libtraceevent: Document tep_load_plugins_hook()

Add description of tep_load_plugins_hook() traceevent API. Updated
library man pages with description of the tep_load_plugins_hook() API.

Link: https://lore.kernel.org/r/CAM9d7cgLBWCrEHwz+Lhv5x5EXGcNWB0QQoeGh3OKh2JfR=dV9Q@mail.gmail.com
Link: https://lore.kernel.org/linux-trace-devel/20200714103027.2477584-2-tz.stoyanov@gmail.com
Link: https://lore.kernel.org/linux-trace-devel/20200716092014.2613403-2-tz.stoyanov@gmail.com

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200722011754.720060785@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/Documentation/libtraceevent-plugins.txt

@@ -3,7 +3,7 @@ libtraceevent(3)
 
 NAME
 ----
-tep_load_plugins, tep_unload_plugins - Load / unload traceevent plugins.
+tep_load_plugins, tep_unload_plugins, tep_load_plugins_hook - Load / unload traceevent plugins.
 
 SYNOPSIS
 --------
@@ -13,6 +13,12 @@ SYNOPSIS
 
 struct tep_plugin_list pass:[*]*tep_load_plugins*(struct tep_handle pass:[*]_tep_);
 void *tep_unload_plugins*(struct tep_plugin_list pass:[*]_plugin_list_, struct tep_handle pass:[*]_tep_);
+void *tep_load_plugins_hook*(struct tep_handle pass:[*]_tep_, const char pass:[*]_suffix_,
+			   void (pass:[*]_load_plugin_)(struct tep_handle pass:[*]tep,
+					       const char pass:[*]path,
+					       const char pass:[*]name,
+					       void pass:[*]data),
+			   void pass:[*]_data_);
 --
 
 DESCRIPTION
@@ -22,11 +28,13 @@ directories. The _tep_ argument is trace event parser context.
 The plugin directories are :
 [verse]
 --
+	- Directories, specified in _tep_->plugins_dir with priority TEP_PLUGIN_FIRST
 	- System's plugin directory, defined at the library compile time. It
 	  depends on the library installation prefix and usually is
 	  _(install_preffix)/lib/traceevent/plugins_
 	- Directory, defined by the environment variable _TRACEEVENT_PLUGIN_DIR_
 	- User's plugin directory, located at _~/.local/lib/traceevent/plugins_
+	- Directories, specified in _tep_->plugins_dir with priority TEP_PLUGIN_LAST
 --
 Loading of plugins can be controlled by the _tep_flags_, using the
 _tep_set_flag()_ API:
@@ -44,6 +52,12 @@ _tep_load_plugins()_. The _tep_ argument is trace event parser context. The
 _plugin_list_ is the list of loaded plugins, returned by
 the _tep_load_plugins()_ function.
 
+The _tep_load_plugins_hook_ function walks through all directories with plugins
+and calls user specified _load_plugin()_ hook for each plugin file. Only files
+with given _suffix_ are considered to be plugins. The _data_ is a user specified
+context, passed to _load_plugin()_. Directories and the walk order are the same
+as in _tep_load_plugins()_ API.
+
 RETURN VALUE
 ------------
 The _tep_load_plugins()_ function returns a list of successfully loaded plugins,
@@ -63,6 +77,15 @@ if (plugins == NULL) {
 }
 ...
 tep_unload_plugins(plugins, tep);
+...
+void print_plugin(struct tep_handle *tep, const char *path,
+		  const char *name, void *data)
+{
+	pritnf("Found libtraceevent plugin %s/%s\n", path, name);
+}
+...
+tep_load_plugins_hook(tep, ".so", print_plugin, NULL);
+...
 --
 
 FILES

tools/lib/traceevent/event-plugin.c

@@ -543,6 +543,22 @@ load_plugins_dir(struct tep_handle *tep, const char *suffix,
 	closedir(dir);
 }
 
+/**
+ * tep_load_plugins_hook - call a user specified callback to load a plugin
+ * @tep: handler to traceevent context
+ * @suffix: filter only plugin files with given suffix
+ * @load_plugin: user specified callback, called for each plugin file
+ * @data: custom context, passed to @load_plugin
+ *
+ * Searches for traceevent plugin files and calls @load_plugin for each
+ * The order of plugins search is:
+ *  - Directories, specified in @tep->plugins_dir and priority TEP_PLUGIN_FIRST
+ *  - Directory, specified at compile time with PLUGIN_TRACEEVENT_DIR
+ *  - Directory, specified by environment variable TRACEEVENT_PLUGIN_DIR
+ *  - In user's home: ~/.local/lib/traceevent/plugins/
+ *  - Directories, specified in @tep->plugins_dir and priority TEP_PLUGIN_LAST
+ *
+ */
 void tep_load_plugins_hook(struct tep_handle *tep, const char *suffix,
 			   void (*load_plugin)(struct tep_handle *tep,
 					       const char *path,