[U-Boot] FDT howto

[Simon Glass]

Hi Jagan,

On Sun, Feb 24, 2013 at 8:19 AM, Jagan Teki <jagannadh.teki at gmail.com> wrote:
> Hi Simon,
>
> Thanks for your response, please find my below comments.
>
> On Sun, Feb 24, 2013 at 9:24 PM, Simon Glass <sjg at chromium.org> wrote:
>> Hi Jagan,
>>
>> On Thu, Feb 21, 2013 at 9:03 AM, Jagan Teki <jagannadh.teki at gmail.com> wrote:
>>> Hi All,
>>>
>>> I am planning to use devicetree on u-boot.
>>> I have an experience to work with devicetree on Linux.
>>>
>>> For u-boot, I have read doc from doc/README.fdt-control.
>>> I see some dts usages on tegra boards.
>>>
>>> I have lot of confusions with the concept itself.
>>>
>>> Is Linux and u-boot devicetree concept and build system are same?
>>
>> I don't really understand this question sorry. U-Boot and Linux use
>> the device tree mainly for run-time configuration of drivers, so that
>> the same driver code can operate on different boards.
>
> I am some how confusing the fdt usage in u-boot.
> Because when compared to Linux, u-boot fdt setup mandatory to require
> the CONFIG_DEFAULT_DEVICE_TREE
> on the config file [from your previous comments].
>
> is this the only difference when compared to Linux i guess..is it?
> Linux defconfig file does need to hot-code
> the dts.

Yes - that is a difference. Linux provides a way to build .dts files
but it is not mandatory. At present it is mandatory with U-Boot, and
only one file is built. It can easily be ignored though.

>
>>
>>>
>>> Suppose I have 4 boards J1, J2, J3 & J4 on my soc "emb".of vendor "vast"
>>>    For this requirement I have
>>>    board/vast/dts/J1.dts
>>>    board/vast/dts/J2.dts
>>>    board/vast/dts/J3.dts
>>>    board/vast/dts/J4.dts
>>>
>>>    include/configs/emb_common.h ==> single configuration of all SOC
>>> needed definitions
>>>    like defconfig in Linux.
>>>
>>>    do I need any more files?
>>
>> That's enough for the basics I think.
>
> Is dtsi file require to add it on arch folder along with above.

If your architecture is not one of those already supported (like arm
tegra/exynos and x86) then yes you need to add this file in
arch/<arch>/dts. What architecture are you using?

>
>>
>>>
>>>    In emb_common.h i am defining
>>>    CONFIG_OF_SEPARATE is it sufficient?
>>
>> And CONFIG_OF_CONTROL
>>
>>>
>>>    My plan is to build u-boot and then build the dtb with specific
>>> board and then combine.
>>
>> That's fine, and is how we do things on Chromium also. U-Boot tries to
>> build an FDT even with CONFIG_OF_SEPARATE, so you need to put a
>> default device tree file in your emb_common.h file that it can find.
>> But you can ignore it, and for flashing your boards just use
>> u-boot.bin plus whatever .dtb you want to select.
>
> So I will add the fdt support and then build the u-boot.
> Is there any separate u-boot build command to build dts file.
> My plan is to combine both u-boot and dtb.

You can run dtc yourself if you like - see Makefile/dts for how it is
done there. Once you have the .dtb binary, the easiest thing is to add
it to the end of u-boot.bin, as described in README.fdt-control.

>
> Thanks,
> Jagan.
>
>>
>>>
>>> I saw that all tegra boards config files are defining
>>> CONFIG_DEFAULT_DEVICE_TREE if ie. the case dts is a compile time
>>> option right.
>>> am i correct?
>>
>> Yes - see above. This is convenient for testing and development.
>>
>>>
>>> Please let me know your inputs.
>>
>> Regards,
>> Simon
>>
>>>
>>> --
>>> Thanks,
>>> Jagan.