DTB/DTBO Partitions

If your DTB/DTBO is in a unique partition, e.g. dtb and dtbo partition, use the following table structure and header format:

Figure 1. Example dtb/dtbo partition layout (for AVB signature, see Security).

Data structures

The dt_table_header is only for the dtb/dtbo partition; you CANNOT append this format after the end of image.gz. If you have a single DTB/DTBO, you must still use this format (and the dt_entry_size in dt_table_header is 1).

#define DT_TABLE_MAGIC 0xd7b7ab1e

struct dt_table_header {
  uint32_t magic;             // DT_TABLE_MAGIC
  uint32_t total_size;        // includes dt_table_header + all dt_table_entry
                              // and all dtb/dtbo
  uint32_t header_size;       // sizeof(dt_table_header)

  uint32_t dt_entry_size;     // sizeof(dt_table_entry)
  uint32_t dt_entry_count;    // number of dt_table_entry
  uint32_t dt_entries_offset; // offset to the first dt_table_entry
                              // from head of dt_table_header

  uint32_t page_size;         // flash page size we assume
  uint32_t reserved[1];       // must be zero
};

struct dt_table_entry {
  uint32_t dt_size;
  uint32_t dt_offset;         // offset from head of dt_table_header

  uint32_t id;                // optional, must be zero if unused
  uint32_t rev;               // optional, must be zero if unused
  uint32_t custom[4];         // optional, must be zero if unused
};

To read all dt_table_entry, use the dt_entry_size, dt_entry_count, and dt_entries_offset. Example:

my_read(entries_buf,
        header_addr + header->dt_entries_offset,
        header->dt_entry_size * header->dt_entry_count);

The id, rev, custom in dt_table_entry are optional hardware identifications of the device tree the bootloader can use to efficiently identify the DTB/DTBO to load. If the bootloader requires additional information, put it in the DTB/DTBO where bootloader can read it by parsing DTB/DTBO (see the sample code below).

Sample code

The following sample code checks the hardware identification in bootloader.

  • The check_dtbo() function checks the hardware identification. It first checks the data in struct dt_table_entry (id, rev, etc.). If this data is not enough, it loads dtb data into memory and checks the value in dtb.
  • The values of my_hw_information and soc_id properties are parsed in the root node (example in my_dtbo_1.dts).
    [my_dtbo_1.dts]
    /dts-v1/;
    /plugin/;
    
    / {
      /* As DTS design, these properties only for loader, won't overlay */
      compatible = "board_manufacturer,board_model";
    
      /* These properties are examples */
      board_id = <0x00010000>;
      board_rev = <0x00010001>;
      another_hw_information = "some_data";
      soc_id = <0x68000000>;
      ...
    };
    
    &device@0 {
      value = <0x1>;
      status = "okay";
    };
    
    
    [my_bootloader.c]
    int check_dtbo(const dt_table_entry *entry, uint32_t header_addr) {
      ...
      if (entry->id != ... || entry->rev != ...) {
        ...
      }
      ...
      void * fdt_buf = my_load_dtb(header_addr + entry->dt_offset, entry->dt_size);
      int root_node_off = fdt_path_offset(fdt_buf, "/");
      ...
      const char *my_hw_information =
        (const char *)fdt_getprop(fdt_buf, root_node_off, "my_hw_information", NULL);
      if (my_hw_information != NULL && strcmp(my_hw_information, ...) != 0) {
        ...
      }
      const fdt32_t *soc_id = fdt_getprop(fdt_buf, root_node_off, "soc_id", NULL);
      if (soc_id != NULL && *soc_id != ...) {
        ...
      }
      ...
    }
    

mkdtimg

mkdtimg is a tool for creating dtb/dtbo images (source code at system/libufdt in AOSP). mkdtimg supports several commands, including create, cfg_create, and dump.

create

Use the create command to create a dtb/dtbo image:

mkdtimg create <image_filename> (<global-option>...) \
    <ftb1_filename> (<entry1_option>...) \
    <ftb2_filename> (<entry2_option>...) \
    ...

ftbX_filename generates a dt_table_entry in the image. entryX_options are the values to assign to dt_table_entry. These values can be any of the following:

--id=<number|path>
--rev=<number|path>
--custom0=<number|path>
--custom1=<number|path>
--custom2=<number|path>
--custom3=<number|path>

Number values can be a 32-bit digit (such as 68000) or a hex number (such as 0x6800). Alternatively, you can specify a path using the format:

<full_node_path>:<property_name>

For example, /board/:id. mkdtimg reads the value from the path in the DTB/DTBO file and assigns the value (32-bit) to a relative property in dt_table_entry. Alternatively, you can give a global_option as a default option for all entries. The default value of page_size in dt_table_header is 2048; use global_option --page_size=<number> to assign a different value.

Example:

[board1.dts]
/dts-v1/;
/plugin/;

/ {
  compatible = "board_manufacturer,board_model";
  board_id = <0x00010000>;
  board_rev = <0x00010001>;
  another_hw_information = "some_data";
  ...
};

&device@0 {
  value = <0x1>;
  status = "okay";
};


mkdtimg create dtbo.img --id=/:board_id --custom0=0xabc \
  board1.dtbo \
  board2.dtbo --id=0x6800 \
  board3.dtbo --id=0x6801 --custom0=0x123
  • First dt_table_entry (board1.dtbo) id is 0x00010000 and custom[0] is 0x00000abc.
  • Second id is 0x00006800 and custom[0] is 0x00000abc.
  • Third id is 0x00006801 and custom[0] is 0x00000123.
  • All others use the default value (0).

cfg_create

The cfg_create command creates an image with a config file in the following format:

# global options
  <global_option>
  ...
# entries
<ftb1_filename>     # comment
  <entry1_option>   # comment
  ...
<ftb2_filename>
  <entry2_option>
  ...
...

Options global_option and entryX_option must start with one or more space characters (these options are the same as create options, without the -- prefix). Empty lines or lines beginning with # are ignored.

Example:

[dtboimg.cfg]
# global options
  id=/:board_id
  rev=/:board_rev
  custom0=0xabc

board1.dtbo

board2.dtbo
  id=0x6800       # override the value of id in global options

board2.dtbo
  id=0x6801       # override the value of id in global options
  custom0=0x123   # override the value of custom0 in global options


mkdtimg cfg_create dtbo.img dtboimg.cfg

mkdtimg does not handle alignment for .dtb/.dtbo files but rather appends them to the image. When you use dtc to compile .dts to .dtb/.dtbo, you must add option -a. For example, adding the option -a 4 adds padding so the size of .dtb/.dtbo aligns to 4 bytes.

Several DT table entries can share a .dtb/.dtbo. If you use the same filename for different entries, it stores only one content in the image with same dt_offset and dt_size. This is useful when using different hardware with identical DTs.

dump

For dtb/dtbo images, use the dump command to print the information in the image. Example:

mkdtimg dump dtbo.img
dt_table_header:
               magic = d7b7ab1e
          total_size = 1300
         header_size = 32
       dt_entry_size = 32
      dt_entry_count = 3
   dt_entries_offset = 32
           page_size = 2048
         reserved[0] = 00000000
dt_table_entry[0]:
             dt_size = 380
           dt_offset = 128
                  id = 00010000
                 rev = 00010001
           custom[0] = 00000abc
           custom[1] = 00000000
           custom[2] = 00000000
           custom[3] = 00000000
           (FDT)size = 380
     (FDT)compatible = board_manufacturer,board_model
...