drm/atomic: Unconfuse the old_state mess in commmit_tail

I totally butcherd the job on typing the kernel-doc for these, and no
one realized. Noticed by Russell. Maarten has a more complete approach
to this confusion, by making it more explicit what the new/old state
is, instead of this magic switching behaviour.

v2:
- Liviu pointed out that wait_for_fences is even more magic. Leave
that as @state, and document @pre_swap better.
- While at it, patch in header for the reference section.
- Fix spelling issues Russell noticed.

v3: Fix up the @pre_swap note (Liviu): Also s/synchronous/blocking/,
since async flip is something else than non-blocking.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Fixes: 9f2a7950e77a ("drm/atomic-helper: nonblocking commit support")
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tomeu Vizoso <tomeu.vizoso@gmail.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161121171802.24147-1-daniel.vetter@ffwll.ch

Documentation/gpu/drm-kms-helpers.rst

@@ -63,6 +63,9 @@ Atomic State Reset and Initialization
 .. kernel-doc:: drivers/gpu/drm/drm_atomic_helper.c
    :doc: atomic state reset and initialization
 
+Helper Functions Reference
+--------------------------
+
 .. kernel-doc:: include/drm/drm_atomic_helper.h
    :internal:
 

drivers/gpu/drm/drm_atomic_helper.c

@@ -1006,13 +1006,21 @@ EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
  * drm_atomic_helper_wait_for_fences - wait for fences stashed in plane state
  * @dev: DRM device
  * @state: atomic state object with old state structures
- * @pre_swap: if true, do an interruptible wait
+ * @pre_swap: If true, do an interruptible wait, and @state is the new state.
+ * 	Otherwise @state is the old state.
  *
  * For implicit sync, driver should fish the exclusive fence out from the
  * incoming fb's and stash it in the drm_plane_state.  This is called after
  * drm_atomic_helper_swap_state() so it uses the current plane state (and
  * just uses the atomic state to find the changed planes)
  *
+ * Note that @pre_swap is needed since the point where we block for fences moves
+ * around depending upon whether an atomic commit is blocking or
+ * non-blocking. For async commit all waiting needs to happen after
+ * drm_atomic_helper_swap_state() is called, but for synchronous commits we want
+ * to wait **before** we do anything that can't be easily rolled back. That is
+ * before we call drm_atomic_helper_swap_state().
+ *
  * Returns zero if success or < 0 if dma_fence_wait() fails.
  */
 int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
@@ -1147,7 +1155,7 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks);
 
 /**
  * drm_atomic_helper_commit_tail - commit atomic update to hardware
- * @state: new modeset state to be committed
+ * @old_state: atomic state object with old state structures
  *
  * This is the default implemenation for the ->atomic_commit_tail() hook of the
  * &drm_mode_config_helper_funcs vtable.
@@ -1158,53 +1166,53 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks);
  *
  * For drivers supporting runtime PM the recommended sequence is instead ::
  *
- *     drm_atomic_helper_commit_modeset_disables(dev, state);
+ *     drm_atomic_helper_commit_modeset_disables(dev, old_state);
  *
- *     drm_atomic_helper_commit_modeset_enables(dev, state);
+ *     drm_atomic_helper_commit_modeset_enables(dev, old_state);
  *
- *     drm_atomic_helper_commit_planes(dev, state,
+ *     drm_atomic_helper_commit_planes(dev, old_state,
  *                                     DRM_PLANE_COMMIT_ACTIVE_ONLY);
  *
  * for committing the atomic update to hardware.  See the kerneldoc entries for
  * these three functions for more details.
  */
-void drm_atomic_helper_commit_tail(struct drm_atomic_state *state)
+void drm_atomic_helper_commit_tail(struct drm_atomic_state *old_state)
 {
-	struct drm_device *dev = state->dev;
+	struct drm_device *dev = old_state->dev;
 
-	drm_atomic_helper_commit_modeset_disables(dev, state);
+	drm_atomic_helper_commit_modeset_disables(dev, old_state);
 
-	drm_atomic_helper_commit_planes(dev, state, 0);
+	drm_atomic_helper_commit_planes(dev, old_state, 0);
 
-	drm_atomic_helper_commit_modeset_enables(dev, state);
+	drm_atomic_helper_commit_modeset_enables(dev, old_state);
 
-	drm_atomic_helper_commit_hw_done(state);
+	drm_atomic_helper_commit_hw_done(old_state);
 
-	drm_atomic_helper_wait_for_vblanks(dev, state);
+	drm_atomic_helper_wait_for_vblanks(dev, old_state);
 
-	drm_atomic_helper_cleanup_planes(dev, state);
+	drm_atomic_helper_cleanup_planes(dev, old_state);
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_tail);
 
-static void commit_tail(struct drm_atomic_state *state)
+static void commit_tail(struct drm_atomic_state *old_state)
 {
-	struct drm_device *dev = state->dev;
+	struct drm_device *dev = old_state->dev;
 	struct drm_mode_config_helper_funcs *funcs;
 
 	funcs = dev->mode_config.helper_private;
 
-	drm_atomic_helper_wait_for_fences(dev, state, false);
+	drm_atomic_helper_wait_for_fences(dev, old_state, false);
 
-	drm_atomic_helper_wait_for_dependencies(state);
+	drm_atomic_helper_wait_for_dependencies(old_state);
 
 	if (funcs && funcs->atomic_commit_tail)
-		funcs->atomic_commit_tail(state);
+		funcs->atomic_commit_tail(old_state);
 	else
-		drm_atomic_helper_commit_tail(state);
+		drm_atomic_helper_commit_tail(old_state);
 
-	drm_atomic_helper_commit_cleanup_done(state);
+	drm_atomic_helper_commit_cleanup_done(old_state);
 
-	drm_atomic_state_put(state);
+	drm_atomic_state_put(old_state);
 }
 
 static void commit_work(struct work_struct *work)
@@ -1498,10 +1506,10 @@ static struct drm_crtc_commit *preceeding_commit(struct drm_crtc *crtc)
 
 /**
  * drm_atomic_helper_wait_for_dependencies - wait for required preceeding commits
- * @state: new modeset state to be committed
+ * @old_state: atomic state object with old state structures
  *
  * This function waits for all preceeding commits that touch the same CRTC as
- * @state to both be committed to the hardware (as signalled by
+ * @old_state to both be committed to the hardware (as signalled by
  * drm_atomic_helper_commit_hw_done) and executed by the hardware (as signalled
  * by calling drm_crtc_vblank_send_event on the event member of
  * &drm_crtc_state).
@@ -1509,7 +1517,7 @@ static struct drm_crtc_commit *preceeding_commit(struct drm_crtc *crtc)
  * This is part of the atomic helper support for nonblocking commits, see
  * drm_atomic_helper_setup_commit() for an overview.
  */
-void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *state)
+void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *old_state)
 {
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *crtc_state;
@@ -1517,7 +1525,7 @@ void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *state)
 	int i;
 	long ret;
 
-	for_each_crtc_in_state(state, crtc, crtc_state, i) {
+	for_each_crtc_in_state(old_state, crtc, crtc_state, i) {
 		spin_lock(&crtc->commit_lock);
 		commit = preceeding_commit(crtc);
 		if (commit)
@@ -1548,7 +1556,7 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_dependencies);
 
 /**
  * drm_atomic_helper_commit_hw_done - setup possible nonblocking commit
- * @state: new modeset state to be committed
+ * @old_state: atomic state object with old state structures
  *
  * This function is used to signal completion of the hardware commit step. After
  * this step the driver is not allowed to read or change any permanent software
@@ -1561,15 +1569,15 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_dependencies);
  * This is part of the atomic helper support for nonblocking commits, see
  * drm_atomic_helper_setup_commit() for an overview.
  */
-void drm_atomic_helper_commit_hw_done(struct drm_atomic_state *state)
+void drm_atomic_helper_commit_hw_done(struct drm_atomic_state *old_state)
 {
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *crtc_state;
 	struct drm_crtc_commit *commit;
 	int i;
 
-	for_each_crtc_in_state(state, crtc, crtc_state, i) {
-		commit = state->crtcs[i].commit;
+	for_each_crtc_in_state(old_state, crtc, crtc_state, i) {
+		commit = old_state->crtcs[i].commit;
 		if (!commit)
 			continue;
 
@@ -1584,16 +1592,16 @@ EXPORT_SYMBOL(drm_atomic_helper_commit_hw_done);
 
 /**
  * drm_atomic_helper_commit_cleanup_done - signal completion of commit
- * @state: new modeset state to be committed
+ * @old_state: atomic state object with old state structures
  *
- * This signals completion of the atomic update @state, including any cleanup
- * work. If used, it must be called right before calling
+ * This signals completion of the atomic update @old_state, including any
+ * cleanup work. If used, it must be called right before calling
  * drm_atomic_state_put().
  *
  * This is part of the atomic helper support for nonblocking commits, see
  * drm_atomic_helper_setup_commit() for an overview.
  */
-void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *state)
+void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *old_state)
 {
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *crtc_state;
@@ -1601,8 +1609,8 @@ void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *state)
 	int i;
 	long ret;
 
-	for_each_crtc_in_state(state, crtc, crtc_state, i) {
-		commit = state->crtcs[i].commit;
+	for_each_crtc_in_state(old_state, crtc, crtc_state, i) {
+		commit = old_state->crtcs[i].commit;
 		if (WARN_ON(!commit))
 			continue;
 

include/drm/drm_modeset_helper_vtables.h

@@ -999,10 +999,14 @@ struct drm_mode_config_helper_funcs {
 	 * to implement blocking and nonblocking commits easily. It is not used
 	 * by the atomic helpers
 	 *
-	 * This hook should first commit the given atomic state to the hardware.
-	 * But drivers can add more waiting calls at the start of their
-	 * implementation, e.g. to wait for driver-internal request for implicit
-	 * syncing, before starting to commit the update to the hardware.
+	 * This function is called when the new atomic state has already been
+	 * swapped into the various state pointers. The passed in state
+	 * therefore contains copies of the old/previous state. This hook should
+	 * commit the new state into hardware. Note that the helpers have
+	 * already waited for preceeding atomic commits and fences, but drivers
+	 * can add more waiting calls at the start of their implementation, e.g.
+	 * to wait for driver-internal request for implicit syncing, before
+	 * starting to commit the update to the hardware.
 	 *
 	 * After the atomic update is committed to the hardware this hook needs
 	 * to call drm_atomic_helper_commit_hw_done(). Then wait for the upate