a question about falcon mode

[Alex G.]

On 11/26/21 4:36 PM, Abder wrote:
> Hi Alex,
> 
> Just a quick remarque that intrigued me:
> 
> Le jeu. 25 nov. 2021 à 15:57, Alex G. <mr.nuke.me at gmail.com> a écrit :
>>
>> On 11/25/21 1:07 AM, Chan Kim wrote:
>>> Hello all,
>>>
>>> I'm trying to implement falcon mode for our board. Then should I first
>>> implement the normal mode(spl + proper)?
>>>
>>> It looks like so while I'm reading doc/README.falcon. (It says, after
>>> loading kernel, DT etc. I should give 'spl export' command).
>>>
>>
>> Falcon mode is a bit board dependent.  There are a couple of ways you
>> could go about this.
>>
>> The first is to have an "fdtargs" partition. This is where "spl export"
>> comes in. Once you run "spl export", it will give a modified dtb at
>> "$fdtargsaddr". It's that DTB that you need to write to your ftdargs
>> partition. For example:
>>
>>       > spl export fdt $loadaddr - $fdt_addr_r
>>       > mmc write $fdtargsaddr 0x9800 0x8000
>>
>> In this example the ftdargs partition starts at sector 0x9800, and is
>> 0x800 sectors long.
>>
>>
>> The second option is to forget about "spl export" and "fdtargs", and
>> package your kernel, devicetree, and overlays in a FIT container. You'd
>> make sure to enable SPL_LOAD_FIT_APPLY_OVERLAY. There isn't much more to
>> this other than the usual gotcha's with FIT and overlays.
>>
> 
> Do you mean by this that the SPL has the capability to generate the
> "fdtargs" by it self (if we provide it with the dtb in the fitImage) ?
> 
> Form my last experience with the falcon mode, I had a - not sure -
> conclusion that the only way to generate the "fdtargs" is by using the
> "spl export" command from uboot cmdline !
> because the reality of the fdtargs blob, as its name indicates, is not
> just the fdt but it has also the bootargs (inside the chosen node )
> that are required by the kernel. So if you give only the DTB to the
> SPL it will not work - to my knowledge -, cuz the data that will be
> passed to the kernel needs to contain also the bootargs !
> 
> Can you please confirm to me if this capability is implemented on the
> SPL and that we can actually forget about the "spl export" command ?

It might not be obvious that an overlay can contain the "/chosen" node 
with the appropriate bootargs:

/dts-v1/;
/plugin/;
/ {
	fragment at 1 {
		target-path = "/chosen";
		__overlay__ {
			bootargs = "root=blablabla console=ttyeS0";
		};
	};
};

> Thanks
> And apologies Chan for jumping on your thread,
> 
> 
> Best regards,
> --
> Abder
>