async def read(self, addr, length):
-        i2c_addr, addr_bytes = self._carry_addr(addr)
-
-        # Note that even if this is a 1-byte address EEPROM and we write 2 bytes here, we will not
-        # overwrite the contents, since the actual write is only initiated on stop, not repeated
-        # start condition.
-        self._log("i2c-addr=%#04x addr=%#06x", i2c_addr, addr)
-        result = await self.lower.write(i2c_addr, addr_bytes)
-        if result is False:
-            self._log("unacked")
-            return None
-
-        self._log("read=%d", length)
-        data = await self.lower.read(i2c_addr, length, stop=True)
-        if data is None:
-            self._log("unacked")
-        else:
-            self._log("data=<%s>", data.hex())
-        return data
+        chunks = []
+
+        while length > 0:
+            i2c_addr, addr_bytes = self._carry_addr(addr)
+
+            # Our lower layer can't do reads of 64K and higher, so use 32K chunks.
+            chunk_size = min(length, 0x8000)
+
+            # Note that even if this is a 1-byte address EEPROM and we write 2 bytes here,
+            # we will not overwrite the contents, since the actual write is only initiated
+            # on stop, not repeated start condition.
+            self._log("i2c-addr=%#04x addr=%#06x", i2c_addr, addr)
+            result = await self.lower.write(i2c_addr, addr_bytes)
+            if result is False:
+                self._log("unacked")
+                return None
+
+            self._log("read=%d", chunk_size)
+            chunk = await self.lower.read(i2c_addr, chunk_size, stop=True)
+            if chunk is None:
+                self._log("unacked")
+            else:
+                self._log("chunk=<%s>", chunk.hex())
+                chunks.append(chunk)
+
+            length -= chunk_size
+            addr   += chunk_size
+
+        return b"".join(chunks)
 
     async def write(self, addr, data):
         while len(data) > 0: