drm/doc: document recommended component helper usage

Now that component has docs it's worth spending a few words and
hyperlinks on recommended best practices in drm.

v2: Add another item that component shouldn't be preferred over
drm_bridge/panel and similar subsystems already providing specialized
support for specific components (Laurent). Also convert to bullet
list.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190212164615.13370-1-daniel.vetter@ffwll.ch

Documentation/driver-api/component.rst

@@ -1,3 +1,5 @@
+.. _component:
+
 ======================================
 Component Helper for Aggregate Drivers
 ======================================

Documentation/gpu/drm-internals.rst

@@ -93,6 +93,11 @@ Device Instance and Driver Handling
 Driver Load
 -----------
 
+Component Helper Usage
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_drv.c
+   :doc: component helper usage recommendations
 
 IRQ Helper Library
 ~~~~~~~~~~~~~~~~~~

drivers/gpu/drm/drm_drv.c

@@ -456,6 +456,31 @@ static void drm_fs_inode_free(struct inode *inode)
 	}
 }
 
+/**
+ * DOC: component helper usage recommendations
+ *
+ * DRM drivers that drive hardware where a logical device consists of a pile of
+ * independent hardware blocks are recommended to use the :ref:`component helper
+ * library<component>`. For consistency and better options for code reuse the
+ * following guidelines apply:
+ *
+ *  - The entire device initialization procedure should be run from the
+ *    &component_master_ops.master_bind callback, starting with drm_dev_init(),
+ *    then binding all components with component_bind_all() and finishing with
+ *    drm_dev_register().
+ *
+ *  - The opaque pointer passed to all components through component_bind_all()
+ *    should point at &struct drm_device of the device instance, not some driver
+ *    specific private structure.
+ *
+ *  - The component helper fills the niche where further standardization of
+ *    interfaces is not practical. When there already is, or will be, a
+ *    standardized interface like &drm_bridge or &drm_panel, providing its own
+ *    functions to find such components at driver load time, like
+ *    drm_of_find_panel_or_bridge(), then the component helper should not be
+ *    used.
+ */
+
 /**
  * drm_dev_init - Initialise new DRM device
  * @dev: DRM device