sg2042: trm: document spi flash layout (eg for bypassing SCP bootrom)

[zamaudio]

I noticed there is an option to reset the sg2042 and make it bypass the SCP bootrom but boot directly from SPI flash (64MiB).
Can we get the spi flash layout documented in system.rst TODO item so we can write a custom bios? Thanks

[zamaudio]

In absence of documentation, I found this C program that appears to create a bootable spi flash image, but I haven't tested it.

https://github.com/sophgo/bootloader-arm64/blob/master/scripts/gen_spi_flash.c

[orlitzky]

I somewhat reverse engineered the format here: https://wiki.gentoo.org/wiki/Milk-V_Pioneer_Box#The_long_version

I think this should be part of the ZSBL documentation, because gen_spi_flash.c is basically putting things where ZSBL expects them to be. In fact, gen_spi_flash.cshould be part of the ZSBL repo.

[zamaudio]

Well done!