IB/ipath: Correctly describe workaround for TID write chip bug

This is a comment change, only, correcting the comment to match the
implemented workaround, rather than the original workaround, and
clarifying why it's needed.
Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/ipath/ipath_iba6120.c

@@ -1143,11 +1143,14 @@ static void ipath_pe_put_tid(struct ipath_devdata *dd, u64 __iomem *tidptr,
 			pa |= 2 << 29;
 	}
 
-	/* workaround chip bug 9437 by writing each TID twice
-	 * and holding a spinlock around the writes, so they don't
-	 * intermix with other TID (eager or expected) writes
-	 * Unfortunately, this call can be done from interrupt level
-	 * for the port 0 eager TIDs, so we have to use irqsave
+	/*
+	 * Workaround chip bug 9437 by writing the scratch register
+	 * before and after the TID, and with an io write barrier.
+	 * We use a spinlock around the writes, so they can't intermix
+	 * with other TID (eager or expected) writes (the chip bug
+	 * is triggered by back to back TID writes). Unfortunately, this
+	 * call can be done from interrupt level for the port 0 eager TIDs,
+	 * so we have to use irqsave locks.
 	 */
 	spin_lock_irqsave(&dd->ipath_tid_lock, flags);
 	ipath_write_kreg(dd, dd->ipath_kregs->kr_scratch, 0xfeeddeaf);