firewire: ohci: properly clear posted write errors

To remove the error information from the controller's queue and to allow
more posted writes, the driver has to read the failed posted write
address before clearing the postedWriteErr interrupt bit.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

(Stefan R:) The spec is somewhat fuzzy about the actual requirements.
To err on the safe side, let's do these two read accesses.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/ohci.c

@@ -1790,8 +1790,12 @@ static irqreturn_t irq_handler(int irq, void *data)
 	if (!event || !~event)
 		return IRQ_NONE;
 
-	/* busReset must not be cleared yet, see OHCI 1.1 clause 7.2.3.2 */
-	reg_write(ohci, OHCI1394_IntEventClear, event & ~OHCI1394_busReset);
+	/*
+	 * busReset and postedWriteErr must not be cleared yet
+	 * (OHCI 1.1 clauses 7.2.3.2 and 13.2.8.1)
+	 */
+	reg_write(ohci, OHCI1394_IntEventClear,
+		  event & ~(OHCI1394_busReset | OHCI1394_postedWriteErr));
 	log_irqs(event);
 
 	if (event & OHCI1394_selfIDComplete)
@@ -1831,8 +1835,13 @@ static irqreturn_t irq_handler(int irq, void *data)
 		fw_error("Register access failure - "
 			 "please notify linux1394-devel@lists.sf.net\n");
 
-	if (unlikely(event & OHCI1394_postedWriteErr))
+	if (unlikely(event & OHCI1394_postedWriteErr)) {
+		reg_read(ohci, OHCI1394_PostedWriteAddressHi);
+		reg_read(ohci, OHCI1394_PostedWriteAddressLo);
+		reg_write(ohci, OHCI1394_IntEventClear,
+			  OHCI1394_postedWriteErr);
 		fw_error("PCI posted write error\n");
+	}
 
 	if (unlikely(event & OHCI1394_cycleTooLong)) {
 		if (printk_ratelimit())