[PATCH v3 2/4] state: add CONFIG_STATE_OVERLAY to inject a state node via devicetree overlay

[chalianis1@gmail.com]

From: Chali Anis <chalianis1@gmail.com>

Until now, a "barebox,state" node had to be part of a board's own,
statically compiled-in devicetree source. That's a hard requirement
for external build systems (Yocto, buildroot, ...) that want to add a
state layout without carrying a board-specific dts patch.

Add CONFIG_STATE_OVERLAY, which compiles an externally supplied
devicetree overlay (.dtso, pointed to by CONFIG_STATE_OVERLAY_DTS)
into the barebox binary and applies it to barebox's own live
devicetree at postcore_initcall time, mirroring how
CONFIG_EXTERNAL_DTS_FRAGMENTS already lets an external build system
inject plain dts fragments. Once applied, the resulting node is
picked up by the regular state probing like any statically defined
one. This selects CONFIG_OF_OVERLAY_LIVE, required so &label
references in the overlay (e.g. to an existing backend partition)
resolve against the base devicetree's __symbols__ node.

Not every target has a live devicetree by postcore_initcall time,
though, so guard against that explicitly and skip cleanly rather than
calling into the overlay code with a NULL root. Once applied, call
of_alias_scan() so the overlay's /aliases entry becomes visible the
same way a live overlay applied via the interactive of_overlay command
already does. Also select CONFIG_OFDEVICE: registering a live
devicetree root at all, on targets with no firmware-supplied one of
their own, depends on it.

Assisted-by: Claude Sonnet 5
Signed-off-by: Chali Anis <chalianis1@gmail.com>
---
 .../bindings/barebox/barebox,state.rst        |  9 ++++
 Documentation/user/state.rst                  | 32 +++++++++++++++
 common/Kconfig                                | 41 +++++++++++++++++++
 common/state/Makefile                         | 20 +++++++++
 common/state/state_overlay.c                  | 29 +++++++++++++
 5 files changed, 131 insertions(+)
 create mode 100644 common/state/state_overlay.c

diff --git a/Documentation/devicetree/bindings/barebox/barebox,state.rst b/Documentation/devicetree/bindings/barebox/barebox,state.rst
index 390e148a2879..36b1d9acb038 100644
--- a/Documentation/devicetree/bindings/barebox/barebox,state.rst
+++ b/Documentation/devicetree/bindings/barebox/barebox,state.rst
@@ -23,6 +23,15 @@ Required Properties
 * additionally a *state* node must have an alias in the ``/aliases`` node pointing
   to it.
 
+.. note:: A *state* node does not have to be part of the board's static
+   devicetree source. It can instead be added at runtime via a devicetree
+   overlay, see :ref:`CONFIG_STATE_OVERLAY <state_overlay>`. In that case,
+   the node referenced by ``backend`` must still exist in the board's own
+   devicetree source under a stable, well-known *label* (not merely an
+   ``/aliases`` entry), because overlay phandle resolution works by
+   resolving ``&label`` references against the base devicetree's
+   ``__symbols__`` node, which requires ``CONFIG_OF_OVERLAY_LIVE``.
+
 .. _barebox,state_magic:
 
 The ``magic`` property is a unique number which identifies the *state* variable
diff --git a/Documentation/user/state.rst b/Documentation/user/state.rst
index d97ba4e9f157..a03670dfa68e 100644
--- a/Documentation/user/state.rst
+++ b/Documentation/user/state.rst
@@ -759,6 +759,38 @@ content, its backend-type and *state* variable layout.
 		};
 	};
 
+.. _state_overlay:
+
+Devicetree Overlay based State Node
+------------------------------------
+
+Normally the *state* node is part of the board's own, statically compiled-in
+devicetree source. ``CONFIG_STATE_OVERLAY`` allows a *state* node to instead
+be added at runtime, via a devicetree overlay that is compiled into the
+barebox binary and applied to barebox's own live devicetree during boot.
+Once applied, the resulting node is picked up by the regular *state* probing
+just like a statically defined one, and is fixed up into whatever devicetree
+barebox eventually boots (internal or external), without requiring any
+board-specific code.
+
+This is primarily meant for use by an external build system (Yocto,
+buildroot, ...) that wants to inject a state layout without patching the
+board's dts: set ``CONFIG_STATE_OVERLAY=y`` and point
+``CONFIG_STATE_OVERLAY_DTS`` at the ``.dtso`` overlay file's path, similar to
+how ``CONFIG_EXTERNAL_DTS_FRAGMENTS`` works for regular dts fragments. As
+with that option, it's not intended to be set in barebox's own defconfig
+files.
+
+Because the overlay is applied to barebox's *live* devicetree, its
+``backend`` phandle can only resolve references to nodes that already exist
+in the board's own devicetree source, and only if that devicetree carries a
+``__symbols__`` node - i.e. ``CONFIG_OF_OVERLAY_LIVE`` must be enabled
+(``CONFIG_STATE_OVERLAY`` selects it automatically). This means the
+referenced backend node needs a stable, well-known *label* defined in the
+board's own devicetree source, not merely an ``/aliases`` entry - the
+overlay itself then only needs to add the *state* node and its alias,
+referencing that existing label.
+
 Frontend
 --------
 
diff --git a/common/Kconfig b/common/Kconfig
index 85df7f7daec6..abe7d100150c 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1351,6 +1351,47 @@ config STATE_BACKWARD_COMPATIBLE
 	  compatibility with the state framework of barebox <= v2016.08.0. Newer
 	  revisions expect an additional 'meta header' and fail otherwise.
 
+config STATE_OVERLAY
+	bool "apply an external devicetree overlay to add a state node"
+	depends on STATE
+	select OF_OVERLAY
+	select OF_OVERLAY_LIVE
+	select OFDEVICE
+	help
+	  Compile an externally supplied devicetree overlay (.dtso) into the
+	  barebox binary and apply it to barebox's own live devicetree at
+	  boot, in order to add a "barebox,state" node (and its /aliases
+	  entry) that isn't part of the board's own compiled-in devicetree.
+
+	  This selects CONFIG_OF_OVERLAY_LIVE, required so the board's own
+	  built-in devicetree carries a __symbols__ node, needed to resolve
+	  &label references from the overlay back into the base devicetree
+	  (e.g. a reference to a backend partition already defined in the
+	  board's static dts).
+
+	  This also selects CONFIG_OFDEVICE: registering a live devicetree
+	  root at all, on targets with no firmware-supplied one of their
+	  own, depends on it.
+
+	  See CONFIG_STATE_OVERLAY_DTS to specify the overlay source file.
+
+config STATE_OVERLAY_DTS
+	string "external state overlay .dtso file"
+	depends on STATE_OVERLAY
+	help
+	  Path to a devicetree overlay source file (.dtso) that will be
+	  compiled and linked into the barebox image and applied to the
+	  live devicetree at boot to add a "barebox,state" node.
+
+	  As with CONFIG_EXTERNAL_DTS_FRAGMENTS, this is not intended to be
+	  put into Barebox's defconfig files. It's an external build
+	  system's job, like Yocto or buildroot, to inject a state overlay
+	  file from outside the Barebox source tree.
+
+	  Any backend node referenced from the overlay via &label must
+	  already exist in the board's own devicetree source, under a
+	  stable, well-known label (not merely an /aliases entry).
+
 config BOOTCHOOSER
 	bool "bootchooser infrastructure"
 	select BOOT
diff --git a/common/state/Makefile b/common/state/Makefile
index 93215dd06921..a906c66a0747 100644
--- a/common/state/Makefile
+++ b/common/state/Makefile
@@ -7,3 +7,23 @@ obj-y += backend_format_raw.o
 obj-y += backend_storage.o
 obj-y += backend_bucket_direct.o
 obj-$(CONFIG_MTD) += backend_bucket_circular.o
+
+# External state devicetree overlay
+# ---------------------------------------------------------------------------
+state-overlay-dts := $(call remove_quotes,$(CONFIG_STATE_OVERLAY_DTS))
+
+ifdef CONFIG_STATE_OVERLAY
+ifeq ($(state-overlay-dts),)
+$(error CONFIG_STATE_OVERLAY is enabled but CONFIG_STATE_OVERLAY_DTS is empty)
+endif
+ifeq ($(wildcard $(state-overlay-dts)),)
+$(error CONFIG_STATE_OVERLAY_DTS="$(state-overlay-dts)" does not exist)
+endif
+
+obj-y += state_overlay.o state-overlay.dtbo.o
+
+$(obj)/state-overlay.dtbo: $(state-overlay-dts) $(DTC) FORCE
+	$(call if_changed_dep,dtc)
+endif
+
+clean-files += *.dtbo *.dtbo.S .*.dtso
diff --git a/common/state/state_overlay.c b/common/state/state_overlay.c
new file mode 100644
index 000000000000..b3f68eaea4b2
--- /dev/null
+++ b/common/state/state_overlay.c
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <common.h>
+#include <init.h>
+#include <of.h>
+#include <linux/err.h>
+
+extern char __dtbo_state_overlay_start[];
+
+static int state_overlay_apply(void)
+{
+	struct device_node *root = of_get_root_node();
+	int ret;
+
+	if (!root) {
+		pr_err("no live devicetree yet, skipping state overlay\n");
+		return 0;
+	}
+
+	ret = of_overlay_apply_dtbo(root, __dtbo_state_overlay_start);
+	if (ret) {
+		pr_err("failed to apply state overlay: %pe\n", ERR_PTR(ret));
+		return ret;
+	}
+
+	of_alias_scan();
+
+	return 0;
+}
+postcore_initcall(state_overlay_apply);