modbus: Get rid of custom CRC16 function

In previous commits the crc16_ansi() function has been made compliant
with the CRC-16-ANSI aka CRC-16-MODBUS standard. Use that standard
function instead of a custom one.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

subsys/modbus/modbus_serial.c

@@ -26,6 +26,7 @@ LOG_MODULE_REGISTER(modbus_serial, CONFIG_MODBUS_LOG_LEVEL);
 #include <kernel.h>
 #include <string.h>
 #include <sys/byteorder.h>
+#include <sys/crc.h>
 #include <modbus_internal.h>
 
 static void modbus_serial_tx_on(struct modbus_context *ctx)
@@ -238,37 +239,6 @@ static void modbus_ascii_tx_adu(struct modbus_context *ctx)
 }
 #endif
 
-static uint16_t modbus_rtu_crc16(uint8_t *src, size_t length)
-{
-	uint16_t crc = 0xFFFF;
-	uint8_t shiftctr;
-	bool flag;
-	uint8_t *pblock = src;
-
-	while (length > 0) {
-		length--;
-		crc ^= (uint16_t)*pblock++;
-		shiftctr = 8;
-		do {
-			/* Determine if the shift out of rightmost bit is 1 */
-			flag = (crc & 0x0001) ? true : false;
-			/* Shift CRC to the right one bit. */
-			crc >>= 1;
-			/*
-			 * If bit shifted out of rightmost bit was a 1
-			 * exclusive OR the CRC with the generating polynomial.
-			 */
-			if (flag == true) {
-				crc ^= MODBUS_CRC16_POLY;
-			}
-
-			shiftctr--;
-		} while (shiftctr > 0);
-	}
-
-	return crc;
-}
-
 /* Copy Modbus RTU frame and check if the CRC is valid. */
 static int modbus_rtu_rx_adu(struct modbus_context *ctx)
 {
@@ -296,8 +266,8 @@ static int modbus_rtu_rx_adu(struct modbus_context *ctx)
 
 	ctx->rx_adu.crc = sys_get_le16(&cfg->uart_buf[crc_idx]);
 	/* Calculate CRC over address, function code, and payload */
-	calc_crc = modbus_rtu_crc16(&cfg->uart_buf[0],
-				    cfg->uart_buf_ctr - sizeof(ctx->rx_adu.crc));
+	calc_crc = crc16_ansi(&cfg->uart_buf[0],
+			      cfg->uart_buf_ctr - sizeof(ctx->rx_adu.crc));
 
 	if (ctx->rx_adu.crc != calc_crc) {
 		LOG_WRN("Calculated CRC does not match received CRC");
@@ -320,8 +290,7 @@ static void rtu_tx_adu(struct modbus_context *ctx)
 
 	memcpy(data_ptr, ctx->tx_adu.data, ctx->tx_adu.length);
 
-	ctx->tx_adu.crc = modbus_rtu_crc16(&cfg->uart_buf[0],
-					     ctx->tx_adu.length + 2);
+	ctx->tx_adu.crc = crc16_ansi(&cfg->uart_buf[0], ctx->tx_adu.length + 2);
 	sys_put_le16(ctx->tx_adu.crc,
 		     &cfg->uart_buf[ctx->tx_adu.length + 2]);
 	tx_bytes += 2;