1 files changed, 36 insertions, 15 deletions

diff --git a/arch/arm/mach-mvebu/coherency.c b/arch/arm/mach-mvebu/coherency.c
index 044b51185fcc..1163a3e9accd 100644
--- a/arch/arm/mach-mvebu/coherency.c
+++ b/arch/arm/mach-mvebu/coherency.c
@@ -361,30 +361,51 @@ static int coherency_type(void)
 {
 	struct device_node *np;
 	const struct of_device_id *match;
+	int type;
 
-	np = of_find_matching_node_and_match(NULL, of_coherency_table, &match);
-	if (np) {
-		int type = (int) match->data;
+	/*
+	 * The coherency fabric is needed:
+	 * - For coherency between processors on Armada XP, so only
+	 *   when SMP is enabled.
+	 * - For coherency between the processor and I/O devices, but
+	 *   this coherency requires many pre-requisites (write
+	 *   allocate cache policy, shareable pages, SMP bit set) that
+	 *   are only meant in SMP situations.
+	 *
+	 * Note that this means that on Armada 370, there is currently
+	 * no way to use hardware I/O coherency, because even when
+	 * CONFIG_SMP is enabled, is_smp() returns false due to the
+	 * Armada 370 being a single-core processor. To lift this
+	 * limitation, we would have to find a way to make the cache
+	 * policy set to write-allocate (on all Armada SoCs), and to
+	 * set the shareable attribute in page tables (on all Armada
+	 * SoCs except the Armada 370). Unfortunately, such decisions
+	 * are taken very early in the kernel boot process, at a point
+	 * where we don't know yet on which SoC we are running.
 
-		/* Armada 370/XP coherency works in both UP and SMP */
-		if (type == COHERENCY_FABRIC_TYPE_ARMADA_370_XP)
-			return type;
+	 */
+	if (!is_smp())
+		return COHERENCY_FABRIC_TYPE_NONE;
 
-		/* Armada 375 coherency works only on SMP */
-		else if (type == COHERENCY_FABRIC_TYPE_ARMADA_375 && is_smp())
-			return type;
+	np = of_find_matching_node_and_match(NULL, of_coherency_table, &match);
+	if (!np)
+		return COHERENCY_FABRIC_TYPE_NONE;
 
-		/* Armada 380 coherency works only on SMP */
-		else if (type == COHERENCY_FABRIC_TYPE_ARMADA_380 && is_smp())
-			return type;
-	}
+	type = (int) match->data;
 
-	return COHERENCY_FABRIC_TYPE_NONE;
+	of_node_put(np);
+
+	return type;
 }
 
+/*
+ * As a precaution, we currently completely disable hardware I/O
+ * coherency, until enough testing is done with automatic I/O
+ * synchronization barriers to validate that it is a proper solution.
+ */
 int coherency_available(void)
 {
-	return coherency_type() != COHERENCY_FABRIC_TYPE_NONE;
+	return false;
 }
 
 int __init coherency_init(void)