device: support for mutable devices

Add support for mutable devices. Mutable devices are those which
can be modified after declaration, in-place, in kernel mode.

In order for a device to be mutable, the following must be true

  * `CONFIG_DEVICE_MUTABLE` must be y-selected
  * the Devicetree bindings for the device must include
    `mutable.yaml`
  * the Devicetree node must include the `zephyr,mutable` property

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

cmake/linker_script/common/common-ram.cmake

@@ -128,3 +128,7 @@ if(CONFIG_USB_HOST_STACK)
   zephyr_iterable_section(NAME usbh_contex GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
   zephyr_iterable_section(NAME usbh_class_data GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
 endif()
+
+if(CONFIG_DEVICE_MUTABLE)
+  zephyr_iterable_section(NAME device_mutable GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
+endif()

dts/bindings/base/mutable.yaml

@@ -0,0 +1,13 @@
+# Copyright (c) 2023, Meta
+# SPDX-License-Identifier: Apache-2.0
+
+# Properties for Mutable devices
+
+properties:
+  zephyr,mutable:
+    type: boolean
+    description: |
+      True iff the device structure may be mutated.
+
+      Inherit this binding for devices that are runtime-modifiable, in-place.
+      This places the device structure into SRAM rather than Flash.

include/zephyr/device.h

@@ -41,6 +41,10 @@ extern "C" {
  */
 #define Z_DEVICE_DEPS_ENDS INT16_MAX
 
+/** @brief Determine if a DT node is mutable */
+#define Z_DEVICE_IS_MUTABLE(node_id)                                                               \
+	COND_CODE_1(IS_ENABLED(CONFIG_DEVICE_MUTABLE), (DT_PROP(node_id, zephyr_mutable)), (0))
+
 /** @endcond */
 
 /**
@@ -924,12 +928,13 @@ static inline bool z_impl_device_is_ready(const struct device *dev)
  * @param api Reference to device API.
  * @param ... Optional dependencies, manually specified.
  */
-#define Z_DEVICE_BASE_DEFINE(node_id, dev_id, name, pm, data, config, level,   \
-			     prio, api, state, deps)                           \
+#define Z_DEVICE_BASE_DEFINE(node_id, dev_id, name, pm, data, config, level, prio, api, state,     \
+			     deps)                                                                 \
 	COND_CODE_1(DT_NODE_EXISTS(node_id), (), (static))                                         \
-	const STRUCT_SECTION_ITERABLE_NAMED(device,                            \
-		Z_DEVICE_SECTION_NAME(level, prio),                            \
-		DEVICE_NAME_GET(dev_id)) =                                     \
+	COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (), (const))                                     \
+	STRUCT_SECTION_ITERABLE_NAMED_ALTERNATE(                                                   \
+		device, COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (device_mutable), (device)),     \
+		Z_DEVICE_SECTION_NAME(level, prio), DEVICE_NAME_GET(dev_id)) =                     \
 		Z_DEVICE_INIT(name, pm, data, config, api, state, deps)
 
 /* deprecated device initialization levels */
@@ -964,11 +969,11 @@ static inline bool z_impl_device_is_ready(const struct device *dev)
 #define Z_DEVICE_INIT_ENTRY_DEFINE(node_id, dev_id, init_fn_, level, prio)                         \
 	Z_DEVICE_LEVEL_CHECK_DEPRECATED_LEVEL(level)                                               \
                                                                                                    \
-	static const Z_DECL_ALIGN(struct init_entry) __used __noasan           \
-		Z_INIT_ENTRY_SECTION(level, prio,                              \
-				     Z_DEVICE_INIT_SUB_PRIO(node_id))          \
+	static const Z_DECL_ALIGN(struct init_entry) __used __noasan Z_INIT_ENTRY_SECTION(         \
+		level, prio, Z_DEVICE_INIT_SUB_PRIO(node_id))                                      \
 		Z_INIT_ENTRY_NAME(DEVICE_NAME_GET(dev_id)) = {                                     \
-			.init_fn = {.dev = (init_fn_)},                        \
+			.init_fn = {COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (.dev_rw), (.dev)) = \
+					    (init_fn_)},                                           \
 			.dev = &DEVICE_NAME_GET(dev_id),                                           \
 	}
 
@@ -1016,7 +1021,8 @@ static inline bool z_impl_device_is_ready(const struct device *dev)
  * that out until after we've built the zephyr image, though.
  */
 #define Z_MAYBE_DEVICE_DECLARE_INTERNAL(node_id)                                                   \
-	extern const struct device DEVICE_DT_NAME_GET(node_id);
+	extern COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (),                                       \
+			   (const)) struct device DEVICE_DT_NAME_GET(node_id);
 
 DT_FOREACH_STATUS_OKAY_NODE(Z_MAYBE_DEVICE_DECLARE_INTERNAL)
 

include/zephyr/init.h

@@ -73,6 +73,17 @@ union init_function {
 	 * @retval -errno If device initialization fails.
 	 */
 	int (*dev)(const struct device *dev);
+#ifdef CONFIG_DEVICE_MUTABLE
+	/**
+	 * Device initialization function (rw).
+	 *
+	 * @param dev Device instance.
+	 *
+	 * @retval 0 On success
+	 * @retval -errno If device initialization fails.
+	 */
+	int (*dev_rw)(struct device *dev);
+#endif
 };
 
 /**
@@ -96,7 +107,12 @@ struct init_entry {
 	 * If the init entry belongs to a device, this fields stores a
 	 * reference to it, otherwise it is set to NULL.
 	 */
+	union {
 		const struct device *dev;
+#ifdef CONFIG_DEVICE_MUTABLE
+		struct device *dev_rw;
+#endif
+	};
 };
 
 /** @cond INTERNAL_HIDDEN */

include/zephyr/linker/common-ram.ld

@@ -137,6 +137,10 @@
 	ITERABLE_SECTION_RAM(zbus_channel_observation_mask, 1)
 #endif /* CONFIG_ZBUS */
 
+#if defined(CONFIG_DEVICE_MUTABLE)
+	ITERABLE_SECTION_RAM(device_mutable, 4)
+#endif
+
 #ifdef CONFIG_USERSPACE
 	_static_kernel_objects_end = .;
 #endif

kernel/Kconfig

@@ -1274,6 +1274,13 @@ config DEVICE_DEPS_DYNAMIC
 	  Option that makes it possible to manipulate device dependencies at
 	  runtime.
 
+config DEVICE_MUTABLE
+	bool "Mutable devices [EXPERIMENTAL]"
+	select EXPERIMENTAL
+	help
+	  Support mutable devices. Mutable devices are instantiated in SRAM
+	  instead of Flash and are runtime modifiable in kernel mode.
+
 endmenu
 
 rsource "Kconfig.vm"