IB/mthca: Use mmiowb() to avoid firmware commands getting jumbled up

Firmware commands are sent to the HCA by writing multiple words to a
command register block.  Access to this block of registers is
serialized with a mutex.  However, on large SGI systems, problems were
seen with multiple CPUs issuing FW commands at the same time, because
the writes to the register block may be reordered within the system
interconnect and reach the HCA in a different order than they were
issued (even with the mutex).  Fix this by adding an mmiowb() before
dropping the mutex.
Tested-by: Arthur Kepner <akepner@sgi.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_cmd.c

@@ -290,6 +290,12 @@ static int mthca_cmd_post(struct mthca_dev *dev,
 		err = mthca_cmd_post_hcr(dev, in_param, out_param, in_modifier,
 					 op_modifier, op, token, event);
 
+	/*
+	 * Make sure that our HCR writes don't get mixed in with
+	 * writes from another CPU starting a FW command.
+	 */
+	mmiowb();
+
 	mutex_unlock(&dev->cmd.hcr_mutex);
 	return err;
 }