Fix generic Linux SPI write_readinto slice

Author: makermelissa

src/adafruit_blinka/microcontroller/generic_linux/spi.py

@@ -113,9 +113,7 @@ def readinto(self, buf, start=0, end=None, write_value=0):
     def write_readinto(
         self, buffer_out, buffer_in, out_start=0, out_end=None, in_start=0, in_end=None
     ):
-        """Perform a half-duplex write from buffer_out and then
-        read data into buffer_in
-        """
+        """Write to and read from SPI at the same time."""
         if buffer_out is None or buffer_in is None:
             return
         if len(buffer_out) < 1 or len(buffer_in) < 1:
@@ -132,7 +130,7 @@ def write_readinto(
             self._spi.max_speed_hz = self.baudrate
             self._spi.mode = self.mode
             self._spi.bits_per_word = self.bits
-            data = self._spi.transfer(list(buffer_out[out_start : out_end + 1]))
+            data = self._spi.transfer(list(buffer_out[out_start:out_end]))
             for i in range((in_end - in_start)):
                 buffer_in[i + in_start] = data[i]
             # self._spi.close()

tests/test_generic_linux_spi.py

@@ -0,0 +1,62 @@
+# SPDX-FileCopyrightText: 2026 Melissa LeBlanc-Williams for Adafruit Industries
+#
+# SPDX-License-Identifier: MIT
+import sys
+import types
+from unittest import mock
+
+
+class FakePureIOSPI:
+    last_instance = None
+
+    def __init__(self, device):
+        self.device = device
+        self.max_speed_hz = None
+        self.mode = None
+        self.bits_per_word = None
+        self.transfers = []
+        FakePureIOSPI.last_instance = self
+
+    def transfer(self, data):
+        self.transfers.append(data)
+        return [value + 0x10 for value in data]
+
+
+class FakeDetector:
+    class Chip:
+        id = "test-chip"
+
+    class Board:
+        id = "test-board"
+
+    chip = Chip()
+    board = Board()
+
+
+fake_platformdetect = types.ModuleType("adafruit_platformdetect")
+fake_platformdetect.Detector = lambda: FakeDetector()
+sys.modules.setdefault("adafruit_platformdetect", fake_platformdetect)
+
+fake_pureio = types.ModuleType("Adafruit_PureIO")
+fake_pureio.spi = types.SimpleNamespace(SPI=FakePureIOSPI)
+sys.modules.setdefault("Adafruit_PureIO", fake_pureio)
+
+# pylint: disable=wrong-import-position
+from adafruit_blinka.microcontroller.generic_linux import spi as generic_linux_spi  # noqa: E402
+
+
+@mock.patch.object(generic_linux_spi.spi, "SPI", FakePureIOSPI)
+def test_write_readinto_uses_exclusive_out_end():
+    FakePureIOSPI.last_instance = None
+    spi = generic_linux_spi.SPI((0, 0))
+    spi.init(baudrate=1000000, polarity=1, phase=1, bits=8)
+
+    buffer_out = bytearray([0x10, 0x20, 0x30, 0x40])
+    buffer_in = bytearray([0x00, 0x00, 0x00, 0x00])
+
+    spi.write_readinto(
+        buffer_out, buffer_in, out_start=1, out_end=3, in_start=1, in_end=3
+    )
+
+    assert FakePureIOSPI.last_instance.transfers == [[0x20, 0x30]]
+    assert buffer_in == bytearray([0x00, 0x30, 0x40, 0x00])