can: mcp251xfd: mcp251xfd_tx_obj_from_skb(): don't copy data for RTR CAN frames in TX-path

In Classical CAN there are RTR frames. RTR frames have the RTR bit set, may
have a dlc != 0, but contain no data.

This patch optimizes the TX-path to not copy any data for RTR frames.

Link: https://lore.kernel.org/r/20210114153448.1506901-5-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

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

@@ -2304,7 +2304,7 @@ mcp251xfd_tx_obj_from_skb(const struct mcp251xfd_priv *priv,
 	union mcp251xfd_tx_obj_load_buf *load_buf;
 	u8 dlc;
 	u32 id, flags;
-	int pad_len, len;
+	int len_sanitized = 0, len;
 
 	if (cfd->can_id & CAN_EFF_FLAG) {
 		u32 sid, eid;
@@ -2331,6 +2331,8 @@ mcp251xfd_tx_obj_from_skb(const struct mcp251xfd_priv *priv,
 
 	if (cfd->can_id & CAN_RTR_FLAG)
 		flags |= MCP251XFD_OBJ_FLAGS_RTR;
+	else
+		len_sanitized = canfd_sanitize_len(cfd->len);
 
 	/* CANFD */
 	if (can_is_canfd_skb(skb)) {
@@ -2356,14 +2358,18 @@ mcp251xfd_tx_obj_from_skb(const struct mcp251xfd_priv *priv,
 	memcpy(hw_tx_obj->data, cfd->data, cfd->len);
 
 	/* Clear unused data at end of CAN frame */
-	pad_len = can_fd_dlc2len(dlc) - cfd->len;
-	if (MCP251XFD_SANITIZE_CAN && pad_len)
+	if (MCP251XFD_SANITIZE_CAN && len_sanitized) {
+		int pad_len;
+
+		pad_len = len_sanitized - cfd->len;
+		if (pad_len)
 			memset(hw_tx_obj->data + cfd->len, 0x0, pad_len);
+	}
 
 	/* Number of bytes to be written into the RAM of the controller */
 	len = sizeof(hw_tx_obj->id) + sizeof(hw_tx_obj->flags);
 	if (MCP251XFD_SANITIZE_CAN)
-		len += round_up(can_fd_dlc2len(dlc), sizeof(u32));
+		len += round_up(len_sanitized, sizeof(u32));
 	else
 		len += round_up(cfd->len, sizeof(u32));