OpenSBI firmware with Payload (FW_PAYLOAD) is a firmware which directly includes the binary for the booting stage to follow the 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 the OpenSBI firmware is not capable of loading both the 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 the OpenSBI firmware does not pass a flattened device tree (FDT file). In such a case, a FW_PAYLOAD firmware allows embedding a flattened device tree in the .rodata section of the final firmware.
The FW_PAYLOAD firmware can be enabled by any of the following methods:
FW_PAYLOAD=y on the top level make command line.FW_PAYLOAD=y in the target platform objects.mk configuration file.The compiled FW_PAYLOAD firmware ELF file is named fw_payload.elf. Its expanded image file is fw_payload.bin. Both files are created in the platform-specific build directory under the build/platform/<platform_subdir>/firmware directory.
A FW_PAYLOAD firmware is built according to configuration parameters and options. These configuration parameters can be defined using either the top level make command line or the target platform objects.mk configuration file. The parameters currently defined are as follows:
FW_PAYLOAD_OFFSET - Offset from the opensbi load address where the payload binary will be linked in the final FW_PAYLOAD firmware binary image. This configuration parameter is mandatory if FW_PAYLOAD_ALIGN is not defined. Compilation errors will result from an incorrect definition of FW_PAYLOAD_OFFSET or of FW_PAYLOAD_ALIGN, or if neither of these parameters are defined.
FW_PAYLOAD_ALIGN - Address alignment constraint where the payload binary will be linked after the end of the base firmware binary in the final FW_PAYLOAD firmware binary image. This configuration parameter is mandatory if FW_PAYLOAD_OFFSET is not defined. If both FW_PAYLOAD_OFFSET and FW_PAYLOAD_ALIGN are defined, FW_PAYLOAD_OFFSET is used and FW_PAYLOAD_ALIGN is ignored.
FW_PAYLOAD_PATH - Path to the image file of the next booting stage binary. If this option is not provided then a simple test payload is automatically generated and used as a payload. This test payload executes an infinite while (1) loop after printing a message on the platform console.
FW_PAYLOAD_FDT_ADDR - Address where the FDT passed by the prior booting stage or specified by the FW_FDT_PATH parameter and embedded in the .rodata section will be placed before executing the next booting stage, that is, the payload firmware. If this option is not provided, then the firmware will pass the FDT address passed by the previous booting stage to the next booting stage.
FW_PAYLOAD_FDT_OFFSET - Address offset from the opensbi load address where the FDT will be passed to the next booting stage. This offset is used as relocatable address of the FDT passed to the next booting stage. If FW_PAYLOAD_FDT_ADDR is also defined, the firmware will prefer FW_PAYLOAD_FDT_ADDR.
The qemu/virt platforms illustrate how to configure and use a FW_PAYLOAD firmware. Detailed information regarding these platforms can be found in the platform documentation files.