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

[Marco Felsch <m.felsch@pengutronix.de>]

On 26-02-06, Ahmad Fatoum wrote:
> 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);

This would require, that the 'fdt' element is always the last element.
We could add a comment, but it's rather fragile IMHO.

>                 };
>         };
> };
> static_assert(sizeof(struct imx_scratch_space)
>           - offsetof(struct imx_scratch_space, fdt)
> 			<= PBL_OPTEE_DTB_MAX_SIZE);

This would introduce a very strict dependency for CONFIG_SCRATCH_SIZE to
the not always visible PBL_OPTEE_DTB_MAX_SIZE. As you said:

> >> 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.

Therefore I would like to use your name suggestion
PBL_OPTEE_DTB_MAX_SIZE but keep it loose like RK does.

> The benefits I see:
>   - Just one argument to tune (CONFIG_SCRATCH_SIZE) like we already use
>     for Rockchip

Rockchip is using two Kconfig switch too (ARCH_ROCKCHIP_ATF_FDT_SIZE).

>   - 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

Good points!

> Would this work for you?

Please see above.

Regards,
  Marco


> 
> Cheers,
> Ahmad
> 
> 
> > 
> > Regards,
> >   Marco
> > 
> > 
> >>
> >>
> >> Cheers,
> >> Ahmad
> >>
> >> -- 
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> >>
> >>
> > 
> 
> -- 
> Pengutronix e.K.                  |                             |
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> 
> 

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