[PATCHv3 2/5] ARM: mvebu: disable I/O coherency on non-SMP situations on Armada 370/375/38x/XP

[Thomas Petazzoni]

Enabling the hardware I/O coherency on Armada 370, Armada 375, Armada
38x and Armada XP requires a certain number of conditions:

 - On Armada 370, the cache policy must be set to write-allocate.

 - On Armada 375, 38x and XP, the cache policy must be set to
   write-allocate, the pages must be mapped with the shareable
   attribute, and the SMP bit must be set

Currently, on Armada XP, when CONFIG_SMP is enabled, those conditions
are met. However, when Armada XP is used in a !CONFIG_SMP kernel, none
of these conditions are met. With Armada 370, the situation is worse:
since the processor is single core, regardless of whether CONFIG_SMP
or !CONFIG_SMP is used, the cache policy will be set to write-back by
the kernel and not write-allocate.

Since solving this problem turns out to be quite complicated, and we
don't want to let users with a mainline kernel known to have
infrequent but existing data corruptions, this commit proposes to
simply disable hardware I/O coherency in situations where it is known
not to work.

And basically, the is_smp() function of the kernel tells us whether it
is OK to enable hardware I/O coherency or not, so this commit slightly
refactors the coherency_type() function to return
COHERENCY_FABRIC_TYPE_NONE when is_smp() is false, or the appropriate
type of the coherency fabric in the other case.

Thanks to this, the I/O coherency fabric will no longer be used at all
in !CONFIG_SMP configurations. It will continue to be used in
CONFIG_SMP configurations on Armada XP, Armada 375 and Armada 38x
(which are multiple cores processors), but will no longer be used on
Armada 370 (which is a single core processor).

In the process, it simplifies the implementation of the
coherency_type() function, and adds a missing call to of_node_put().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Fixes: e60304f8cb7bb545e79fe62d9b9762460c254ec2 ("arm: mvebu: Add hardware I/O Coherency support")
Cc: <stable at vger.kernel.org> # v3.8+
Acked-by: Gregory CLEMENT <gregory.clement at free-electrons.com>
---
 arch/arm/mach-mvebu/coherency.c | 44 ++++++++++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 14 deletions(-)

diff --git a/arch/arm/mach-mvebu/coherency.c b/arch/arm/mach-mvebu/coherency.c
index 044b511..c31f4c0 100644
--- a/arch/arm/mach-mvebu/coherency.c
+++ b/arch/arm/mach-mvebu/coherency.c
@@ -361,25 +361,41 @@ 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;
+
+	of_node_put(np);
 
-	return COHERENCY_FABRIC_TYPE_NONE;
+	return type;
 }
 
 int coherency_available(void)
-- 
2.0.0