[U-Boot-Users] Ideas on U-Boot configuration with FDT

Wolfgang Grandegger wg at grandegger.com
Mon May 21 19:08:19 CEST 2007

Timur Tabi wrote:
> Wolfgang Grandegger wrote:
>> Jerry Van Baren wrote:
>>> Timur Tabi wrote:
>>>> Jerry Van Baren wrote:
>>>>> Yes, that is what "fdt addr" does today, it tells the u-boot innards 
>>>>> where the blob is (sets/changes the blob location).  The user can 
>>>>> download a new blob and switch to using it via the "fdt addr" command.
>>>> By "change", I thought you meant that "fdt addr" would actually move 
>>>> the device tree, since that's technically changing the address.  
>>>> Perhaps you meant "set the address"?
>>> Yes.
>>>>> The user can use the "fdt addr" command to set/change the location 
>>>>> of the blob.  Moving blobs is not out of scope, it is what "fdt 
>>>>> move" does. 
>>>> How is "fdt move" different than cp.b followed by another "fdt addr"?
>>> Conceptually the same thing but easier: you don't have to know (guess) 
>>> the blob size because fdt_open_into() gets the size from the source blob.
>> Summing up, brainstorming a bit more, here are my revised ideas:
>> The following definitions control the FDT usage in U-Boot:
>>     If defined, "fdt" is set to that address at compile time. The
>>         FDT can be used from the early beginning of the boot.
>>     If defined, the env variable "fdtaddr" is looked up early in the
>>         boot and "fdt" is set accordingly. This allows to hold more than
>>         one blob in FLASH and select one via env setting. This would
>>         allow for _one_ combination of images of U-Boot + Linux + Blobs
>>         for a _set_ of boards.
> If CFG_FDT_ADDR_BY_ENV is *not* defined, should the FDT code then set that variable?

Yes, then the address is set to CFG_FDT_ADDR_XXX at compile time.

>>     If defined, the blob is moved to RAM after relocation for
>>         further preparation or for performance reasons. "fdt" is re-set
>>         accordingly. The blob is then ready and in place for booting
>>         Linux. If CFG_FDT_ADDR_RAM is set to 0, the blob will be copied
>>     to a default location, e.g. before the initrd location.
> I think the FDT blob should *always* be copied to RAM.
>> A board-specific checkboad function is called as early as possible to 
>> verify the FDT.
>> This should also make Timur happy, as he has the choice, e.g. read the 
>> FDT solely from FLASH. 
> I think I may have changed my mind a bit.  I'm not so much concerned about read-only FDT 
> as I want to automate the process of copying it to RAM.

As a result of discussion, in the meantime I have now also a better idea 
on how to implement FDT for U-Boot configuration. Going to make a demo 
for my Icecube board a.s.a.p.


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