The boot flow is maskrom -> idbloader -> uboot -> linux.
The idbloader in rock5b is a image which is consisted with uboot-spl and a ddr initialization program, and this image is located in the 64th sector of the flash, the maskrom will load and execute it, then the uboot-spl will jump to the uboot, if there is a uboot in your sd card the spl will jump to the uboot in your card, the priority of the uboots is defined in the dts
u-boot,spl-boot-order = &sdmmc, &sdhci, &spi_nand, &spi_nor;
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