Fix LIN checksum calculation

include/linbus.h

@@ -19,6 +19,7 @@
 #ifndef LINBUS_H
 #define LINBUS_H
 
+#include <stdint.h>
 
 class LinBus
 {
@@ -41,8 +42,6 @@ class LinBus
          uint16_t pin;
       };
 
-      static uint8_t Checksum(uint8_t pid, uint8_t* data, int len);
-      static uint8_t Parity(uint8_t id);
 
       static const HwInfo hwInfo[];
       static const int payloadIndex = 3;
@@ -51,6 +50,10 @@ class LinBus
       const HwInfo* hw;
       uint8_t sendBuffer[11];
       uint8_t recvBuffer[12];
+
+   public:
+      static uint8_t Checksum(uint8_t pid, uint8_t* data, int len);
+      static uint8_t Parity(uint8_t id);
 };
 
 #endif // LINBUS_H

src/linbus.cpp

@@ -152,7 +152,7 @@ uint8_t LinBus::Checksum(uint8_t pid, uint8_t* data, int len)
    for (int i = 0; i < len; i++)
    {
       uint16_t tmp = (uint16_t)checksum + (uint16_t)data[i];
-      if (tmp > 256) tmp -= 255;
+      if (tmp >= 256) tmp -= 255;
       checksum = tmp;
    }
    return checksum ^ 0xff;

test/Makefile

@@ -4,12 +4,12 @@ CAN_SIGNED ?= 0
 CC		= gcc
 CPP		= g++
 LD		= g++
-CFLAGS    = -std=c99 -ggdb -DSTM32F1 -DCAN_SIGNED=$(CAN_SIGNED) -Itest-include -I../include -I../../libopencm3/include
-CPPFLAGS    = -ggdb -DSTM32F1 -DCAN_SIGNED=$(CAN_SIGNED) -Itest-include -I../include -I../../libopencm3/include
+CFLAGS    = -std=c99 -ggdb -fpermissive -DSTM32F1 -DCAN_SIGNED=$(CAN_SIGNED) -Itest-include -I../include -I../../libopencm3/include
+CPPFLAGS    = -ggdb -fpermissive -DSTM32F1 -DCAN_SIGNED=$(CAN_SIGNED) -Itest-include -I../include -I../../libopencm3/include
 LDFLAGS     = -g
 BINARY		= test_libopeninv
 OBJS		= test_main.o fu.o test_fu.o test_fp.o my_fp.o my_string.o params.o \
-			  stub_canhardware.o test_canmap.o canmap.o \
+			  stub_canhardware.o test_canmap.o canmap.o test_linbus.o linbus.o \
 			  stub_libopencm3.o
 VPATH = ../src ../libopeninv/src
 

test/stub_libopencm3.c

@@ -56,3 +56,94 @@ uint32_t crc_calculate_block(uint32_t *datap, int size)
 void crc_reset(void)
 {
 }
+
+void gpio_set_mode(uint32_t gpioport, uint8_t mode, uint8_t cnf, uint16_t gpios)
+{
+}
+
+void usart_set_baudrate(uint32_t usart, uint32_t baud)
+{
+}
+
+void usart_set_databits(uint32_t usart, uint32_t bits)
+{
+}
+
+void usart_set_stopbits(uint32_t usart, uint32_t stopbits)
+{
+}
+
+void usart_set_mode(uint32_t usart, uint32_t mode)
+{
+}
+
+void usart_set_parity(uint32_t usart, uint32_t parity)
+{
+}
+
+void usart_set_flow_control(uint32_t usart, uint32_t flowcontrol)
+{
+}
+
+void usart_enable_tx_dma(uint32_t usart)
+{
+}
+
+void usart_enable_rx_dma(uint32_t usart)
+{
+}
+
+void usart_enable(uint32_t usart)
+{
+}
+
+void dma_channel_reset(uint32_t dma, uint8_t channel)
+{
+}
+
+void dma_set_read_from_memory(uint32_t dma, uint8_t channel)
+{
+}
+
+void dma_set_peripheral_address(uint32_t dma, uint8_t channel, uint32_t address)
+{
+}
+
+void dma_set_memory_address(uint32_t dma, uint8_t channel, uint32_t address)
+{
+}
+
+void dma_set_peripheral_size(uint32_t dma, uint8_t channel,
+			     uint32_t peripheral_size)
+{
+}
+
+void dma_set_memory_size(uint32_t dma, uint8_t channel, uint32_t mem_size)
+{
+}
+
+void dma_enable_memory_increment_mode(uint32_t dma, uint8_t channel)
+{
+}
+
+void dma_set_number_of_data(uint32_t dma, uint8_t channel, uint16_t number)
+{
+}
+
+uint16_t dma_get_number_of_data(uint32_t dma, uint8_t channel)
+{
+    return 0x5a;
+}
+
+void dma_clear_interrupt_flags(uint32_t dma, uint8_t channel,
+			       uint32_t interrupts)
+{
+}
+
+void dma_enable_channel(uint32_t dma, uint8_t channel)
+{
+}
+
+void dma_disable_channel(uint32_t dma, uint8_t channel)
+{
+}

test/test_linbus.cpp

@@ -0,0 +1,95 @@
+/*
+ * This file is part of the libopeninv project.
+ *
+ * Copyright (C) 2025 David J. Fiddes <D.J@fiddes.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linbus.h>
+#include "test.h"
+
+class LinBusTest : public UnitTest
+{
+public:
+   LinBusTest(const std::list<VoidFunction> *cases) : UnitTest(cases) {}
+};
+
+static void TestProtectedIdentifierGeneration()
+{
+   // Examples from 2.8.2 TABLE OF VALID FRAME IDENTIFIERS in
+   // Lin Protocol Specification Revision 2.1
+
+   ASSERT(LinBus::Parity(0x00) == 0x80);
+   ASSERT(LinBus::Parity(0x17) == 0x97);
+   ASSERT(LinBus::Parity(0x2B) == 0x2B);
+   ASSERT(LinBus::Parity(0x3B) == 0xFB);
+}
+
+static void TestSpecChecksumExample()
+{
+   // Example from 2.8.2 EXAMPLE OF CHECKSUM CALCULATION in
+   // Lin Protocol Specification Revision 2.1
+
+   uint8_t data[] = {0x55, 0x93, 0xe5};
+   ASSERT(LinBus::Checksum(0x4a, data, sizeof(data)) == 0xe6);
+}
+
+static void TestSyntheticLinStatusResponse1()
+{
+   // Synthesized using https://github.com/gicking/LIN_slave_portable_Arduino
+   // captured and verified using Saleae Logic analyzer
+   // and https://linchecksumcalculator.machsystems.cz/
+
+   uint8_t data[] = {0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17};
+   ASSERT(LinBus::Checksum(LinBus::Parity(0x30), data, sizeof(data)) == 0x72);
+}
+
+static void TestSyntheticLinStatusResponse2()
+{
+   // Synthesized using https://linchecksumcalculator.machsystems.cz/
+
+   uint8_t data[] = {0x01, 0x02, 0x03, 0x04, 0xaa, 0x55, 0xaa, 0x55};
+   ASSERT(LinBus::Checksum(0xf0, data, sizeof(data)) == 0x05);
+}
+
+static void TestVAGPTCHeaterStatusResponse1()
+{
+   // Chosen at random from
+   // https://raw.githubusercontent.com/Tom-evnut/ID3-LIN-Bus/refs/heads/main/Lin%20Logs/PTC%20Cabin%20Heat%20on%20off.csv
+
+   uint8_t data[] = {0x54, 0x69, 0x55, 0x00, 0xCA, 0x10, 0x00, 0x04};
+
+   ASSERT(LinBus::Checksum(LinBus::Parity(0x0B), data, sizeof(data)) == 0x82);
+}
+
+static void TestVAGPTCHeaterStatusResponse2()
+{
+   // Chosen at random from
+   // https://raw.githubusercontent.com/Tom-evnut/ID3-LIN-Bus/refs/heads/main/Lin%20Logs/PTC%20Cabin%20Heat%20on%20off.csv
+
+   uint8_t data[] = {0xFF, 0x0F, 0x80, 0x64, 0x8B, 0xC9, 0xDD, 0x47};
+
+   ASSERT(LinBus::Checksum(LinBus::Parity(0x37), data, sizeof(data)) == 0x5A);
+}
+
+// This line registers the test
+REGISTER_TEST(
+    LinBusTest,
+    TestProtectedIdentifierGeneration,
+    TestSpecChecksumExample,
+    TestSyntheticLinStatusResponse1,
+    TestVAGPTCHeaterStatusResponse1,
+    TestVAGPTCHeaterStatusResponse2,
+    TestSyntheticLinStatusResponse2);