[U-Boot] Selecting from multiple device trees at runtime

[Stephen Warren]

On 01/08/2013 09:51 AM, Simon Glass wrote:
> Hi Stephen,
> 
> On Tue, Jan 8, 2013 at 8:42 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 01/07/2013 08:16 PM, Simon Glass wrote:
>>> Hi,
>>>
>>> On Mon, Jan 7, 2013 at 2:21 PM, Curt Brune <curt at cumulusnetworks.com> wrote:
>>>>
>>>>
>>>> On 01/07/2013 12:12 PM, Wolfgang Denk wrote:
>>>>>
>>>>> Dear Curt Brune,
>>>>>
>>>>> In message <50EB0D92.2020707 at cumulusnetworks.com> you wrote:
>>>>>>
>>>>>>
>>>>>> What I would love is to have a single multi-file uImage I could use on
>>>>>> all my platforms.  The idea is to introduce a new image type that is a
>>>>>> list of device tree blobs.
>>>>>
>>>>>
>>>>> In addition to the file system based approach suggested by Stephen,
>>>>> you should have a look into using FIT images (see doc/uImage.FIT/ ).
>>>>> One of the reasons for creating these was to deal with situations
>>>>> exactly as you describe...
>>>>
>>>>
>>>> I think that will work perfectly.  Thank you for the suggestion.
>>>
>>> Note also there is code in mainline now to select the correct FDT from
>>> a list of them in a FIT. based on the model name. Then it can pass
>>> this to the kernel. So if you have a way of getting the model name in
>>> U-Boot, it might just work.
>>
>> Hmmm. What's the model name compared against? U-Boot board name variable
>> would be nice!
> 
> At the moment it compares against the model in the U-Boot FDT
> (CONFIG_OF_CONTROL). When flashing a board, you pack u-boot.bin with
> the selected .dtb file containing this model name. Then when U-Boot
> runs it knows what its model is.
> 
> You could do what you describe, but it is then a compile-time check, I think.
> 
> There could be other ways to decide on the model name, such as looking
> at strapping GPIOs, for example:
> 
> static const char *detect_model(void)
> {
>     if (gpio_get_value(36))
>         return "snow";
>     else
>         return "flax";
> }

Right - I believe the TI guys introduced the board_name variable or
similar to indicate the runtime-detected board ID for this purpose
(whereas the board variable I introduced is the board U-Boot was
compiled for). It'd be nice to be able to select a DTB from FIT based on
$board_name instead of U-Boot's own DTB's model given all this. Do the
sub-images in the FIT image have filenames that could be selected as
e.g. ${soc}-${board}.dtb? Using $board or $board_name would also help
where U-Boot doesn't use a DT itself.