[PATCHv5 2/4] ARM: mm: add support for HW coherent systems in PL310

[Thomas Petazzoni]

When a PL310 cache is used on a system that provides hardware
coherency, the outer cache sync operation is useless, and can be
skipped. Moreover, on some systems, it is harmful as it causes
deadlocks between the Marvell coherency mechanism, the Marvell PCIe
controller and the Cortex-A9.

To avoid this, this commit introduces a new Device Tree property
'arm,io-coherent' for the L2 cache controller node, valid only for the
PL310 cache. It identifies the usage of the PL310 cache in an I/O
coherent configuration. Internally, it makes the driver disable the
outer cache sync operation.

Note that technically speaking, a fully coherent system wouldn't
require any of the other .outer_cache operations. However, in
practice, when booting secondary CPUs, these are not yet coherent, and
therefore a set of cache maintenance operations are necessary at this
point. This explains why we keep the other .outer_cache operations and
only ->sync is disabled.

While in theory any write to a PL310 register could cause the
deadlock, in practice, disabling ->sync is sufficient to workaround
the deadlock, since the other cache maintenance operations are only
used in very specific situations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
---
 Documentation/devicetree/bindings/arm/l2cc.txt |  3 +++
 arch/arm/mm/cache-l2x0.c                       | 13 +++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/devicetree/bindings/arm/l2cc.txt b/Documentation/devicetree/bindings/arm/l2cc.txt
index b513cb8..af527ee 100644
--- a/Documentation/devicetree/bindings/arm/l2cc.txt
+++ b/Documentation/devicetree/bindings/arm/l2cc.txt
@@ -40,6 +40,9 @@ Optional properties:
 - arm,filter-ranges : <start length> Starting address and length of window to
   filter. Addresses in the filter window are directed to the M1 port. Other
   addresses will go to the M0 port.
+- arm,io-coherent : indicates that the system is operating in an hardware
+  I/O coherent mode. Valid only when the arm,pl310-cache compatible
+  string is used.
 - interrupts : 1 combined interrupt.
 - cache-id-part: cache id part number to be used if it is not present
   on hardware
diff --git a/arch/arm/mm/cache-l2x0.c b/arch/arm/mm/cache-l2x0.c
index 7abde2ce..30f4476 100644
--- a/arch/arm/mm/cache-l2x0.c
+++ b/arch/arm/mm/cache-l2x0.c
@@ -1005,6 +1005,19 @@ int __init l2x0_of_init(u32 aux_val, u32 aux_mask)
 
 	of_init = true;
 	memcpy(&outer_cache, &data->outer_cache, sizeof(outer_cache));
+
+	/*
+	 * outer sync operations are not needed when the system is I/O
+	 * coherent, and potentially harmful in certain situations
+	 * (PCIe/PL310 deadlock on Armada 375/38x due to hardware I/O
+	 * coherency). The other operations are kept because they are
+	 * infrequent (therefore do not cause the deadlock) and needed
+	 * for secondary CPU boot and other power management
+	 * activities.
+	 */
+	if (of_property_read_bool(np, "arm,io-coherent"))
+		outer_cache.sync = NULL;
+
 	l2x0_init(l2x0_base, aux_val, aux_mask);
 
 	return 0;
-- 
1.9.3