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

Wolfgang Grandegger wg at grandegger.com
Sun May 20 10:36:05 CEST 2007

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 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.

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. With this scheme, I also do not see the real need 
for fdt commands to manipulate the blob in RAM. Read-only support would 
be a nice to have for debugging, though.

Please comment.


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