[PATCH v5] Documentation, dt, numa: Add note to empty NUMA node

[Gavin Shan <gshan@redhat.com>]

The empty memory nodes, where no memory resides in, are allowed.
For these empty memory nodes, the 'len' of 'reg' property is zero.
The NUMA node IDs are still valid and parsed, but memory may be
added to them through hotplug afterwards. I finds difficulty to
get where it's properly documented.

So lets add a section for empty memory nodes in NUMA binding
document. Also, the 'unit-address', equivalent to 'base-address'
in the 'reg' property of these empty memory nodes is suggested to
be the summation of highest memory address plus the NUMA node ID.

Signed-off-by: Gavin Shan <gshan@redhat.com>
---
v5: Separate section for empty memory node
---
 Documentation/devicetree/bindings/numa.txt | 61 +++++++++++++++++++++-
 1 file changed, 60 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/numa.txt b/Documentation/devicetree/bindings/numa.txt
index 21b35053ca5a..230c734af948 100644
--- a/Documentation/devicetree/bindings/numa.txt
+++ b/Documentation/devicetree/bindings/numa.txt
@@ -103,7 +103,66 @@ Example:
 		};
 
 ==============================================================================
-4 - Example dts
+4 - Empty memory nodes
+==============================================================================
+
+Empty memory nodes, which no memory resides in, are allowed. The 'length'
+field of the 'reg' property is zero, but the 'base-address' is a dummy
+address and invalid. The 'base-address' could be the summation of highest
+memory address plus the NUMA node ID. However, the NUMA node IDs and
+distance maps are still valid and memory may be added into them through
+hotplug afterwards.
+
+Example:
+
+	memory@0 {
+		device_type = "memory";
+		reg = <0x0 0x0 0x0 0x80000000>;
+		numa-node-id = <0>;
+	};
+
+	memory@0x80000000 {
+		device_type = "memory";
+		reg = <0x0 0x80000000 0x0 0x80000000>;
+		numa-node-id = <1>;
+	};
+
+	/* Empty memory node */
+	memory@0x100000002 {
+		device_type = "memory";
+		reg = <0x1 0x2 0x0 0x0>;
+		numa-node-id = <2>;
+	};
+
+	/* Empty memory node */
+	memory@0x100000003 {
+		device_type = "memory";
+		reg = <0x1 0x3 0x0 0x0>;
+		numa-node-id = <3>;
+	};
+
+	distance-map {
+		compatible = "numa-distance-map-v1";
+		distance-matrix = <0 0  10>,
+				  <0 1  20>,
+				  <0 2  40>,
+				  <0 3  20>,
+				  <1 0  20>,
+				  <1 1  10>,
+				  <1 2  20>,
+				  <1 3  40>,
+				  <2 0  40>,
+				  <2 1  20>,
+				  <2 2  10>,
+				  <2 3  20>,
+				  <3 0  20>,
+				  <3 1  40>,
+				  <3 2  20>,
+				  <3 3  10>;
+	};
+
+==============================================================================
+5 - Example dts
 ==============================================================================
 
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-- 
2.23.0