platform/chrome: cros_ec_proto: check for NULL transfer function

As new transfer mechanisms are added to the EC codebase, they may
not support v2 of the EC protocol.

If the v3 initial handshake transfer fails, the kernel will try
and call cmd_xfer as a fallback. If v2 is not supported, cmd_xfer
will be NULL, and the code will end up causing a kernel panic.

Add a check for NULL before calling the transfer function, along
with a helpful comment explaining how one might end up in this
situation.
Signed-off-by: Enrico Granata <egranata@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

drivers/platform/chrome/cros_ec_proto.c

@@ -56,6 +56,17 @@ static int send_command(struct cros_ec_device *ec_dev,
 	else
 		xfer_fxn = ec_dev->cmd_xfer;
 
+	if (!xfer_fxn) {
+		/*
+		 * This error can happen if a communication error happened and
+		 * the EC is trying to use protocol v2, on an underlying
+		 * communication mechanism that does not support v2.
+		 */
+		dev_err_once(ec_dev->dev,
+			     "missing EC transfer API, cannot send command\n");
+		return -EIO;
+	}
+
 	ret = (*xfer_fxn)(ec_dev, msg);
 	if (msg->result == EC_RES_IN_PROGRESS) {
 		int i;