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docs/firmware: Update FW_PAYLOAD documentation

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Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
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docs/firmware/fw_payload.md
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OpenSBI Firmware with Payload (FW_PAYLOAD)
OpenSBI Firmware with Payload *FW_PAYLOAD*
==========================================
The **OpenSBI firmware with Payload (FW_PAYLOAD)** is a firmware which
includes the next booting stage binary (i.e. bootloader/kernel) as a
payload embedded in the OpenSBI firmware binary image.
OpenSBI **firmware with Payload (FW_PAYLOAD)** is a firmware which directly
includes the binary for the booting stage to follow OpenSBI firmware execution.
Typically, this payload will be a bootloader or an OS kernel.
This *FW_PAYLOAD* firmware is particularly useful when the booting stage
prior to OpenSBI firmware is not capable of loading the OpenSBI firmware
and the booting stage after OpenSBI firmware separately.
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.
It is also possible that the booting stage prior to OpenSBI firmware
does not pass a *flattened device tree (FDT)*. In this case, a
*FW_PAYLOAD* firmware allows embedding a flattened device tree in
the .text section of the final 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.
How to Enable?
--------------
Enabling *FW_PAYLOAD* compilation
---------------------------------
The *FW_PAYLOAD* firmware can be enabled by any of the following methods:
The *FW_PAYLOAD* firmware can be enabled by any of the following methods.
1. Passing `FW_PAYLOAD=y` command-line parameter to top-level `make`
2. Setting `FW_PAYLOAD=y` in platform `config.mk`
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 needs to be built according to some predefined
configuration options to work correctly. These configuration details can
be passed as parameters to the top-level `make` command or can be defined
in a platform *config.mk* build configuration file.
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.
The following are the build configuration parameters for a *FW_PAYLOAD*
firmware:
* **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_OFFSET** - Offset from *FW_TEXT_BASE* where the payload
binary will be linked in the final *FW_PAYLOAD* firmware binary image.
<<<<<<< HEAD
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 *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* ignored.
* **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 and should not be
defined otherwise.
* **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_PATH** - Path to the next booting stage binary image
file. If this option is not provided then a simple test payload is
automatically generated, executing a `while (1)` loop.
* **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** - Path to an external flattened device tree
binary file to be embedded in the *.text* section of the final
*FW_PAYLOAD* 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** - 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 zero as the FDT address to the next booting stage.
* **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 zero as the FDT address to the
next booting stage.
*FW_PAYLOAD* Example
--------------------
The *[qemu/virt]* and *[qemu/sifive_u]* platforms illustrates how to configure
and use a *FW_PAYLOAD* firmware. Detailed information regarding these platforms
can be found in the platforms documentation files.
The *kendryte/k210* platform also enables build of a *FW_PAYLOAD* using an
internally defined device tree file (*FW_PAYLOAD_FDT*).
[qemu/virt]: ../platform/qemu_virt.md
[qemu/sifive_u]: ../platform/qemu_sifive_u.md