OpenSBI Firmware with Payload FW_PAYLOAD

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 .text section of the final firmware.

Enabling FW_PAYLOAD compilation

The FW_PAYLOAD firmware can be enabled by any of the following methods:

  1. Specifying FW_PAYLOAD=y on the top level make command line.
  2. Specifying FW_PAYLOAD=y in the target platform config.mk configuration file.

The compiled FW_PAYLOAD firmware ELF file is named fw_jump.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.

Configuration Options

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 config.mk configuration file. The parameters currently defined are as follows:

  • FW_PAYLOAD_OFFSET - Offset from FW_TEXT_BASE 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_PATH - Path to an external flattened device tree binary file to be embedded in the .text section of the final firmware. If this option is not provided and no internal device tree file is specified by the platform (c.f. FW_PAYLOAD_FDT), then the firmware will expect the FDT to be passed as an argument by the prior booting stage.

  • FW_PAYLOAD_FDT - Path to an internal flattened device tree binary file defined by the platform code. The file name must match the DTB file name specified in the platform objects.mk file with the platform-dtb-y entry. This option results in FW_PAYLOAD_FDT_PATH to be automatically set. Specifying FW_PAYLOAD_FDT_PATH on the make command line disables FW_PAYLOAD_FDT and the command line specified device tree binary file is used for building the final firmware.

  • FW_PAYLOAD_FDT_ADDR - Address where the FDT passed by the prior booting stage or specified by the FW_PAYLOAD_FDT_PATH parameter and embedded in the .text 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 Example

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.

The kendryte/k210 platform also enables a build of a FW_PAYLOAD using an internally defined device tree file (FW_PAYLOAD_FDT).