+ /*
+ * Update request index to mailbox4 (Request Queue In).
+ * The mmiowb() ensures that this write is ordered with writes by other
+ * CPUs. Without the mmiowb(), it is possible for the following:
+ * CPUA posts write of index 5 to mailbox4
+ * CPUA releases host lock
+ * CPUB acquires host lock
+ * CPUB posts write of index 6 to mailbox4
+ * On PCI bus, order reverses and write of 6 posts, then index 5,
+ * causing chip to issue full queue of stale commands
+ * The mmiowb() prevents future writes from crossing the barrier.
+ * See Documentation/DocBook/deviceiobook.tmpl for more information.
+ */