upl: Add initial documentation
Add some documentation to explain the basic concept along with a link
to the full spec.
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/upl.rst b/doc/usage/cmd/upl.rst
index a64063d..8d6ea5d 100644
--- a/doc/usage/cmd/upl.rst
+++ b/doc/usage/cmd/upl.rst
@@ -16,8 +16,8 @@
-----------
The *upl* command is used to test U-Boot's support for the Universal Payload
-Specification (UPL) firmware standard. It allows creation of a fake handoff for
-use in testing.
+Specification (UPL) firmware standard (see :doc:`../upl`). It allows creation of
+a fake handoff for use in testing.
upl write
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 4246c06..b058c22 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -15,6 +15,7 @@
cmdline
semihosting
measured_boot
+ upl
Shell commands
--------------
diff --git a/doc/usage/upl.rst b/doc/usage/upl.rst
new file mode 100644
index 0000000..3c4a10c
--- /dev/null
+++ b/doc/usage/upl.rst
@@ -0,0 +1,46 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Universal Payload
+-----------------
+
+`Universal Payload (UPL) <https://universalpayload.github.io/spec/index.html>`_
+is an Industry Standard for firmware components. UPL
+is designed to improve interoperability within the firmware industry, allowing
+mixing and matching of projects with less friction and fewer project-specific
+implementations. UPL is cross-platform, supporting ARM, x86 and RISC-V
+initially.
+
+UPL is defined in termns of two firmware components:
+
+`Platform Init`
+ Perhaps initial setup of the hardware and jumps to the payload.
+
+`Payload`
+ Selects the OS to boot
+
+In practice UPL can be used to handle any number of handoffs through the
+firmware startup process, with one program acting as platform init and another
+acting as the payload.
+
+UPL provides a standard for three main pieces:
+
+- file format for the payload, which may comprise multiple images to load
+- handoff format for the information the payload needs, such as serial port,
+ memory layout, etc.
+- machine state and register settings at the point of handoff
+
+See also the :doc:`cmd/upl`.
+
+UPL in U-Boot
+~~~~~~~~~~~~~
+
+U-Boot supports:
+
+- writing a UPL handoff (devicetree) in SPL
+- reading a UPL handoff in U-Boot proper
+- creating a FIT
+
+There are some new FIT features in UPL which are not yet supported in U-Boot.
+
+.. sectionauthor:: Simon Glass <sjg@chromium.org>
+.. July 2024