[PATCH v2] Documentation, dt, numa: Add note to empty NUMA node

[Gavin Shan <gshan@redhat.com>]

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

So lets add note to empty NUMA nodes in the NUMA binding doc.

Signed-off-by: Gavin Shan <gshan@redhat.com>
---
v2: Update to address Randy's comments
---
 Documentation/devicetree/bindings/numa.txt | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/devicetree/bindings/numa.txt b/Documentation/devicetree/bindings/numa.txt
index 21b35053ca5a..08e361f9954c 100644
--- a/Documentation/devicetree/bindings/numa.txt
+++ b/Documentation/devicetree/bindings/numa.txt
@@ -109,6 +109,10 @@ Example:
 Dual socket system consists of 2 boards connected through ccn bus and
 each board having one socket/soc of 8 cpus, memory and pci bus.
 
+Note that the empty NUMA nodes, which no memory resides in period, are
+allowed. Their NUMA node IDs are still valid so that memory can be added
+into these NUMA nodes through hotplug afterwards.
+
 	memory@c00000 {
 		device_type = "memory";
 		reg = <0x0 0xc00000 0x0 0x80000000>;
-- 
2.23.0