[PATCH v2 5/5] drm: don't block fb changes for async plane updates

[Helen Koike <helen.koike@collabora.com>]

In the case of a normal sync update, the preparation of framebuffers (be
it calling drm_atomic_helper_prepare_planes() or doing setups with
drm_framebuffer_get()) are performed in the new_state and the respective
cleanups are performed in the old_state.

In the case of async updates, the preparation is also done in the
new_state but the cleanups are done in the new_state (because updates
are performed in place, i.e. in the current state).

The current code blocks async udpates when the fb is changed, turning
async updates into sync updates, slowing down cursor updates and
introducing regressions in igt tests with errors of type:

"CRITICAL: completed 97 cursor updated in a period of 30 flips, we
expect to complete approximately 15360 updates, with the threshold set
at 7680"

Fb changes in async updates were prevented to avoid the following scenario:

- Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1
- Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2
- Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong)
Where we have a single call to prepare fb2 but double cleanup call to fb2.

To solve the above problems, instead of blocking async fb changes, we
place the old framebuffer in the new_state object, so when the code
performs cleanups in the new_state it will cleanup the old_fb and we
will have the following scenario instead:

- Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup
- Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1
- Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2

Where calls to prepare/cleanup are balanced.

Cc: <stable@vger.kernel.org> # v4.14+
Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>

---
Hello,

As mentioned in the cover letter, I tested in almost all platforms with
igt plane_cursor_legacy and kms_cursor_legacy and I didn't see any
regressions. But I couldn't test on MSM and AMD because I don't have
the hardware I would appreciate if anyone could help me testing those.

Thanks!
Helen

Changes in v2:
- Change the order of the patch in the series, add this as the last one.
- Add documentation
- s/ballanced/balanced

 drivers/gpu/drm/drm_atomic_helper.c      | 20 ++++++++++----------
 include/drm/drm_modeset_helper_vtables.h |  5 +++++
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 540a77a2ade9..e7eb96f1efc2 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1608,15 +1608,6 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
 	    old_plane_state->crtc != new_plane_state->crtc)
 		return -EINVAL;
 
-	/*
-	 * FIXME: Since prepare_fb and cleanup_fb are always called on
-	 * the new_plane_state for async updates we need to block framebuffer
-	 * changes. This prevents use of a fb that's been cleaned up and
-	 * double cleanups from occuring.
-	 */
-	if (old_plane_state->fb != new_plane_state->fb)
-		return -EINVAL;
-
 	funcs = plane->helper_private;
 	if (!funcs->atomic_async_update)
 		return -EINVAL;
@@ -1657,6 +1648,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
 	int i;
 
 	for_each_new_plane_in_state(state, plane, plane_state, i) {
+		struct drm_framebuffer *new_fb = plane_state->fb;
+		struct drm_framebuffer *old_fb = plane->state->fb;
+
 		funcs = plane->helper_private;
 		funcs->atomic_async_update(plane, plane_state);
 
@@ -1665,11 +1659,17 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
 		 * plane->state in-place, make sure at least common
 		 * properties have been properly updated.
 		 */
-		WARN_ON_ONCE(plane->state->fb != plane_state->fb);
+		WARN_ON_ONCE(plane->state->fb != new_fb);
 		WARN_ON_ONCE(plane->state->crtc_x != plane_state->crtc_x);
 		WARN_ON_ONCE(plane->state->crtc_y != plane_state->crtc_y);
 		WARN_ON_ONCE(plane->state->src_x != plane_state->src_x);
 		WARN_ON_ONCE(plane->state->src_y != plane_state->src_y);
+
+		/*
+		 * Make sure the FBs have been swapped so that cleanups in the
+		 * new_state performs a cleanup in the old FB.
+		 */
+		WARN_ON_ONCE(plane_state->fb != old_fb);
 	}
 }
 EXPORT_SYMBOL(drm_atomic_helper_async_commit);
diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index cfb7be40bed7..ce582e8e8f2f 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -1174,6 +1174,11 @@ struct drm_plane_helper_funcs {
 	 * current one with the new plane configurations in the new
 	 * plane_state.
 	 *
+	 * Drivers should also swap the framebuffers between plane state
+	 * and new_state. This is required because prepare and cleanup calls
+	 * are performed on the new_state object, then to cleanup the old
+	 * framebuffer, it needs to be placed inside the new_state object.
+	 *
 	 * FIXME:
 	 *  - It only works for single plane updates
 	 *  - Async Pageflips are not supported yet
-- 
2.20.1