aarch64: add function to get PSCI method from device tree

We need to get the "method" property of the psci node in the device tree to
determine if we should use hvc or smc to issue PSCI commands.

https://www.kernel.org/doc/Documentation/devicetree/bindings/arm/psci.txt

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Message-Id: <20210219215249.25453-3-stewart.hildebrand@dornerworks.com>

arch/aarch64/arch-dtb.cc

@@ -639,6 +639,34 @@ bool dtb_get_vmm_is_xen()
     return fdt_node_offset_by_compatible(dtb, -1, "xen,xen") >= 0;
 }
 
+const char *dtb_get_psci_method()
+{
+    const char *psci_compatible[] = {
+            "arm,psci",
+            "arm,psci-0.2",
+            "arm,psci-1.0",
+    };
+    unsigned int i;
+    int node;
+
+    for (i = 0; i < sizeof(psci_compatible)/sizeof(psci_compatible[0]); i++) {
+        node = fdt_node_offset_by_compatible(dtb, -1, psci_compatible[i]);
+        if (node >= 0)
+            break;
+    }
+
+    if (node < 0) {
+        return NULL;
+    }
+
+    const char *method = (char *)fdt_getprop(dtb, node, "method", NULL);
+    if (!method) {
+        return NULL;
+    }
+
+    return method;
+}
+
 void  __attribute__((constructor(init_prio::dtb))) dtb_setup()
 {
     void *olddtb;

arch/aarch64/arch-dtb.hh

@@ -153,4 +153,11 @@ bool dtb_get_pci_irqmap(u32 *bdfs, int *irq_ids, int n);
 
 bool dtb_get_vmm_is_xen();
 
+/* const char *dtb_get_psci_method();
+ *
+ * Returns the PSCI method, "hvc" or "smc", if found in the device tree.
+ * Returne NULL otherwise.
+ */
+const char *dtb_get_psci_method();
+
 #endif /* ARCH_DTB_HH */