BSP architecture
The RITY BSP has the following booting architecture:
MediaTek SoC’s ROM code
The MediaTek SoC ROM code will try to boot from different medias:
eMMC
NAND
USB
UFS
The RITY SDK only allows to boot from eMMC. NAND and UFS are not yet supported. USB is only used as part of the flashing process but cannot be used alone to boot to a Linux userspace.
eMMC Boot
When booting the SoC will chose which media to boot from. If the eMMC boot is chosen the bootrom will look at the first boot partition to look for a second stage bootloader.
In case the ROM code fails to find a valid boot partition or valid second stage bootloader, it will automatically go into USB Boot.
USB Boot
In RITY, the ROM code USB boot is used only for flashing.
BL2 (TF-A)
The second stage bootloader is based on Trusted Firmware A (TF-A). The RITY BSP ships pre-built binaries for second stage bootloaders.
BL2 first initializes the hardware:
Initialize the system timer
Initialize and turn on the PLLs
Initialize the PMIC wrap component in order to be able to talk to the PMIC
Sends a baseband power-up signal to the PMIC to notify him of a sucessfull power up.
Initialize the DDR
Initialize the eMMC
Once the hardware is initialized, BL2 will read 4MB of memory stored at offset 0x80000 of the eMMC. This section should contains the fip.bin file.
Firmware Image Package (FIP) is a packaging format used by TF-A to package firmware images. In RITY the FIP contains BL31, BL32, BL33 and optionally some certificates when secure boot is enabled.
BL2 will read the fip package and execute each of the binary it contains.
BL31 (TF-A)
BL31 is provided by TF-A and provides the PSCI routines. Unlike BL2 which won’t get used anymore once BL33 is loaded, BL31 will stay in DDR until a power off or a reboot.
Once BL31 is finished to initialize, it will jump back to BL2.
Source code: https://gitlab.com/mediatek/aiot/bsp/trusted-firmware-a
BL32 (OP-TEE)
BL32 is the secure OS that runs in TrustZone. RITY BSP is using by default OP-TEE trusted OS.
Like BL31, OP-TEE OS stays in DDR until a power off or a reboot.
Source code: https://gitlab.com/mediatek/aiot/bsp/optee-os
BL33 (U-Boot)
U-Boot is the third and last stage bootloader in the RITY boot architecture. It is used to load and boot the kernel from the kernel partition.
In RITY the kernel must be encapsulated into a Flattened Image Tree (FIT) image. The FIT image contains the kernel binary, a Device Tree Blob (DTB), and optionally some Device Tree Blob Overlay (DTBO).
The DTBOs are used to provide optional features to a board, for instance for cameras, display, or any daughterboard. U-Boot looks at its boot_conf environment variable in order to know which (if any) overlay it must load and merge to the main DTB. It is possible to set this boot_conf variable from the U-Boot shell, or when flashing.
U-Boot can also be used to flash (via fastboot), or to boot the kernel and/or rootfs using an USB connection. Please refer to the boards documentation in order to know which features are supported for your specific board.
Source code: https://gitlab.com/mediatek/aiot/bsp/u-boot
Linux
The Linux kernel is the main OS running. It will load the root filesystem (rootfs) and starts running the init process.
The following versions of Linux are currently supported:
Kernel version |
Branch name |
|---|---|
v5.10 |
mtk-v5.10 |
v5.15 |
mtk-v5.15-dev |
By default the latest Linux release will be built, if you wan to use an older supported kernel you can set the PREFERRED_VERSION_linux-mtk variable in your local.conf.
For example if you want to use the v5.10 branch you can set:
PREFERRED_VERSION_linux-mtk = "5.10%"