can: mcp251xfd: mcp251xfd_handle_rxif_ring_uinc(): factor out in separate function

This is a preparation patch.

Sending the UINC messages followed by incrementing the tail pointer
will be called in more than one place in upcoming patches, so factor
this out into a separate function.

Also make mcp251xfd_handle_rxif_ring_uinc() safe to be called with a
"len" of 0.
Tested-by: Stefan Althöfer <Stefan.Althoefer@janztec.com>
Tested-by: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-rx.c

@@ -197,6 +197,37 @@ mcp251xfd_rx_obj_read(const struct mcp251xfd_priv *priv,
 	return err;
 }
 
+static int
+mcp251xfd_handle_rxif_ring_uinc(const struct mcp251xfd_priv *priv,
+				struct mcp251xfd_rx_ring *ring,
+				u8 len)
+{
+	int offset;
+	int err;
+
+	if (!len)
+		return 0;
+
+	/* Increment the RX FIFO tail pointer 'len' times in a
+	 * single SPI message.
+	 *
+	 * Note:
+	 * Calculate offset, so that the SPI transfer ends on
+	 * the last message of the uinc_xfer array, which has
+	 * "cs_change == 0", to properly deactivate the chip
+	 * select.
+	 */
+	offset = ARRAY_SIZE(ring->uinc_xfer) - len;
+	err = spi_sync_transfer(priv->spi,
+				ring->uinc_xfer + offset, len);
+	if (err)
+		return err;
+
+	ring->tail += len;
+
+	return 0;
+}
+
 static int
 mcp251xfd_handle_rxif_ring(struct mcp251xfd_priv *priv,
 			   struct mcp251xfd_rx_ring *ring)
@@ -210,8 +241,6 @@ mcp251xfd_handle_rxif_ring(struct mcp251xfd_priv *priv,
 		return err;
 
 	while ((len = mcp251xfd_get_rx_linear_len(ring))) {
-		int offset;
-
 		rx_tail = mcp251xfd_get_rx_tail(ring);
 
 		err = mcp251xfd_rx_obj_read(priv, ring, hw_rx_obj,
@@ -227,22 +256,9 @@ mcp251xfd_handle_rxif_ring(struct mcp251xfd_priv *priv,
 				return err;
 		}
 
-		/* Increment the RX FIFO tail pointer 'len' times in a
-		 * single SPI message.
-		 *
-		 * Note:
-		 * Calculate offset, so that the SPI transfer ends on
-		 * the last message of the uinc_xfer array, which has
-		 * "cs_change == 0", to properly deactivate the chip
-		 * select.
-		 */
-		offset = ARRAY_SIZE(ring->uinc_xfer) - len;
-		err = spi_sync_transfer(priv->spi,
-					ring->uinc_xfer + offset, len);
+		err = mcp251xfd_handle_rxif_ring_uinc(priv, ring, len);
 		if (err)
 			return err;
-
-		ring->tail += len;
 	}
 
 	return 0;