doc: document system workqueue stack size calculation

Make note of the new method of determining the system workqueue size in
the migration guide and the workqueue docs.

Signed-off-by: Jordan Yates <[email protected]>

Author: JordanYates

doc/kernel/services/threads/workqueue.rst

@@ -215,6 +215,23 @@ use of it.
     for example, if the new work items perform blocking operations that
     would delay other system workqueue processing to an unacceptable degree.
 
+Stack Size
+==========
+
+By default the stack size of the system workqueue is determined co-operatively
+among all users of the workqueue. The build system compares the value of all
+Kconfig symbols matching the pattern
+``CONFIG_SYSTEM_WORKQUEUE_MIN_STACK_SIZE_*`` and selects the largest value as
+the stack size. :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE` is
+included in this calculation by its value being copied into
+:kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_MIN_STACK_SIZE_DEFAULT`.
+
+.. note::
+    This behaviour can be disabled through
+    :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE_IGNORE_MIN`, in which
+    case the value of :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE` will
+    be used directly.
+
 How to Use Workqueues
 *********************
 

doc/releases/migration-guide-4.2.rst

@@ -26,6 +26,15 @@ Build System
 Kernel
 ******
 
+* The stack size of the system workqueue is now computed co-operatively
+  among users as the maximum value of any Kconfig options whose name starts
+  with ``CONFIG_SYSTEM_WORKQUEUE_MIN_STACK_SIZE_``. The existing
+  :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE` symbol value is copied
+  to :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_MIN_STACK_SIZE_DEFAULT` so that
+  it is included in the calculation without any application changes.
+  The previous behaviour can be selected with
+  :kconfig:option:`CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE_IGNORE_MIN`.
+
 Boards
 ******