of: base: Skip CPU nodes with "fail"/"fail-..." status

Allow fully disabling CPU nodes using status = "fail".

This allows a bootloader to change the number of available CPUs (for
example when a common DTS is used for SoC variants with different numbers
of cores) without deleting the nodes altogether, which could require
additional fixups to avoid dangling phandle references.

Unknown status values (everything that is not "okay"/"ok", "disabled" or
"fail"/"fail-...") will continue to be interpreted like "disabled",
meaning that the CPU can be enabled during boot.

References:
- https://www.spinics.net/lists/devicetree-spec/msg01007.html
- https://github.com/devicetree-org/dt-schema/pull/61

Link: https://lore.kernel.org/all/CAL_Jsq+1LsTBdVaODVfmB0eme2jMpNL4VgKk-OM7rQWyyF0Jbw@mail.gmail.com/Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Tested-by: Sai Prakash Ranjan <quic_saipraka@quicinc.com>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Link: https://lore.kernel.org/r/20211122114536.2981-1-matthias.schiffer@ew.tq-group.comSigned-off-by: Rob Herring <robh@kernel.org>

drivers/of/base.c

@@ -650,6 +650,28 @@ bool of_device_is_available(const struct device_node *device)
 }
 EXPORT_SYMBOL(of_device_is_available);
 
+/**
+ *  __of_device_is_fail - check if a device has status "fail" or "fail-..."
+ *
+ *  @device: Node to check status for, with locks already held
+ *
+ *  Return: True if the status property is set to "fail" or "fail-..." (for any
+ *  error code suffix), false otherwise
+ */
+static bool __of_device_is_fail(const struct device_node *device)
+{
+	const char *status;
+
+	if (!device)
+		return false;
+
+	status = __of_get_property(device, "status", NULL);
+	if (status == NULL)
+		return false;
+
+	return !strcmp(status, "fail") || !strncmp(status, "fail-", 5);
+}
+
 /**
  *  of_device_is_big_endian - check if a device has BE registers
  *
@@ -796,6 +818,9 @@ EXPORT_SYMBOL(of_get_next_available_child);
  * of_get_next_cpu_node - Iterate on cpu nodes
  * @prev:	previous child of the /cpus node, or NULL to get first
  *
+ * Unusable CPUs (those with the status property set to "fail" or "fail-...")
+ * will be skipped.
+ *
  * Return: A cpu node pointer with refcount incremented, use of_node_put()
  * on it when done. Returns NULL when prev is the last child. Decrements
  * the refcount of prev.
@@ -817,6 +842,8 @@ struct device_node *of_get_next_cpu_node(struct device_node *prev)
 		of_node_put(node);
 	}
 	for (; next; next = next->sibling) {
+		if (__of_device_is_fail(next))
+			continue;
 		if (!(of_node_name_eq(next, "cpu") ||
 		      __of_node_is_type(next, "cpu")))
 			continue;