drm/atomic-helper: nonblocking commit support

Design ideas:

- split up the actual commit into different phases, and have
  completions for each of them. This will be useful for the future
  when we want to interleave phases much more aggressively, for e.g.
  queue depth > 1. For not it's just a minimal optimization compared
  to current common nonblocking implementation patterns from drivers,
  which all stall for the entire commit to complete, including vblank
  waits and cleanups.

- Extract a separate atomic_commit_hw hook since that's the part most
  drivers will need to overwrite, hopefully allowing even more shared
  code.

- Enforce EBUSY seamntics by attaching one of the completions to the
  flip_done vblank event. Side benefit of forcing atomic drivers using
  these helpers to implement event handlign at least semi-correct. I'm
  evil that way ;-)

- Ridiculously modular, as usual.

- The main tracking unit for a commit stays struct drm_atomic_state,
  and the ownership rules for that are unchanged. Ownership still
  gets transferred to the driver (and subsequently to the worker) on
  successful commits. What is added is a small, per-crtc, refcounted
  structure to track pending commits called struct drm_crtc_commit.
  No actual state is attached to that though, it's purely for ordering
  and waiting.

- Dependencies are implicitly handled by assuming that any CRTC part
  of &drm_atomic_state is a dependency, and that the current commit
  must wait for any commits to complete on those CRTC. This way
  drivers can easily add more depencies using
  drm_atomic_get_crtc_state(), which is very natural since in most
  case a dependency exists iff there's some bit of state that needs to
  be cross checked.

  Removing depencies is not possible, drivers simply need to be
  careful to not include every CRTC in a commit if that's not
  necessary. Which is a good idea anyway, since that also avoids
  ww_mutex lock contention.

- Queue depth > 1 sees some prep work in this patch by adding a stall
  paramater to drm_atomic_helper_swap_states(). To be able to push
  commits entirely free-standing and in a deeper queue through the
  back-end the driver must not access any obj->state pointers. This
  means we need to track the old state in drm_atomic_state (much
  easier with the consolidated arrays), and pass them all explicitly
  to driver backends (this will be serious amounts of churn).

  Once that's done stall can be set to false in swap_states.

v2: Dont ask for flip_done signalling when the CRTC is off and stays
off: Drivers don't handle events in that case. Instead complete right
away. This way future commits don't need to have special-case logic,
but can keep blocking for the flip_done completion.

v3: Tons of fixes:
- Stall for preceeding commit for real, not the current one by
  accident.
- Add WARN_ON in case drivers don't fire the drm event.
- Don't double-free drm events.

v4: Make legacy cursor not stall.

v5: Extend the helper hook to cover the entire commit tail. Some
drivers need special code for cleanup and vblank waiting, this makes
it a bit more useful. Inspired by the rockchip driver.

v6: Add WARN_ON to catch drivers who forget to send out the
drm event.

v7: Fixup the stalls in swap_state for real!!

v8:
- Fixup trailing whitespace, spotted by Maarten.
- Actually wait for flip_done in cleanup_done, like the comment says
  we should do. Thanks a lot for Tomeu for helping with debugging this
  on.

v9: Now with awesome kerneldoc!

v10: Split out drm_crtc_commit tracking infrastructure.

v:
- Add missing static (Gustavo).
- Split out the sync functions, only do the actual nonblocking
  logic in this patch (Maarten).

Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Tested-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tomeu Vizoso <tomeu.vizoso@gmail.com>
Cc: Daniel Stone <daniels@collabora.com>
Tested-by: Liviu Dudau <Liviu.Dudau@arm.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Testcase: igt/kms_flip/*
Testcase: igt/kms_cursor*
Testcase: igt/kms*plane*
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-10-git-send-email-daniel.vetter@ffwll.ch

1 files changed, 98 insertions, 46 deletions

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 6dee09cfbf93..6a13df8691d4 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1120,22 +1120,17 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks);
 
 /**
- * drm_atomic_helper_commit - commit validated state object
- * @dev: DRM device
- * @state: the driver state object
- * @nonblocking: whether nonblocking behavior is requested.
+ * drm_atomic_helper_commit_tail - commit atomic update to hardware
+ * @state: new modeset state to be committed
  *
- * This function commits a with drm_atomic_helper_check() pre-validated state
- * object. This can still fail when e.g. the framebuffer reservation fails. For
- * now this doesn't implement nonblocking commits.
+ * This is the default implemenation for the ->atomic_commit_tail() hook of the
+ * &drm_mode_config_helper_funcs vtable.
  *
- * Note that right now this function does not support nonblocking commits, hence
- * driver writers must implement their own version for now. Also note that the
- * default ordering of how the various stages are called is to match the legacy
- * modeset helper library closest. One peculiarity of that is that it doesn't
- * mesh well with runtime PM at all.
+ * Note that the default ordering of how the various stages are called is to
+ * match the legacy modeset helper library closest. One peculiarity of that is
+ * that it doesn't mesh well with runtime PM at all.
  *
- * For drivers supporting runtime PM the recommended sequence is
+ * For drivers supporting runtime PM the recommended sequence is instead ::
  *
  *     drm_atomic_helper_commit_modeset_disables(dev, state);
  *
@@ -1143,7 +1138,73 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks);
  *
  *     drm_atomic_helper_commit_planes(dev, state, true);
  *
- * See the kerneldoc entries for these three functions for more details.
+ * 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)
+{
+	struct drm_device *dev = state->dev;
+
+	drm_atomic_helper_commit_modeset_disables(dev, state);
+
+	drm_atomic_helper_commit_planes(dev, state, false);
+
+	drm_atomic_helper_commit_modeset_enables(dev, state);
+
+	drm_atomic_helper_commit_hw_done(state);
+
+	drm_atomic_helper_wait_for_vblanks(dev, state);
+
+	drm_atomic_helper_cleanup_planes(dev, state);
+}
+EXPORT_SYMBOL(drm_atomic_helper_commit_tail);
+
+static void commit_tail(struct drm_atomic_state *state)
+{
+	struct drm_device *dev = state->dev;
+	struct drm_mode_config_helper_funcs *funcs;
+
+	funcs = dev->mode_config.helper_private;
+
+	drm_atomic_helper_wait_for_fences(dev, state);
+
+	drm_atomic_helper_wait_for_dependencies(state);
+
+	if (funcs && funcs->atomic_commit_tail)
+		funcs->atomic_commit_tail(state);
+	else
+		drm_atomic_helper_commit_tail(state);
+
+	drm_atomic_helper_commit_cleanup_done(state);
+
+	drm_atomic_state_free(state);
+}
+
+static void commit_work(struct work_struct *work)
+{
+	struct drm_atomic_state *state = container_of(work,
+						      struct drm_atomic_state,
+						      commit_work);
+	commit_tail(state);
+}
+
+/**
+ * drm_atomic_helper_commit - commit validated state object
+ * @dev: DRM device
+ * @state: the driver state object
+ * @nonblock: whether nonblocking behavior is requested.
+ *
+ * This function commits a with drm_atomic_helper_check() pre-validated state
+ * object. This can still fail when e.g. the framebuffer reservation fails. This
+ * function implements nonblocking commits, using
+ * drm_atomic_helper_setup_commit() and related functions.
+ *
+ * Note that right now this function does not support nonblocking commits, hence
+ * driver writers must implement their own version for now.
+ *
+ * Committing the actual hardware state is done through the
+ * ->atomic_commit_tail() callback of the &drm_mode_config_helper_funcs vtable,
+ * or it's default implementation drm_atomic_helper_commit_tail().
  *
  * RETURNS:
  * Zero for success or -errno.
@@ -1154,13 +1215,12 @@ int drm_atomic_helper_commit(struct drm_device *dev,
 {
 	int ret;
 
-	if (nonblock)
-		return -EBUSY;
-
 	ret = drm_atomic_helper_setup_commit(state, nonblock);
 	if (ret)
 		return ret;
 
+	INIT_WORK(&state->commit_work, commit_work);
+
 	ret = drm_atomic_helper_prepare_planes(dev, state);
 	if (ret)
 		return ret;
@@ -1187,27 +1247,16 @@ int drm_atomic_helper_commit(struct drm_device *dev,
 	 * update. Which is important since compositors need to figure out the
 	 * composition of the next frame right after having submitted the
 	 * current layout.
+	 *
+	 * NOTE: Commit work has multiple phases, first hardware commit, then
+	 * cleanup. We want them to overlap, hence need system_unbound_wq to
+	 * make sure work items don't artifically stall on each another.
 	 */
 
-	drm_atomic_helper_wait_for_fences(dev, state);
-
-	drm_atomic_helper_wait_for_dependencies(state);
-
-	drm_atomic_helper_commit_modeset_disables(dev, state);
-
-	drm_atomic_helper_commit_planes(dev, state, false);
-
-	drm_atomic_helper_commit_modeset_enables(dev, state);
-
-	drm_atomic_helper_commit_hw_done(state);
-
-	drm_atomic_helper_wait_for_vblanks(dev, state);
-
-	drm_atomic_helper_cleanup_planes(dev, state);
-
-	drm_atomic_helper_commit_cleanup_done(state);
-
-	drm_atomic_state_free(state);
+	if (nonblock)
+		queue_work(system_unbound_wq, &state->commit_work);
+	else
+		commit_tail(state);
 
 	return 0;
 }
@@ -1216,12 +1265,7 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
 /**
  * DOC: implementing nonblocking commit
  *
- * For now the atomic helpers don't support nonblocking commit directly. If
- * there is real need it could be added though, using the dma-buf fence
- * infrastructure for generic synchronization with outstanding rendering.
- *
- * For now drivers have to implement nonblocking commit themselves, with the
- * following sequence being the recommended one:
+ * Nonblocking atomic commits have to be implemented in the following sequence:
  *
  * 1. Run drm_atomic_helper_prepare_planes() first. This is the only function
  * which commit needs to call which can fail, so we want to run it first and
@@ -1233,10 +1277,14 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
  * cancelled updates. Note that it is important to ensure that the framebuffer
  * cleanup is still done when cancelling.
  *
- * For sufficient parallelism it is recommended to have a work item per crtc
- * (for updates which don't touch global state) and a global one. Then we only
- * need to synchronize with the crtc work items for changed crtcs and the global
- * work item, which allows nice concurrent updates on disjoint sets of crtcs.
+ * Asynchronous workers need to have sufficient parallelism to be able to run
+ * different atomic commits on different CRTCs in parallel. The simplest way to
+ * achive this is by running them on the &system_unbound_wq work queue. Note
+ * that drivers are not required to split up atomic commits and run an
+ * individual commit in parallel - userspace is supposed to do that if it cares.
+ * But it might be beneficial to do that for modesets, since those necessarily
+ * must be done as one global operation, and enabling or disabling a CRTC can
+ * take a long time. But even that is not required.
  *
  * 3. The software state is updated synchronously with
  * drm_atomic_helper_swap_state(). Doing this under the protection of all modeset
@@ -1249,6 +1297,10 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
  * commit helpers: a) pre-plane commit b) plane commit c) post-plane commit and
  * then cleaning up the framebuffers after the old framebuffer is no longer
  * being displayed.
+ *
+ * The above scheme is implemented in the atomic helper libraries in
+ * drm_atomic_helper_commit() using a bunch of helper functions. See
+ * drm_atomic_helper_setup_commit() for a starting point.
  */
 
 static int stall_checks(struct drm_crtc *crtc, bool nonblock)