[PATCH v2 1/2] of/fdt: export fdt blob as /sys/firmware/fdt

Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
that was passed to the kernel by the bootloader. This allows userland
applications such as kexec to access the raw binary. The blob needs to
be preserved as early as possible by calling preserve_fdt().

The fact that this node does not reside under /sys/firmware/device-tree
is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
communicate just the UEFI and ACPI entry points, but the FDT is never
unflattened and used to configure the system.

Signed-off-by: Ard Biesheuvel <redacted>
---
  drivers/of/fdt.c       | 34 ++++++++++++++++++++++++++++++++++
  include/linux/of_fdt.h |  2 ++
  2 files changed, 36 insertions(+)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index d1ffca8b34ea..e9ee3d5f7ea4 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c

@@ -22,6 +22,7 @@
  #include <linux/libfdt.h>
  #include <linux/debugfs.h>
  #include <linux/serial_core.h>
+#include <linux/sysfs.h>

  #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
  #include <asm/page.h>

@@ -1103,4 +1104,37 @@ static int __init of_flat_dt_debugfs_export_fdt(void)
  module_init(of_flat_dt_debugfs_export_fdt);
  #endif

+static u8 *raw_fdt_copy;
+
+void __init preserve_fdt(void)
+{
+	u32 fdt_size;
+
+	fdt_size = fdt_totalsize(initial_boot_params);
+	raw_fdt_copy = memcpy(__va(memblock_alloc(fdt_size, PAGE_SIZE)),
+			      initial_boot_params, fdt_size);
+}
+
+#ifdef CONFIG_SYSFS
+static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
+			       struct bin_attribute *bin_attr,
+			       char *buf, loff_t off, size_t count)
+{
+	memcpy(buf, raw_fdt_copy + off, count);