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OpenSBI Platform Firmwares

OpenSBI provides firmware builds for specific platforms. Different types of firmwares are supported to deal with the differences between different platforms early boot stage. All firmwares will execute the same initialization procedure of the platform hardware according to the platform specific code as well as OpenSBI generic library code. The supported firmwares type will differ in how the arguments passed by the platform early boot stage are handled, as well as how the boot stage following the firmware will be handled and executed.

The previous booting stage will pass information via the following registers of RISC-V CPU:

  • hartid via a0 register
  • device tree blob address in memory via a1 register. The address must be aligned to 8 bytes.

OpenSBI currently supports three different types of firmwares.

Firmware with Dynamic Information (FW_DYNAMIC)

The FW_DYNAMIC firmware gets information about the next booting stage entry, e.g. a bootloader or an OS kernel, from previous booting stage at runtime.

A FW_DYNAMIC firmware is particularly useful when the booting stage executed prior to OpenSBI firmware is capable of loading both the OpenSBI firmware and the booting stage binary to follow OpenSBI firmware.

Firmware with Jump Address (FW_JUMP)

The FW_JUMP firmware assumes a fixed address of the next booting stage entry, e.g. a bootloader or an OS kernel, without directly including the binary code for this next stage.

A FW_JUMP firmware is particularly useful when the booting stage executed prior to OpenSBI firmware is capable of loading both the OpenSBI firmware and the booting stage binary to follow OpenSBI firmware.

Firmware with Payload (FW_PAYLOAD)

The FW_PAYLOAD firmware directly includes the binary code for the booting stage to follow OpenSBI firmware execution. Typically, this payload will be a bootloader or an OS kernel.

A FW_PAYLOAD firmware is particularly useful when the booting stage executed prior to OpenSBI firmware is not capable of loading both OpenSBI firmware and the booting stage to follow OpenSBI firmware.

A FW_PAYLOAD firmware is also useful for cases where the booting stage prior to OpenSBI firmware does not pass a flattened device tree (FDT file). In such case, a FW_PAYLOAD firmware allows embedding a flattened device tree in the .rodata section of the final firmware.

Firmware Configuration and Compilation

All firmware types support the following common compile time configuration parameters:

  • FW_TEXT_START - Defines the execution address of the OpenSBI firmware. This configuration parameter is mandatory.
  • FW_FDT_PATH - Path to an external flattened device tree binary file to be embedded in the .rodata section of the final firmware. If this option is not provided then the firmware will expect the FDT to be passed as an argument by the prior booting stage.
  • FW_FDT_PADDING - Optional zero bytes padding to the embedded flattened device tree binary file specified by FW_FDT_PATH option.
  • FW_PIC - "FW_PIC=y" generates position independent executable firmware images. OpenSBI can run at arbitrary address with appropriate alignment. Therefore, the original relocation mechanism ("FW_PIC=n") will be skipped. In other words, OpenSBI will directly run at the load address without any code movement. This option requires a toolchain with PIE support, and it is on by default.

Additionally, each firmware type as a set of type specific configuration parameters. Detailed information for each firmware type can be found in the following documents.

  • FW_DYNAMIC: The Firmware with Dynamic Information (FW_DYNAMIC) is described in more details in the file fw_dynamic.md.
  • FW_JUMP: The Firmware with Jump Address (FW_JUMP) is described in more details in the file fw_jump.md.
  • FW_PAYLOAD: The Firmware with Payload (FW_PAYLOAD) is described in more details in the file fw_payload.md.

Providing different payloads to OpenSBI Firmware

OpenSBI firmware can accept various payloads using a compile time option. Typically, these payloads refer to the next stage boot loader (e.g. U-Boot) or operating system kernel images (e.g. Linux). By default, OpenSBI automatically provides a test payload if no specific payload is specified at compile time.

To specify a payload at compile time, the make variable FW_PAYLOAD_PATH is used.

make PLATFORM=<platform_subdir> FW_PAYLOAD_PATH=<payload path>

The instructions to build each payload is different and the details can be found in the docs/firmware/payload_<payload_name>.md files.

Options for OpenSBI Firmware behaviors

An optional compile time flag FW_OPTIONS can be used to control the OpenSBI firmware run-time behaviors.

make PLATFORM=<platform_subdir> FW_OPTIONS=<options>

FW_OPTIONS is a bitwise or'ed value of various options, eg: FW_OPTIONS=0x1 stands for disabling boot prints from the OpenSBI library.

For all supported options, please check "enum sbi_scratch_options" in the include/sbi/sbi_scratch.h header file.