From: Sinan Kaya <redacted>
Date: Fri, 23 Mar 2018 12:51:47 -0400

It could if txdata->tx_db was not a union. There is a data dependency
between txdata->tx_db.data.prod and txdata->tx_db.raw. 

So, no reordering.

I don't see it that way, the code requires that:

 	txdata->tx_db.data.prod += nbd;

is visible before the doorbell update.

barrier() doesn't provide that.

Neither does writel_relaxed().  However plain writel() does.

Therefore the code is only correct as-is, and your change potentially
adds a reordering problem.