[U-Boot] [PATCH v4 2/6] fdt: Add support for embedded device tree (CONFIG_OF_EMBED)

[Dirk Behme <dirk.behme@googlemail.com>]

Dear Simon,

On 15.10.2011 17:48, Simon Glass wrote:
> This new option allows U-Boot to embed a binary device tree into its image
> to allow run-time control of peripherals. This device tree is for U-Boot's
> own use and is not necessarily the same one as is passed to the kernel.
>
> The device tree compiler output should be placed in the $(obj)
> rooted tree. Since $(OBJCOPY) insists on adding the path to the
> generated symbol names, to ensure consistency it should be
> invoked from the directory where the .dtb file is located and
> given the input file name without the path.
>
> This commit contains my entry for the ugliest Makefile / shell interaction
> competition.
>
> Signed-off-by: Simon Glass<sjg@chromium.org>
> ---
...
> diff --git a/doc/README.fdt-control b/doc/README.fdt-control
> new file mode 100644
> index 0000000..3f8bb5a
> --- /dev/null
> +++ b/doc/README.fdt-control
...

Reading your README.fdt-control and browsing the source code a little 
for the CONFIG_OF_* options I found two questions. Sorry if I haven't 
read carefully enough ;)

> +Device Tree Control in U-Boot
> +=============================
> +
> +This feature provides for run-time configuration of U-Boot via a flat
> +device tree (fdt). U-Boot configuration has traditionally been done
> +using CONFIG options in the board config file. This feature aims to
> +make it possible for a single U-Boot binary to support multiple boards,
> +with the exact configuration of each board controlled by a flat device
> +tree (fdt). This is the approach recently taken by the ARM Linux kernel
> +and has been used by PowerPC for some time.
> +
> +The fdt is a convenient vehicle for implementing run-time configuration
> +for three reasons. Firstly it is easy to use, being a simple text file.
> +It is extensible since it consists of nodes and properties in a nice
> +hierarchical format.
> +
> +Finally, there is already excellent infrastructure for the fdt: a
> +compiler checks the text file and converts it to a compact binary
> +format, and a library is already available in U-Boot (libfdt) for
> +handling this format.
> +
> +The dts directory contains a Makefile for building the device tree blob
> +and embedding it in your U-Boot image. This is useful since it allows
> +U-Boot to configure itself according to what it finds there. If you have
> +a number of similar boards with different peripherals, you can describe
> +the features of each board in the device tree file, and have a single
> +generic source base.
> +
> +To enable this feature, add CONFIG_OF_CONTROL to your board config file.
> +
> +
...
> +Where do I get an fdt file for my board?
> +----------------------------------------
> +
> +You may find that the Linux kernel has a suitable file. Look in the
> +kernel source in arch/<arch>/boot/dts.
> +
> +If not you might find other boards with suitable files that you can
> +modify to your needs. Look in the board directories for files with a
> +.dts extension.
> +
> +Failing that, you could write one from scratch yourself!
> +
> +
> +Configuration
> +-------------
> +
> +Use:
> +
> +#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
> +
> +to set the filename of the device tree source. Then put your device tree
> +file into
> +
> +	board/<vendor>/dts/<name>.dts
> +
> +This should include your CPU or SOC's device tree file, placed in
> +arch/<arch>/dts, and then make any adjustments required. The name of this
> +is CONFIG_ARCH_DEVICE_TREE.dts.
> +
> +If CONFIG_OF_EMBED is defined, then it will be picked up and built into
> +the U-Boot image (including u-boot.bin).
> +
> +If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
> +a u-boot.dtb file alongside u-boot.bin. A common approach is then to
> +join the two:
> +
> +	cat u-boot.bin u-boot.dtb>image.bin
> +
> +and then flash image.bin onto your board.

1. Above, the README.fdt-control talks about "make it possible for a 
single U-Boot binary to support multiple boards". Now, if I understood 
correctly, the 'Configuration' section above describes how to 
include/append *one* dtb. But wouldn't we need to include *multiple* 
dtbs to support multiple boards?

2. The section 'Where do I get an fdt file for my board' talks about 
taking the fdt from the Linux kernel. This is fine. Is U-Boot able to 
pass the included/appended (OF_EMBED/OF_SEPARATE) dtb to the Linux 
kernel, then, too? Or is the included/appended dtb only used for the 
U-Boot configuration? And the one passed to the Linux kernel has to be 
taken from anywhere else, additionally?

Many thanks and best regards

Dirk

P.S.: Any idea about

http://lists.denx.de/pipermail/u-boot/2012-March/120916.html

?