component: Add documentation

While typing these I think doing an s/component_master/aggregate/
would be useful:
- it's shorter :-)
- I think component/aggregate is much more meaningful naming than
  component/puppetmaster or something like that. At least to my
  English ear "aggregate" emphasizes much more the "assemble a pile of
  things into something bigger" aspect, and there's not really much
  of a control hierarchy between aggregate and constituing components.

But that's way more than a quick doc typing exercise ...

Thanks to Ram for commenting on an initial draft of these docs.

v2: Review from Rafael:
- git add Documenation/driver-api/component.rst
- lots of polish to the wording + spelling fixes.

v3: Review from Russell:
- s/framework/helper
- clarify the documentation for component_match_add functions.

v4: Remove a few superflous "This".

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: "C, Ramalingam" <ramalingam.c@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207232759.14553-1-daniel.vetter@ffwll.ch

include/linux/component.h

@@ -4,11 +4,31 @@
 
 #include <linux/stddef.h>
 
+
 struct device;
 
+/**
+ * struct component_ops - callbacks for component drivers
+ *
+ * Components are registered with component_add() and unregistered with
+ * component_del().
+ */
 struct component_ops {
+	/**
+	 * @bind:
+	 *
+	 * Called through component_bind_all() when the aggregate driver is
+	 * ready to bind the overall driver.
+	 */
 	int (*bind)(struct device *comp, struct device *master,
 		    void *master_data);
+	/**
+	 * @unbind:
+	 *
+	 * Called through component_unbind_all() when the aggregate driver is
+	 * ready to bind the overall driver, or when component_bind_all() fails
+	 * part-ways through and needs to unbind some already bound components.
+	 */
 	void (*unbind)(struct device *comp, struct device *master,
 		       void *master_data);
 };
@@ -21,8 +41,42 @@ void component_unbind_all(struct device *master, void *master_data);
 
 struct master;
 
+/**
+ * struct component_master_ops - callback for the aggregate driver
+ *
+ * Aggregate drivers are registered with component_master_add_with_match() and
+ * unregistered with component_master_del().
+ */
 struct component_master_ops {
+	/**
+	 * @bind:
+	 *
+	 * Called when all components or the aggregate driver, as specified in
+	 * the match list passed to component_master_add_with_match(), are
+	 * ready. Usually there are 3 steps to bind an aggregate driver:
+	 *
+	 * 1. Allocate a structure for the aggregate driver.
+	 *
+	 * 2. Bind all components to the aggregate driver by calling
+	 *    component_bind_all() with the aggregate driver structure as opaque
+	 *    pointer data.
+	 *
+	 * 3. Register the aggregate driver with the subsystem to publish its
+	 *    interfaces.
+	 *
+	 * Note that the lifetime of the aggregate driver does not align with
+	 * any of the underlying &struct device instances. Therefore devm cannot
+	 * be used and all resources acquired or allocated in this callback must
+	 * be explicitly released in the @unbind callback.
+	 */
 	int (*bind)(struct device *master);
+	/**
+	 * @unbind:
+	 *
+	 * Called when either the aggregate driver, using
+	 * component_master_del(), or one of its components, using
+	 * component_del(), is unregistered.
+	 */
 	void (*unbind)(struct device *master);
 };
 
@@ -38,6 +92,22 @@ void component_match_add_release(struct device *master,
 	void (*release)(struct device *, void *),
 	int (*compare)(struct device *, void *), void *compare_data);
 
+/**
+ * component_match_add - add a compent match
+ * @master: device with the aggregate driver
+ * @matchptr: pointer to the list of component matches
+ * @compare: compare function to match against all components
+ * @compare_data: opaque pointer passed to the @compare function
+ *
+ * Adds a new component match to the list stored in @matchptr, which the @master
+ * aggregate driver needs to function. The list of component matches pointed to
+ * by @matchptr must be initialized to NULL before adding the first match.
+ *
+ * The allocated match list in @matchptr is automatically released using devm
+ * actions.
+ *
+ * See also component_match_add_release().
+ */
 static inline void component_match_add(struct device *master,
 	struct component_match **matchptr,
 	int (*compare)(struct device *, void *), void *compare_data)