doc: reference: add discussion of terms that define API behavior

doc/reference/index.rst

@@ -23,3 +23,201 @@ API Reference
    usb/index.rst
    usermode/index.rst
    runtime_conf/index.rst
+
+Terminology
+***********
+
+The following terms may be used as shorthand API tags to indicate the
+allowed calling context (thread, ISR, pre-kernel), the effect of a call
+on the current thread state, and other behavioral characteristics.
+
+:ref:`api_term_reschedule`
+   if executing the function reaches a reschedule point
+:ref:`api_term_sleep`
+   if executing the function can cause the invoking thread to sleep
+:ref:`api_term_no-wait`
+   if a parameter to the function can prevent the invoking thread from
+   trying to sleep
+:ref:`api_term_isr-ok`
+   if the function can always be safely called from interrupt context
+   even if it may return an error in that case
+:ref:`api_term_pre-kernel-ok`
+   if the function can be safely called before the kernel has been fully
+   initialized, even if it may return an error in that case
+:ref:`api_term_async`
+   if the function may return before the operation it initializes is
+   complete (i.e. function return and operation completion are
+   asynchronous)
+:ref:`api_term_supervisor`
+   if the calling thread must have supervisor privileges to execute the
+   function
+
+Details on the behavioral impact of each attribute are in the following
+sections.
+
+.. _api_term_reschedule:
+
+reschedule
+==========
+
+The reschedule attribute is used on a function that can reach a
+:ref:`reschedule point <scheduling_v2>` within its execution.
+
+Details
+-------
+
+The significance of this attribute is that when a rescheduling function
+is invoked by a thread it is possible for that thread to be suspended as
+a consequence of a higher-priority thread being made ready.  Whether the
+suspension actually occurs depends on the operation associated with the
+reschedule point and the relative priorities of the invoking thread and
+the head of the ready queue.
+
+Note that in the case of timeslicing, or reschedule points executed from
+interrupts, any thread may be suspended in any function.
+
+Functions that are not **reschedule** may be invoked from either thread
+or interrupt context.
+
+Functions that are **reschedule** may be invoked from thread context.
+
+Functions that are **reschedule** but not **sleep** may be invoked from
+interrupt context.
+
+.. _api_term_sleep:
+
+sleep
+=====
+
+The sleep attribute is used on a function that can cause the invoking
+thread to sleep.
+
+Explanation
+-----------
+
+This attribute is of relevance specifically when considering
+applications that use only non-preemptible threads, because the kernel
+will not replace a running cooperative-only thread at a reschedule point
+unless that thread has explicitly invoked an operation that caused it to
+sleep.
+
+This attribute does not imply the function will sleep unconditionally,
+but that the operation may require an invoking thread that would have to
+suspend, wait, or invoke :cpp:func:`k_yield()` before it can complete
+its operation.  This behavior may be mediated by **no-wait**.
+
+Functions that are **sleep** are implicitly **reschedule**.
+
+Functions that are **sleep** may be invoked from thread context.
+
+Functions that are **sleep** may be invoked from interrupt and
+pre-kernel contexts if and only if invoked in **no-wait** mode.
+
+.. _api_term_no-wait:
+
+no-wait
+=======
+
+The no-wait attribute is used on a function that is also **sleep** to
+indicate that a parameter to the function can force an execution path
+that will not cause the invoking thread to sleep.
+
+Explanation
+-----------
+
+The paradigmatic case of a no-wait function is a function that takes a
+timeout, to which :c:macro:`K_NO_WAIT` can be passed.  The semantics of
+this special timeout value are to execute the function's operation as
+long as it can be completed immediately, and to return an error code
+rather than sleep if it cannot.
+
+It is use of the no-wait feature that allows functions like
+:cpp:func:`k_sem_take` to be invoked from ISRs, since it is not
+permitted to sleep in interrupt context.
+
+A function with a no-wait path does not imply that taking that path
+guarantees the function is synchronous.
+
+Functions with this attribute may be invoked from interrupt and
+pre-kernel contexts only when the parameter selects the no-wait path.
+
+.. _api_term_isr-ok:
+
+isr-ok
+======
+
+The isr-ok attribute is used on a function to indicate that it can be
+called from interrupt context.  If necessary the function will use
+:cpp:func:`k_is_in_isr` to detect its calling context and force an
+execution path that will not cause the invoking thread to sleep.
+
+Explanation
+-----------
+
+This attribute is intended for **sleep** functions that may be
+indirectly invoked from interrupt context with arguments that could
+attempt to put the invoking thread to sleep, e.g. because the function
+is not **no-wait** or the parameters do not select the no-wait path.
+
+Functions that are **isr-ok** may be always be safely invoked from
+interrupt context, and will return an error if they were unable to
+fulfill their behavior in that context.
+
+.. _api_term_pre-kernel-ok:
+
+pre-kernel-ok
+=============
+
+The pre-kernel-ok attribute is used on a function to indicate that it
+will take reasonable steps to ensure it is safe to invoke before all
+kernel services are started. In some cases the invocation in that
+context may return an error code.
+
+Explanation
+-----------
+
+This attribute is similar to **isr-ok** in function, but is intended for
+use by any API that is expected to be called in :c:func:`DEVICE_INIT()`
+or :c:func:`SYS_INIT()` calls that may be invoked with ``PRE_KERNEL_1``
+or ``PRE_KERNEL_2`` initialization levels.
+
+Generally a function that is **pre-kernel-ok** checks
+:cpp:func:`k_is_pre_kernel` when determining whether it can fulfill its
+required behavior.  In many cases it would also check
+:cpp:func:`k_is_in_isr` so it can be **isr-ok** as well.
+
+.. _api_term_async:
+
+async
+=====
+
+A function is **async** (i.e. asynchronous) if it may return before the
+operation it initiates has completed.  An asynchronous function will
+generally provide a mechanism by which operation completion is reported,
+e.g. a callback or event.
+
+A function that is not asynchronous is synchronous, i.e. the operation
+will always be complete when the function returns.  As most functions
+are synchronous this behavior does not have a distinct attribute to
+identify it.
+
+Explanation
+-----------
+
+Be aware that **async** is orthogonal to context-switching.  Some APIs
+may provide completion information through a callback, but may suspend
+while waiting for the resource necessary to initiate the operation; an
+example is :cpp:func:`spi_transceive_async`.
+
+If a function is both **no-wait** and **async** then selecting the
+no-wait path only guarantees that the function will not sleep.  It does
+not affect whether the operation will be completed before the function
+returns.
+
+.. _api_term_supervisor:
+
+supervisor
+==========
+
+The supervisor attribute is relevant only in user-mode applications, and
+indicates that the function cannot be invoked from user mode.

doc/reference/kernel/scheduling/index.rst

@@ -12,6 +12,17 @@ Concepts
 The scheduler determines which thread is allowed to execute
 at any point in time; this thread is known as the **current thread**.
 
+There are various points in time when the scheduler is given an
+opportunity to change the identity of the current thread.  These points
+are called **reschedule points**. Some potential reschedule points are:
+
+- transition of a thread to the :ref:`ready state <thread_states>`, for
+  example by :cpp:func:`k_sem_give` or :cpp:func:`k_thread_start`
+- transition of a thread from running state to a suspended or waiting
+  state, for example by :cpp:func:`k_sem_take` or :cpp:func:`k_sleep`
+- return to thread context after processing an interrupt
+- when a running thread invokes :cpp:func:`k_yield()`
+
 Whenever the scheduler changes the identity of the current thread,
 or when execution of the current thread is replaced by an ISR,
 the kernel first saves the current thread's CPU register values.

doc/zephyr.doxyfile.in

@@ -1980,6 +1980,7 @@ PREDEFINED             = "CONFIG_ARCH_HAS_CUSTOM_BUSY_WAIT" \
                          "CONFIG_SCHED_DEADLINE" \
                          "CONFIG_SCHED_DEADLINE" \
                          "CONFIG_SMP" \
+                         "CONFIG_SPI_ASYNC" \
                          "CONFIG_SYS_CLOCK_EXISTS" \
                          "CONFIG_SYS_POWER_MANAGEMENT" \
                          "CONFIG_THREAD_CUSTOM_DATA" \