iommu/amd: Add workaround for event log erratum

Due to a recent erratum it can happen that the head pointer
of the event-log is updated before the actual event-log
entry is written. This patch implements the recommended
workaround.

Cc: stable@vger.kernel.org	# all stable kernels
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

drivers/iommu/amd_iommu.c

@@ -450,12 +450,27 @@ static void dump_command(unsigned long phys_addr)
 
 static void iommu_print_event(struct amd_iommu *iommu, void *__evt)
 {
-	u32 *event = __evt;
-	int type  = (event[1] >> EVENT_TYPE_SHIFT)  & EVENT_TYPE_MASK;
-	int devid = (event[0] >> EVENT_DEVID_SHIFT) & EVENT_DEVID_MASK;
-	int domid = (event[1] >> EVENT_DOMID_SHIFT) & EVENT_DOMID_MASK;
-	int flags = (event[1] >> EVENT_FLAGS_SHIFT) & EVENT_FLAGS_MASK;
-	u64 address = (u64)(((u64)event[3]) << 32) | event[2];
+	int type, devid, domid, flags;
+	volatile u32 *event = __evt;
+	int count = 0;
+	u64 address;
+
+retry:
+	type    = (event[1] >> EVENT_TYPE_SHIFT)  & EVENT_TYPE_MASK;
+	devid   = (event[0] >> EVENT_DEVID_SHIFT) & EVENT_DEVID_MASK;
+	domid   = (event[1] >> EVENT_DOMID_SHIFT) & EVENT_DOMID_MASK;
+	flags   = (event[1] >> EVENT_FLAGS_SHIFT) & EVENT_FLAGS_MASK;
+	address = (u64)(((u64)event[3]) << 32) | event[2];
+
+	if (type == 0) {
+		/* Did we hit the erratum? */
+		if (++count == LOOP_TIMEOUT) {
+			pr_err("AMD-Vi: No event written to event log\n");
+			return;
+		}
+		udelay(1);
+		goto retry;
+	}
 
 	printk(KERN_ERR "AMD-Vi: Event logged [");
 
@@ -508,6 +523,8 @@ static void iommu_print_event(struct amd_iommu *iommu, void *__evt)
 	default:
 		printk(KERN_ERR "UNKNOWN type=0x%02x]\n", type);
 	}
+
+	memset(__evt, 0, 4 * sizeof(u32));
 }
 
 static void iommu_poll_events(struct amd_iommu *iommu)