[PATCH v3 2/4] docs: Add OpenSBI DT configuration guide.

[Cheng Yang]

This patch add docs/opensbi_config.md which describes the
"/chosen/opensbi-config" DT node and properties

Signed-off-by: Cheng Yang <yangcheng.work at foxmail.com>
---
 docs/opensbi_config.md | 83 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 docs/opensbi_config.md

diff --git a/docs/opensbi_config.md b/docs/opensbi_config.md
new file mode 100644
index 0000000..0c42443
--- /dev/null
+++ b/docs/opensbi_config.md
@@ -0,0 +1,83 @@
+OpenSBI Device Tree Configuration Guideline
+==================================
+
+Some configurations of OpenSBI's Generic Platform can be described
+in the **device tree (DT) blob** (or flattened device tree) passed
+to the OpenSBI firmwares by the previous booting stage. OpenSBI will
+parse and use these configurations during the boot phase, but delete
+them from the device tree at the end of cold boot.
+
+### OpenSBI Configuration Node
+
+All nodes related to OpenSBI configuration should be under the OpenSBI
+configuration DT node. The **/chosen** DT node is the preferred parent
+of the OpenSBI configuration DT node.
+
+The DT properties of a domain configuration DT node are as follows:
+
+* **compatible** (Mandatory) - The compatible string of the OpenSBI
+  configuration. This DT property should have value *"opensbi,config"*
+
+* **cold-boot-harts** (Optional) - If a platform lacks an override
+  cold_boot_allowed() mechanism, this DT property specifies that a
+  set of harts is permitted to perform a cold boot. Otherwise, all
+  harts are allowed to cold boot.
+
+The OpenSBI Configuration Node will be deleted at the end of cold boot
+(replace the node (subtree) with nop tags).
+
+### Example
+
+```text
+    chosen {
+        opensbi-config {
+            compatible = "opensbi,config";
+            cold-boot-harts = <&cpu1 &cpu2 &cpu3 &cpu4>;
+        };
+    };
+
+    cpus {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        timebase-frequency = <10000000>;
+
+        cpu0: cpu at 0 {
+            device_type = "cpu";
+            reg = <0x00>;
+            compatible = "riscv";
+            ...
+        };
+
+        cpu1: cpu at 1 {
+            device_type = "cpu";
+            reg = <0x01>;
+            compatible = "riscv";
+            ...
+        };
+
+        cpu2: cpu at 2 {
+            device_type = "cpu";
+            reg = <0x02>;
+            compatible = "riscv";
+            ...
+        };
+
+        cpu3: cpu at 3 {
+            device_type = "cpu";
+            reg = <0x03>;
+            compatible = "riscv";
+            ...
+        };
+
+        cpu4: cpu at 4 {
+            device_type = "cpu";
+            reg = <0x04>;
+            compatible = "riscv";
+            ...
+        };
+    };
+
+    uart1: serial at 10011000 {
+        ...
+    };
+```
-- 
2.34.1