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.

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
.text section of the final firmware.

Firmware Configuration and Compilation

All firmware types mandate the definition of the following compile time
configuration parameter.

• FW_TEXT_ADDR - Defines the address at which the previous booting stage
  loads OpenSBI firmware.

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_.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.