Well, my naive thought was “SPI flash” is flash attached via an SPI bus, isn’t it?
Anyway, I think by checking schematics and RK3588S datasheet I got it now and the limitation is the following:
RK3588S while containing plenty of SPI controllers can only boot from:
- SD/MMC Interface
- eMMC Interface
- Flexible Serial Flash Interface (FSPI)
And on RK3588S (and castrated variants like RK3582/RK3583) the eMMC and FSPI data lines are pinmuxed (EMMC_D0 and FSPI_D0 share the pins, EMMC_D1 and FSPI_D1 as well and so on) as such it’s as you said and you either get eMMC or (bootable) SPI flash. And what I called an eMMC to SPI adapter isn’t one but it’s a simple module that works since internal RK3588S 1st stage bootloader is capable to probe the data lines and can work directly with SPI flash inside the eMMC socket.
As for the questions why Radxa guys recommend to flash something to something non-existent or why they want to annoy their 5C customers by not bundling these boards with the SPI module too… ah, nevermind…