loader/efi/fdt: Add fdtdump command to access device tree

The fdtdump command allows dumping arbitrary device tree properties
and saving them to a variable similar to the smbios command.

This is useful in scripts where further actions such as selecting
a kernel or loading another device tree depend on the compatible
or model values of the device tree provided by the firmware.

For now only the root level properties of the dtb are exposed.

Signed-off-by: Tobias Heider <tobias.heider@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/loader/efi/fdt.c

@@ -1,6 +1,7 @@
 /*
  *  GRUB  --  GRand Unified Bootloader
  *  Copyright (C) 2013-2015  Free Software Foundation, Inc.
+ *  Copyright (C) 2024       Canonical, Ltd.
  *
  *  GRUB is free software: you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
@@ -18,9 +19,11 @@
 
 #include <grub/fdt.h>
 #include <grub/mm.h>
+#include <grub/env.h>
 #include <grub/err.h>
 #include <grub/dl.h>
 #include <grub/command.h>
+#include <grub/extcmd.h>
 #include <grub/file.h>
 #include <grub/efi/efi.h>
 #include <grub/efi/fdtload.h>
@@ -36,6 +39,13 @@ static void *fdt;
                              sizeof (FDT_ADDR_CELLS_STRING) + \
                              sizeof (FDT_SIZE_CELLS_STRING))
 
+static const struct grub_arg_option options_fdtdump[] = {
+  {"prop",	'p', 0, N_("Get property."), N_("prop"), ARG_TYPE_STRING},
+  {"set",       '\0', 0, N_("Store the value in the given variable name."),
+                         N_("variable"), ARG_TYPE_STRING},
+  {0, 0, 0, 0, 0, 0}
+};
+
 void *
 grub_fdt_load (grub_size_t additional_size)
 {
@@ -167,10 +177,45 @@ out:
   return grub_errno;
 }
 
+static grub_err_t
+grub_cmd_fdtdump (grub_extcmd_context_t ctxt,
+                 int argc __attribute__ ((unused)),
+                 char **argv __attribute__ ((unused)))
+{
+  struct grub_arg_list *state = ctxt->state;
+  const char *value = NULL;
+  void *fw_fdt;
+
+  fw_fdt = grub_efi_get_firmware_fdt ();
+  if (fw_fdt == NULL)
+      return grub_error (GRUB_ERR_IO,
+                         N_("No device tree found"));
+
+  if (state[0].set)
+      value = grub_fdt_get_prop (fw_fdt, 0, state[0].arg, NULL);
+
+  if (value == NULL)
+    return grub_error (GRUB_ERR_OUT_OF_RANGE,
+                       N_("failed to retrieve the prop field"));
+
+  if (state[1].set)
+    grub_env_set (state[1].arg, value);
+  else
+    grub_printf ("%s\n", value);
+
+  return GRUB_ERR_NONE;
+}
+
 static grub_command_t cmd_devicetree;
+static grub_extcmd_t cmd_fdtdump;
 
 GRUB_MOD_INIT (fdt)
 {
+  cmd_fdtdump =
+    grub_register_extcmd ("fdtdump", grub_cmd_fdtdump, 0,
+                          N_("[-p] [--set variable]"),
+                          N_("Retrieve device tree information."),
+                          options_fdtdump);
   cmd_devicetree =
     grub_register_command_lockdown ("devicetree", grub_cmd_devicetree, 0,
 				    N_("Load DTB file."));
@@ -179,4 +224,5 @@ GRUB_MOD_INIT (fdt)
 GRUB_MOD_FINI (fdt)
 {
   grub_unregister_command (cmd_devicetree);
+  grub_unregister_extcmd (cmd_fdtdump);
 }