libcamera: Add a DebugMetadata helper

Debug metadata often occurs in places where the metadata control list is
not available e.g. in queueRequest() or processStatsBuffer() or even in
a class far away from the metadata handling code. It is therefore
difficult to add debug metadata without adding lots of boilerplate
code. This can be mitigated by recording the metadata and forwarding it
to the metadata control list when it becomes available. To solve the
issue of code that is far away from the metadata context, add a chaining
mechanism to allow loose coupling at runtime.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/debug_controls.h

@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Google Inc.
+ *
+ * Debug metadata helpers
+ */
+
+#pragma once
+
+#include <libcamera/control_ids.h>
+
+namespace libcamera {
+
+class DebugMetadata
+{
+public:
+	DebugMetadata() = default;
+
+	void enableByControl(const ControlList &controls);
+	void enable(bool enable = true);
+	void setParent(DebugMetadata *parent);
+	void moveEntries(ControlList &list);
+
+	template<typename T, typename V>
+	void set(const Control<T> &ctrl, const V &value)
+	{
+		if (parent_) {
+			parent_->set(ctrl, value);
+			return;
+		}
+
+		if (!enabled_)
+			return;
+
+		cache_.set(ctrl, value);
+	}
+
+	void set(unsigned int id, const ControlValue &value);
+
+private:
+	bool enabled_ = false;
+	DebugMetadata *parent_ = nullptr;
+	ControlList cache_;
+};
+
+} /* namespace libcamera */

include/libcamera/internal/meson.build

@@ -14,6 +14,7 @@ libcamera_internal_headers = files([
     'control_serializer.h',
     'control_validator.h',
     'converter.h',
+    'debug_controls.h',
     'delayed_controls.h',
     'device_enumerator.h',
     'device_enumerator_sysfs.h',