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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*).
[qemu/virt]: ../platform/qemu_virt.md
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