zbus: improve the way of storing observers

ZBus stores observers in two ways: statically using a list and dynamically
using a memory slab. Both present limitations. Static observers work only
for channel definition. The dynamic observers rely on a memory slab that
forces the user to manage its size to avoid issues with adding
observers. This commit fixes the static allocation problem by using the
iterable sections for allocating observation data and replacing the VDED
execution sequence since now it is possible to prioritize static observer
execution. All the runtime observers are dynamically allocated on the heap
instead of a specific memory pool.

BREAK changes (only internal, not APIs):

* ZBus channel metadata changed. Remove the observers' static array
pointer. Rename the `runtime_observers` pointer to `observers`. Add
`observer_start_idx` and `observer_end_idx`;
* Change the VDED execution sequence. The position (on definition time),
the priority in conjunction with the lexical order, is considered for
static post-definition time observers. At last, the runtime observer
follows the adding sequence;
* Replace the `CONFIG_ZBUS_RUNTIME_OBSERVERS_POOL_SIZE` with
`CONFIG_ZBUS_RUNTIME_OBSERVERS`.

New APIs:

* New iterable section iterators (for channels and observers) can now
receive a user_data pointer to keep context between the function calls;
* New `ZBUS_LISTENER_DEFINE_WITH_ENABLE(_name, _cb, _enable)` and
`ZBUS_SUBSCRIBER_DEFINE_WITH_ENABLE(_name, _queue_size, enable)` that
enable developers define disabled observers. They need to be enabled
during runtime to receive notifications from the bus;
* `ZBUS_CHAN_ADD_OBS` macro for adding post-definition static observers of
a channel.

Important changes:

* Move the ZBus LD file content to the `common-ram.ld` LD file. That was
necessary to make ZBus compatible with some Xtensa and RISCV boards.

Signed-off-by: Rodrigo Peixoto <[email protected]>

Author: rodrigopex

cmake/linker_script/common/common-ram.cmake

@@ -103,8 +103,8 @@ if(CONFIG_ZTEST_NEW_API)
 endif()
 
 if(CONFIG_ZBUS)
-  zephyr_iterable_section(NAME zbus_channel GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
   zephyr_iterable_section(NAME zbus_observer GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
+  zephyr_iterable_section(NAME zbus_channel_observation_mask GROUP DATA_REGION ${XIP_ALIGN_WITH_INPUT} SUBALIGN 4)
 endif()
 
 if(CONFIG_UVB)

cmake/linker_script/common/common-rom.cmake

@@ -213,3 +213,8 @@ endif()
 if(CONFIG_USBD_MSC_CLASS)
   zephyr_iterable_section(NAME usbd_msc_lun KVMA RAM_REGION GROUP RODATA_REGION SUBALIGN 4)
 endif()
+
+if(CONFIG_ZBUS)
+  zephyr_iterable_section(NAME zbus_channel KVMA RAM_REGION GROUP RODATA_REGION SUBALIGN 4)
+  zephyr_iterable_section(NAME zbus_channel_observation KVMA RAM_REGION GROUP RODATA_REGION SUBALIGN 4)
+endif()

include/zephyr/linker/common-ram.ld

@@ -124,6 +124,11 @@
 	ITERABLE_SECTION_RAM(sensing_sensor, 4)
 #endif /* CONFIG_SENSING */
 
+#if defined(CONFIG_ZBUS)
+	ITERABLE_SECTION_RAM(zbus_observer, 4)
+	ITERABLE_SECTION_RAM(zbus_channel_observation_mask, 1)
+#endif /* CONFIG_ZBUS */
+
 #ifdef CONFIG_USERSPACE
 	_static_kernel_objects_end = .;
 #endif

include/zephyr/linker/common-rom/common-rom-misc.ld

@@ -34,6 +34,11 @@
 	ITERABLE_SECTION_ROM(emul, 4)
 #endif /* CONFIG_EMUL */
 
+#if defined(CONFIG_ZBUS)
+	ITERABLE_SECTION_ROM(zbus_channel, 4)
+	ITERABLE_SECTION_ROM(zbus_channel_observation, 4)
+#endif /* CONFIG_ZBUS */
+
 	SECTION_DATA_PROLOGUE(symbol_to_keep,,)
 	{
 		__symbol_to_keep_start = .;

include/zephyr/zbus/zbus.h

@@ -4,11 +4,8 @@ zephyr_library()
 
 zephyr_library_sources(zbus.c)
 
-if(CONFIG_ZBUS_RUNTIME_OBSERVERS_POOL_SIZE GREATER 0)
+if(CONFIG_ZBUS_RUNTIME_OBSERVERS)
     zephyr_library_sources(zbus_runtime_observers.c)
 endif()
 
-if(CONFIG_ZBUS_STRUCTS_ITERABLE_ACCESS)
-    zephyr_library_sources(zbus_iterable_sections.c)
-    zephyr_linker_sources(DATA_SECTIONS zbus.ld)
-endif()
+zephyr_library_sources(zbus_iterable_sections.c)

subsys/zbus/CMakeLists.txt

@@ -8,25 +8,19 @@ menuconfig ZBUS
 
 if ZBUS
 
-config ZBUS_STRUCTS_ITERABLE_ACCESS
-	bool "Zbus iterable sections support."
-	depends on !XTENSA
-	default y
+config ZBUS_CHANNELS_SYS_INIT_PRIORITY
+	default 5
+	int "The priority used during the SYS_INIT procedure."
 
 config ZBUS_CHANNEL_NAME
 	bool "Channel name field"
 
 config ZBUS_OBSERVER_NAME
 	bool "Observer name field"
 
-config ZBUS_RUNTIME_OBSERVERS_POOL_SIZE
-	int "The size of the runtime observers pool."
-	default 0
-	help
-	  When the size is bigger than zero this feature will be enabled. It applies the Object Pool Pattern,
-	  where the objects in the pool are pre-allocated and can be used and recycled after use. The
-	  technique avoids dynamic allocation and allows the code to increase the number of observers by
-	  only changing a configuration.
+config ZBUS_RUNTIME_OBSERVERS
+	bool "Runtime observers support."
+	default n
 
 config ZBUS_ASSERT_MOCK
 	bool "Zbus assert mock for test purposes."