tracing: add kernel documentation for trace_array_set_clr_event, trace_set_clr_event and supporting functions

As per Linux Kernel documentation guidelines
(https://docs.kernel.org/doc-guide/kernel-doc.html),
<<Every function that is exported to loadable modules using
EXPORT_SYMBOL or EXPORT_SYMBOL_GPL should have a kernel-doc
comment>>;
hence this patch adds detailed kernel-doc headers documentation
for trace_array_set_clr_event(), trace_set_clr_event() and the
main functions in the respective call-trees that support their
functionalities.

For each of the documented functions, as part of the extensive
description, a set of "Function's expectations" and "Assumptions
of Use" are described in a way that facilitate:

1) evaluating the current code and any proposed modification
   to behave as described;

2) writing kernel tests to verify the code to behave as described.

Signed-off-by: Gabriele Paoloni <gpaoloni@redhat.com>

Author: paolonig

kernel/trace/trace_events.c

@@ -763,6 +763,58 @@ void trace_event_enable_tgid_record(bool enable)
 	} while_for_each_event_file();
 }
 
+/**
+ * __ftrace_event_enable_disable - enable or disable a trace event
+ * @file: trace event file associated with the event.
+ * @enable: 0 or 1 respectively to disable/enable the event.
+ * @soft_disable: 1 or 0 respectively to mark if the enable parameter IS or
+ * IS NOT a soft enable/disable.
+ *
+ * Function's expectations:
+ * - If soft_disable is 1 a soft mode reference counter associated with the
+ *   trace event shall be increased or decreased according to the enable
+ *   parameter being 1 (enable) or 0 (disable) respectively.
+ *   If the soft mode reference counter is > 0 before the increase or after
+ *   the decrease, no other actions shall be taken.
+ *
+ * - if soft_disable is 1 and the soft mode reference counter is 0 before
+ *   the increase or after the decrease, an enable value set to 0 or 1 shall
+ *   result in disabling or enabling the use of trace_buffered_event
+ *   respectively.
+ *
+ * - If soft_disable is 1 and enable is 0 and the soft mode reference counter
+ *   reaches zero and if the soft disabled flag is set (i.e. if the event was
+ *   previously enabled with soft_disable = 1), tracing for the trace point
+ *   event shall be disabled and the soft disabled flag shall be cleared.
+ *
+ * - If soft_disable is 0 and enable is 0, tracing for the trace point event
+ *   shall be disabled only if the soft mode reference counter is 0.
+ *   Additionally the soft disabled flag shall be set or cleared according to
+ *   the soft mode reference counter being greater than 0 or 0 respectively.
+ *
+ * - If enable is 1, tracing for the trace point event shall be enabled (if
+ *   previously disabled); in addition, if soft_disable is 1 and the soft mode
+ *   reference counter is 0 before the increase, the soft disabled flag shall
+ *   be set.
+ *
+ * - When enabling or disabling tracing for the trace point event
+ *   the flags associated with comms and tgids shall be checked and, if set,
+ *   respectively tracing of comms and tgdis at sched_switch shall be
+ *   enabled/disabled.
+ *
+ * Assumptions of Use:
+ * - for thread-safe execution, event_mutex shall be locked before calling
+ *   this function;
+ * - the file input pointer is assumed to be a valid one;
+ * - the enable input parameter shall not be set to any value other than 0
+ *   or 1.
+ *
+ * Context: process context.
+ *
+ * Return:
+ * * 0 on success
+ * * any error returned by the event register or unregister callbacks
+ */
 static int __ftrace_event_enable_disable(struct trace_event_file *file,
 					 int enable, int soft_disable)
 {
@@ -1296,8 +1348,46 @@ static void remove_event_file_dir(struct trace_event_file *file)
 	event_file_put(file);
 }
 
-/*
- * __ftrace_set_clr_event(NULL, NULL, NULL, set) will set/unset all events.
+/**
+ * __ftrace_set_clr_event_nolock - enable or disable an event within a system.
+ * @tr: target trace_array containing the events list.
+ * @match: target system or event name (NULL for any).
+ * @sub: target system name (NULL for any).
+ * @event: target event name (NULL for any).
+ * @set: 1 to enable, 0 to disable.
+ * @mod: target module name (NULL for any).
+ *
+ * Function's expectations:
+ * - If mod is set, the mod name shall be sanitized by replacing all '-' with
+ *   '_' to match the modules' naming convention used in the Kernel.
+ *
+ * - From the events' list in the input tr, the ensemble of events to be enabled
+ *   or disabled shall be selected according to the input match, sub, event and
+ *   mod parameters. Each of these parameters, if set, shall restrict the events
+ *   ensemble to those with a matching parameter's name.
+ *
+ * - For each of the selected events the IGNORE_ENABLE flag shall be checked,
+ *   and, if not set, ftrace_event_enable_disable shall be invoked passing the
+ *   input set parameter to either enable or disable the event.
+ *
+ * Assumptions of Use:
+ * - for thread-safe execution, event_mutex shall be locked before calling
+ *   this function;
+ * - the tr input pointer is assumed to be a valid one;
+ * - the set input parameter shall not be set to any value other than 0
+ *   or 1.
+ *
+ * NOTE: __ftrace_set_clr_event_nolock(NULL, NULL, NULL, set, NULL) will
+ * set/unset all events.
+ *
+ * Context: process context.
+ *
+ * Return:
+ * * 0 on success
+ * * %-EINVAL - the input parameters do not match any registered event
+ * * %-ENOMEM -  memory allocation fails for the module pointer
+ * * any value returned by the first call to ftrace_event_enable_disable that
+ * returned an error
  */
 static int
 __ftrace_set_clr_event_nolock(struct trace_array *tr, const char *match,
@@ -1440,16 +1530,32 @@ int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set)
 }
 
 /**
- * trace_set_clr_event - enable or disable an event
- * @system: system name to match (NULL for any system)
- * @event: event name to match (NULL for all events, within system)
- * @set: 1 to enable, 0 to disable
+ * trace_set_clr_event - enable or disable an event within a system.
+ * @system: system name (NULL for any system).
+ * @event: event name (NULL for all events, within system).
+ * @set: 1 to enable, 0 to disable.
  *
  * This is a way for other parts of the kernel to enable or disable
  * event recording.
  *
- * Returns 0 on success, -EINVAL if the parameters do not match any
- * registered events.
+ * Function's expectations:
+ * - This function shall retrieve the pointer of the global trace array (global
+ *   tracer) and pass it, along the rest of input parameters, to
+ *   __ftrace_set_clr_event_nolock.
+ *
+ * - This function shall properly lock/unlock the global event_mutex
+ *   before/after invoking ftrace_set_clr_event_nolock.
+ *
+ * Assumptions of Use:
+ * - the set input parameter shall not be set to any value other than 0
+ *   or 1.
+ *
+ * Context: process context, locks and unlocks event_mutex.
+ *
+ * Return:
+ * * 0 on success
+ * * %-ENODEV - the global tracer cannot be retrieved
+ * * any other error condition returned by __ftrace_set_clr_event_nolock
  */
 int trace_set_clr_event(const char *system, const char *event, int set)
 {
@@ -1463,17 +1569,27 @@ int trace_set_clr_event(const char *system, const char *event, int set)
 EXPORT_SYMBOL_GPL(trace_set_clr_event);
 
 /**
- * trace_array_set_clr_event - enable or disable an event for a trace array.
- * @tr: concerned trace array.
- * @system: system name to match (NULL for any system)
- * @event: event name to match (NULL for all events, within system)
- * @enable: true to enable, false to disable
+ * trace_array_set_clr_event - enable or disable an event within a system for
+ * a trace array.
+ * @tr: input trace array.
+ * @system: system name (NULL for any system).
+ * @event: event name (NULL for all events, within system).
+ * @enable: true to enable, false to disable.
  *
- * This is a way for other parts of the kernel to enable or disable
- * event recording.
+ * This is a way for other parts of the kernel to enable or disable event
+ * recording.
+ *
+ * Function's expectations:
+ * - This function shall properly lock/unlock the global event_mutex
+ *   before/after invoking ftrace_set_clr_event_nolock passing along the same
+ *   input parameters.
+ *
+ * Context: process context, locks and unlocks event_mutex.
  *
- * Returns 0 on success, -EINVAL if the parameters do not match any
- * registered events.
+ * Return:
+ * * 0 on success
+ * * %-ENOENT - the input tr is NULL
+ * * any other error condition returned by __ftrace_set_clr_event_nolock
  */
 int trace_array_set_clr_event(struct trace_array *tr, const char *system,
 		const char *event, bool enable)