Re: [PATCH v2 06/15] ARM: i.MX: scratch: add FDT support

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hi,

On 2/6/26 11:02 AM, Marco Felsch wrote:
>> On 2/4/26 9:01 PM, Marco Felsch wrote:
>>> Add support to store a FDT within the scratch area. The user needs to
>>> query the location and size via imx_scratch_get_fdt() which can be used
>>> afterwards to write the actual FDT into it.
>>>
>>> Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>

>>> +config SCRATCH_FDT_SIZE
>>> +	hex
>>> +	default 0x0
>>> +	prompt "Scratch FDT size"
>>> +	help
>>> +	  The size of the scratch area used as destination to load and optional
>>> +	  decompress the barebox builtin DTB into it. Can be 0x0 if early FDT
>>> +	  support is not requied.
>>> +
>>> +	  If specified, SCRATCH_FDT_SIZE <= SCRATCH_SIZE must be ensured.
>>> +
>>> +	  This option should match the OP-TEE's CFG_DTB_MAX_SIZE configuration
>>> +	  if used by OP-TEE.
>>
>> AFAIU, this is a workaround because OP-TEE doesn't accept outside
>> information about how big the DTB buffer is, just its base address.
> 
> This is correct for the bl_params case. In case of transfer-lists the
> size could be detected by OP-TEE OS.
> 
>> In that case, this should go to security/Kconfig and be named something
>> like PBL_OPTEE_DTB_MAX_SIZE. There's no need to ask every user that
>> configures barebox about this.
> 
> This would bind the FDT support to OP-TEE but the FDT could be used by
> other boot binaries as well.In the speak of ARM, it could be used by
> the TF-A. In the speak of RISC-V, it could be used by opensbi. Therefore
> I made it generic.
> 
> I wanted to avoid multiple Kconfig options to configure the scatch space
> for:
>  - OP-TEE FDT
>  - OpenSBI FDT
>  - TF-A FDT
> 
> I'm currently unsure if dropping the OP-TEE note from the help message
> helps. But I have no hard feelings about PBL_OPTEE_DTB_MAX_SIZE either.

How about:

struct imx_scratch_space {
        union {
                u8 __b[CONFIG_SCRATCH_SIZE];
                struct {
                        u32 bootrom_log[128];
                        u32 reserved[128];
                        struct optee_header optee_hdr;
                        u8 fdt[] __aligned(8);
                };
        };
};
static_assert(sizeof(struct imx_scratch_space)
          - offsetof(struct imx_scratch_space, fdt)
			<= PBL_OPTEE_DTB_MAX_SIZE);

The benefits I see:
  - Just one argument to tune (CONFIG_SCRATCH_SIZE) like we already use
    for Rockchip
  - We still have a dedicated option that can be kept in-sync with the
    exact option value in OP-TEE to find problems at compile-time
  - Only those who actually use OP-TEE will be asked about
    PBL_OPTEE_DTB_MAX_SIZE



Would this work for you?

Cheers,
Ahmad


> 
> Regards,
>   Marco
> 
> 
>>
>>
>> Cheers,
>> Ahmad
>>
>> -- 
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>>
>>
> 

-- 
Pengutronix e.K.                  |                             |
Steuerwalder Str. 21              | http://www.pengutronix.de/  |
31137 Hildesheim, Germany         | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686  | Fax:   +49-5121-206917-5555 |