fix(handoff): correct 8-bit modulo csum calculation

Fix the handoff 8-bit modulo checksum calculation to ensure we never get
a checksum larger than 8 bits. The previous calculation failed to
truncate the sum at the final step in update_checksum

Change-Id: Ice0b72eb139af90f416adeff157d337646d6201a
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>

tools/tlc/tests/test_transfer_list.py

@@ -88,12 +88,19 @@ def test_calculate_te_sum_of_bytes(tag_id, data):
     assert te.sum_of_bytes == csum
 
 
-@pytest.mark.parametrize(("tag_id", "data"), test_entries)
-def test_calculate_tl_checksum(tag_id, data):
+def test_calc_tl_checksum(tmpdir, random_entries):
+    tl_file = tmpdir.join("tl.bin")
+
     tl = TransferList(0x1000)
 
-    tl.add_transfer_entry(tag_id, data)
-    assert tl.sum_of_bytes() == 0
+    for id, data in random_entries(10):
+        tl.add_transfer_entry(id, data)
+
+    assert sum(tl.to_bytes()) % 256 == 0
+
+    # Write the transfer list to a file and check that the sum of bytes is 0
+    tl.write_to_file(tl_file)
+    assert sum(tl_file.read_binary()) % 256 == 0
 
 
 def test_empty_transfer_list_blob(tmpdir):
@@ -129,7 +136,7 @@ def test_write_multiple_tes_to_file(tmpdir, random_entries):
     """Check that we can create a TL with multiple TE's."""
     test_file = tmpdir.join("test_tl_blob.bin")
     tl = TransferList(0x4000)
-    _test_entries = random_entries()
+    _test_entries = list(random_entries())
 
     for tag_id, data in _test_entries:
         tl.add_transfer_entry(tag_id, data)

tools/tlc/tlc/te.py

@@ -48,7 +48,10 @@ class TransferEntry:
 
     @property
     def sum_of_bytes(self) -> int:
-        return (sum(self.header_to_bytes()) + sum(self.data)) % 256
+        return sum(self.to_bytes()) % 256
+
+    def to_bytes(self) -> bytes:
+        return self.header_to_bytes() + self.data
 
     def header_to_bytes(self) -> bytes:
         return self.id.to_bytes(3, "little") + struct.pack(

tools/tlc/tlc/tl.py

@@ -189,13 +189,15 @@ class TransferList:
 
     def update_checksum(self) -> None:
         """Calculates the checksum based on the sum of bytes."""
-        self.checksum = 256 - ((self.sum_of_bytes() - self.checksum) % 256)
+        self.checksum = (256 - (self.sum_of_bytes() - self.checksum)) % 256
+        assert self.checksum <= 0xFF
+
+    def to_bytes(self) -> bytes:
+        return self.header_to_bytes() + b"".join([te.to_bytes() for te in self.entries])
 
     def sum_of_bytes(self) -> int:
         """Sum of all bytes between the base address and the end of that last TE (modulo 0xff)."""
-        return (
-            sum(self.header_to_bytes()) + sum(te.sum_of_bytes for te in self.entries)
-        ) % 256
+        return (sum(self.to_bytes())) % 256
 
     def get_entry(self, tag_id: int) -> Optional[TransferEntry]:
         for te in self.entries: