serdev: make synchronous write return bytes written

Make the synchronous serdev_device_write() helper behave analogous to
the asynchronous serdev_device_write_buf() by returning the number of
bytes written (or rather buffered) also on timeout.

This will allow drivers to distinguish the case where data was partially
written from the case where no data was written.

Also update the only two users that checked the return value.

Signed-off-by: Johan Hovold <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/gnss/serial.c

···
 
 	/* write is only buffered synchronously */
 	ret = serdev_device_write(serdev, buf, count, MAX_SCHEDULE_TIMEOUT);
-	if (ret < 0)
+	if (ret < 0 || ret < count)
 		return ret;
 
 	/* FIXME: determine if interrupted? */

drivers/gnss/sirf.c

···
 
 	/* write is only buffered synchronously */
 	ret = serdev_device_write(serdev, buf, count, MAX_SCHEDULE_TIMEOUT);
-	if (ret < 0)
+	if (ret < 0 || ret < count)
 		return ret;
 
 	/* FIXME: determine if interrupted? */

drivers/tty/serdev/core.c

···
 			unsigned long timeout)
 {
 	struct serdev_controller *ctrl = serdev->ctrl;
+	int written = 0;
 	int ret;
 
 	if (!ctrl || !ctrl->ops->write_buf || !serdev->ops->write_wakeup)
···
 		if (ret < 0)
 			break;
 
+		written += ret;
 		buf += ret;
 		count -= ret;
-
 	} while (count &&
 		 (timeout = wait_for_completion_timeout(&serdev->write_comp,
 							timeout)));
 	mutex_unlock(&serdev->write_lock);
-	return ret < 0 ? ret : (count ? -ETIMEDOUT : 0);
+
+	if (ret < 0)
+		return ret;
+
+	if (timeout == 0 && written == 0)
+		return -ETIMEDOUT;
+
+	return written;
 }
 EXPORT_SYMBOL_GPL(serdev_device_write);