[PATCH V2 Create empty OF root 1/1] of: create empty of root

[Lizhi Hou]

Xilinx Alveo PCIe accelerator cards use flattened device tree to describe
describe apertures in its PCIe BARs. Each device tree node represents an
aperture and each aperture is an hardware unit which requires a driver.
The product detail:
    https://www.xilinx.com/products/boards-and-kits/alveo.html

Thus a base device tree is required. Then the Alveo card driver can load
its flattened device tree and overlay it to the base tree. However device
tree does not always exist. To resolve this, add OF_EMPTY_ROOT config.
When it is selected and there is not a device tree, create an empty device
tree root node.

Signed-off-by: Sonal Santan <sonal.santan@xxxxxxxxxx>
Signed-off-by: Max Zhen <max.zhen@xxxxxxxxxx>
Signed-off-by: Lizhi Hou <lizhi.hou@xxxxxxxxxx>
---
 drivers/of/Kconfig         |  5 ++++
 drivers/of/Makefile        |  1 +
 drivers/of/of_empty_root.c | 51 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+)
 create mode 100644 drivers/of/of_empty_root.c

diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index 80b5fd44ab1c..452d2316b47b 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -94,4 +94,9 @@ config OF_DMA_DEFAULT_COHERENT
 	# arches should select this if DMA is coherent by default for OF devices
 	bool
 
+config OF_EMPTY_ROOT
+	# driver which requires at least the empty base device tree to
+	# overlay its own device nodes should select this.
+	bool
+
 endif # OF
diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index e0360a44306e..c65364f32935 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -12,6 +12,7 @@ obj-$(CONFIG_OF_RESERVED_MEM) += of_reserved_mem.o
 obj-$(CONFIG_OF_RESOLVE)  += resolver.o
 obj-$(CONFIG_OF_OVERLAY) += overlay.o
 obj-$(CONFIG_OF_NUMA) += of_numa.o
+obj-$(CONFIG_OF_EMPTY_ROOT) += of_empty_root.o
 
 ifdef CONFIG_KEXEC_FILE
 ifdef CONFIG_OF_FLATTREE
diff --git a/drivers/of/of_empty_root.c b/drivers/of/of_empty_root.c
new file mode 100644
index 000000000000..5c429c7a27bd
--- /dev/null
+++ b/drivers/of/of_empty_root.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Xilinx, Inc.
+ */
+
+#include <linux/of.h>
+#include <linux/slab.h>
+
+#include "of_private.h"
+
+static int __init of_root_init(void)
+{
+	struct property *prop = NULL;
+	struct device_node *node;
+	__be32 *val = NULL;
+
+	if (of_root)
+		return 0;
+
+	pr_info("Create empty OF root node\n");
+	node = kzalloc(sizeof(*node), GFP_KERNEL);
+	if (!node)
+		return -ENOMEM;
+	of_node_init(node);
+	node->full_name = "/";
+
+	prop = kcalloc(2, sizeof(*prop), GFP_KERNEL);
+	if (!prop)
+		return -ENOMEM;
+
+	val = kzalloc(sizeof(*val), GFP_KERNEL);
+	if (!val)
+		return -ENOMEM;
+	*val = cpu_to_be32(sizeof(void *) / sizeof(u32));
+
+	prop->name = "#address-cells";
+	prop->value = val;
+	prop->length = sizeof(u32);
+	of_add_property(node, prop);
+	prop++;
+	prop->name = "#size-cells";
+	prop->value = val;
+	prop->length = sizeof(u32);
+	of_add_property(node, prop);
+	of_root = node;
+	for_each_of_allnodes(node)
+		__of_attach_node_sysfs(node);
+
+	return 0;
+}
+pure_initcall(of_root_init);
-- 
2.27.0