how to use SPL_MULTI_DTB_FIT?

[Tim Harvey]

On Fri, Oct 23, 2020 at 11:32 AM Tim Harvey <tharvey at gateworks.com> wrote:
>
> Greetings,
>
> I'm working on U-Boot board support for a family of IMX8MM boards and
> need to support multiple DTB's. My boards have an i2c EEPROM that I
> need to read to find the model and other specifics before
> board_fit_config_name_match can do the right thing.
>
> I find that when I enable CONFIG_SPL_MULTI_DTB_FIT I can not access
> i2c within my board_fit_config_name_match. It would appear that
> perhaps the heap isn't setup yet as I get a spew of 'alloc space
> exhausted'.
>
> Any ideas how I can hook a board specific function that can access i2c
> before the call to board_fit_config_name_match in the SPL?
>

I seem to be stuck with a chicken and egg problem here.

I'm using DM_I2C but that requires that I've processed a device-tree
first but my problem is that I need to read an I2C EEPROM before I
know what device-tree to use. So how can I do this without disabling
DM_I2C for U-Boot? I feel like storing model information in an EEPROM
is not that uncommon of a thing.

The 'alloc space exhausted' issue was that malloc space wasn't setup
yet as that is done in spl_early_init().

My board family consists of a SOM and a Baseboard. There is 2 variants
of the SOM currently both based on IMX8MM but differing in memory and
emmc size. There are 4 baseboards the SOM can be married two. EEPROM
data tells me which SOM I have (and its memory size) and which
baseboard I have.

At some point we wish to add an IMX8MP SOM compatible with the same
baseboards but I believe that may mean two different bootloaders as I
don't think IMX8MM is runtime compatible with IMX8MP as far as U-Boot
is concerned.

Perhaps the best approach is to create a small default dtb for the SOM
then handle the baseboard as overlays but I'm not exactly sure how to
handle the overlays. Looking at CONFIG_SPL_LOAD_FIT_APPLY_OVERLAY it
would appear that overlays are added fairly late in the SPL and not
early enough for the SPL to use DM to configure things like SDHC and
PMIC.

Any suggestions are greatly appreciated.

Best Regards,

Tim