drm/i915: kerneldoc for fences

v2: Clarify that this is about fence _registers_. Also clarify that
the fence code revokes cpu ptes and not gtt ptes. Both suggested by
Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Documentation/DocBook/drm.tmpl

@@ -4197,6 +4197,11 @@ int num_ioctls;</synopsis>
         <title>Global GTT views</title>
 !Pdrivers/gpu/drm/i915/i915_gem_gtt.c Global GTT views
 !Idrivers/gpu/drm/i915/i915_gem_gtt.c
+      </sect2>
+      <sect2>
+        <title>Global GTT Fence Handling</title>
+!Pdrivers/gpu/drm/i915/i915_gem_fence.c fence register handling
+!Idrivers/gpu/drm/i915/i915_gem_fence.c
       </sect2>
       <sect2>
         <title>Buffer Object Eviction</title>

drivers/gpu/drm/i915/i915_gem_fence.c

@@ -25,6 +25,36 @@
 #include <drm/i915_drm.h>
 #include "i915_drv.h"
 
+/**
+ * DOC: fence register handling
+ *
+ * Important to avoid confusions: "fences" in the i915 driver are not execution
+ * fences used to track command completion but hardware detiler objects which
+ * wrap a given range of the global GTT. Each platform has only a fairly limited
+ * set of these objects.
+ *
+ * Fences are used to detile GTT memory mappings. They're also connected to the
+ * hardware frontbuffer render tracking and hence interract with frontbuffer
+ * conmpression. Furthermore on older platforms fences are required for tiled
+ * objects used by the display engine. They can also be used by the render
+ * engine - they're required for blitter commands and are optional for render
+ * commands. But on gen4+ both display (with the exception of fbc) and rendering
+ * have their own tiling state bits and don't need fences.
+ *
+ * Also note that fences only support X and Y tiling and hence can't be used for
+ * the fancier new tiling formats like W, Ys and Yf.
+ *
+ * Finally note that because fences are such a restricted resource they're
+ * dynamically associated with objects. Furthermore fence state is committed to
+ * the hardware lazily to avoid unecessary stalls on gen2/3. Therefore code must
+ * explictly call i915_gem_object_get_fence() to synchronize fencing status
+ * for cpu access. Also note that some code wants an unfenced view, for those
+ * cases the fence can be removed forcefully with i915_gem_object_put_fence().
+ *
+ * Internally these functions will synchronize with userspace access by removing
+ * CPU ptes into GTT mmaps (not the GTT ptes themselves) as needed.
+ */
+
 static void i965_write_fence_reg(struct drm_device *dev, int reg,
 				 struct drm_i915_gem_object *obj)
 {
@@ -247,6 +277,17 @@ i915_gem_object_wait_fence(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
+/**
+ * i915_gem_object_put_fence - force-remove fence for an object
+ * @obj: object to map through a fence reg
+ *
+ * This function force-removes any fence from the given object, which is useful
+ * if the kernel wants to do untiled GTT access.
+ *
+ * Returns:
+ *
+ * 0 on success, negative error code on failure.
+ */
 int
 i915_gem_object_put_fence(struct drm_i915_gem_object *obj)
 {
@@ -322,6 +363,10 @@ i915_find_fence_reg(struct drm_device *dev)
  * and tiling format.
  *
  * For an untiled surface, this removes any existing fence.
+ *
+ * Returns:
+ *
+ * 0 on success, negative error code on failure.
  */
 int
 i915_gem_object_get_fence(struct drm_i915_gem_object *obj)
@@ -374,6 +419,21 @@ i915_gem_object_get_fence(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
+/**
+ * i915_gem_object_pin_fence - pin fencing state
+ * @obj: object to pin fencing for
+ *
+ * This pins the fencing state (whether tiled or untiled) to make sure the
+ * object is ready to be used as a scanout target. Fencing status must be
+ * synchronize first by calling i915_gem_object_get_fence():
+ *
+ * The resulting fence pin reference must be released again with
+ * i915_gem_object_unpin_fence().
+ *
+ * Returns:
+ *
+ * True if the object has a fence, false otherwise.
+ */
 bool
 i915_gem_object_pin_fence(struct drm_i915_gem_object *obj)
 {
@@ -390,6 +450,14 @@ i915_gem_object_pin_fence(struct drm_i915_gem_object *obj)
 		return false;
 }
 
+/**
+ * i915_gem_object_unpin_fence - unpin fencing state
+ * @obj: object to unpin fencing for
+ *
+ * This releases the fence pin reference acquired through
+ * i915_gem_object_pin_fence. It will handle both objects with and without an
+ * attached fence correctly, callers do not need to distinguish this.
+ */
 void
 i915_gem_object_unpin_fence(struct drm_i915_gem_object *obj)
 {
@@ -400,6 +468,13 @@ i915_gem_object_unpin_fence(struct drm_i915_gem_object *obj)
 	}
 }
 
+/**
+ * i915_gem_restore_fences - restore fence state
+ * @dev: DRM device
+ *
+ * Restore the hw fence state to match the software tracking again, to be called
+ * after a gpu reset and on resume.
+ */
 void i915_gem_restore_fences(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;