LinuxFDTBlob < DULG

%SECTION0{name=LinuxFDTBlob}% Flattened Device Tree Blob

Linux kernel expects certain information on the hardware that it runs on. For kernels compiled with fdt support, this information has the form of a device tree, which is based on the Open Firmware specification. Bootloaders like U-Boot that do not implement the Open Firmware API, are expected to pass to the kernel a binary form of the flattened device tree, commonly referred to as _FDT blob_ or simply _the blob_.

Device trees are defined in human-readable text files, which are part of the Linux 2.6 source tree. Device tree source for the %BOARDNAME% board is found in =arch/%ARCH%/boot/dts/%BOARD%.dts= file. Before the device tree can be passed to the kernel, it has to be compiled to the binary form by the =dtc= compiler. The =dtc= compiler is included with the Linux kernel since 2.6.25.  Since 2.6.26 there is also a simple makefile rule to generate the blob:

<pre>
make ARCH=%ARCH% CROSS_COMPILE=%CROSS_COMPILE% %BOARD%.dtb
</pre>

After the blob has been compiled, it has to be transferred from
where it was built (="arch/%ARCH%/boot/%BOARD%.dtb"=)
to target's memory, for example over the TFTP protocol using U-Boot's =tftp= command. Then, the blob is passed to the kernel by the =bootm= command, and its address in memory is one of the arguments to =bootm= - refer to the description of this command in UBootCmdGroupExec for more details.

Note that U-Boot makes some automatic modifications to the blob before passing it to the kernel - mainly adding and modifying information that is learnt at run-time.  See the board-specific function ft_board_setup() and related routines.

U-Boot also has provisions to alter a flattened device tree in arbitrary ways from the command line, refer to the description of the =fdt= commands found in UBootCmdFDT.

Notes:
   * Flattened Device Tree custodian's page at http://www.denx.de/wiki/U-Boot/UBootFdtInfo contains useful information, and a number of references.
   * At the time of this writing (September 2007) blob handling is still a very fresh feature and undergoing frequent changes. Reader is encouraged to watch the =u-boot-users= and =linuxppc-dev= mailing lists for important news (required version of the dtc compiler, blob compilation options, flattened device tree source file structure, etc.).
7.1. Introduction		1. Abstract		7.3. Passing Kernel Arguments
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