drm/i915: add some kernel-doc for shrink_pin and friends

Attempt to document shrink_pin and the other relevant interfaces that
interact with it, before we start messing with it.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018091055.1998191-5-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_object_types.h

@@ -488,6 +488,28 @@ struct drm_i915_gem_object {
 		 * instead go through the pin/unpin interfaces.
 		 */
 		atomic_t pages_pin_count;
+
+		/**
+		 * @shrink_pin: Prevents the pages from being made visible to
+		 * the shrinker, while the shrink_pin is non-zero. Most users
+		 * should pretty much never have to care about this, outside of
+		 * some special use cases.
+		 *
+		 * By default most objects will start out as visible to the
+		 * shrinker(if I915_GEM_OBJECT_IS_SHRINKABLE) as soon as the
+		 * backing pages are attached to the object, like in
+		 * __i915_gem_object_set_pages(). They will then be removed the
+		 * shrinker list once the pages are released.
+		 *
+		 * The @shrink_pin is incremented by calling
+		 * i915_gem_object_make_unshrinkable(), which will also remove
+		 * the object from the shrinker list, if the pin count was zero.
+		 *
+		 * Callers will then typically call
+		 * i915_gem_object_make_shrinkable() or
+		 * i915_gem_object_make_purgeable() to decrement the pin count,
+		 * and make the pages visible again.
+		 */
 		atomic_t shrink_pin;
 
 		/**
@@ -549,7 +571,7 @@ struct drm_i915_gem_object {
 		struct i915_gem_object_page_iter get_dma_page;
 
 		/**
-		 * Element within i915->mm.unbound_list or i915->mm.bound_list,
+		 * Element within i915->mm.shrink_list or i915->mm.purge_list,
 		 * locked by i915->mm.obj_lock.
 		 */
 		struct list_head link;

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c

@@ -463,6 +463,16 @@ void i915_gem_shrinker_taints_mutex(struct drm_i915_private *i915,
 
 #define obj_to_i915(obj__) to_i915((obj__)->base.dev)
 
+/**
+ * i915_gem_object_make_unshrinkable - Hide the object from the shrinker. By
+ * default all object types that support shrinking(see IS_SHRINKABLE), will also
+ * make the object visible to the shrinker after allocating the system memory
+ * pages.
+ * @obj: The GEM object.
+ *
+ * This is typically used for special kernel internal objects that can't be
+ * easily processed by the shrinker, like if they are perma-pinned.
+ */
 void i915_gem_object_make_unshrinkable(struct drm_i915_gem_object *obj)
 {
 	struct drm_i915_private *i915 = obj_to_i915(obj);
@@ -513,12 +523,33 @@ static void __i915_gem_object_make_shrinkable(struct drm_i915_gem_object *obj,
 	spin_unlock_irqrestore(&i915->mm.obj_lock, flags);
 }
 
+/**
+ * i915_gem_object_make_shrinkable - Move the object to the tail of the
+ * shrinkable list. Objects on this list might be swapped out. Used with
+ * WILLNEED objects.
+ * @obj: The GEM object.
+ *
+ * MUST only be called on objects which have backing pages.
+ *
+ * MUST be balanced with previous call to i915_gem_object_make_unshrinkable().
+ */
 void i915_gem_object_make_shrinkable(struct drm_i915_gem_object *obj)
 {
 	__i915_gem_object_make_shrinkable(obj,
 					  &obj_to_i915(obj)->mm.shrink_list);
 }
 
+/**
+ * i915_gem_object_make_purgeable - Move the object to the tail of the purgeable
+ * list. Used with DONTNEED objects. Unlike with shrinkable objects, the
+ * shrinker will attempt to discard the backing pages, instead of trying to swap
+ * them out.
+ * @obj: The GEM object.
+ *
+ * MUST only be called on objects which have backing pages.
+ *
+ * MUST be balanced with previous call to i915_gem_object_make_unshrinkable().
+ */
 void i915_gem_object_make_purgeable(struct drm_i915_gem_object *obj)
 {
 	__i915_gem_object_make_shrinkable(obj,