Re: [Freedreno] [PATCH v2 5/5] drm/msm: subclass work object for vblank events

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On 2018-11-07 07:55, Sean Paul wrote:
On Tue, Nov 06, 2018 at 02:36:30PM -0800, Jeykumar Sankaran wrote:
msm maintains a separate structure to define vblank
work definitions and a list to track events submitted
to the workqueue. We can avoid this redundant list
and its protection mechanism, if we subclass the
work object to encapsulate vblank event parameters.

changes in v2:
	- subclass optimization on system wq (Sean Paul)

I wouldn't do it like this, tbh. One problem is that you've lost your
flush() on
unbind, so there's no way to know if you have workers in the wild waiting
to
enable/disable vblank.

Another issues is that AFAICT, we don't need a queue of enables/disables,
but
rather just the last requested state (ie: should we be on or off). So
things
don't need to be this complicated (and we're possibly thrashing vblank
on/off
for no reason).

I'm still of the mind that you should just make this synchronous and be
done
with the threads (especially since we're still uncovering/introducing
races!).

While scoping out the effort to make vblank events synchronous, I found
that the spinlock locking order of vblank request sequence and vblank callback
sequences are the opposite.

In DPU, drm_vblank_enable acquires vblank_time_lock before registering
the crtc to encoder which happens after acquiring encoder_spinlock. But
the vblank_callback acquires encoder_spinlock before accessing the registered crtc and calling into drm_vblank_handler which tries to acquire vblank_time_lock.
Acquiring both vblank_time_lock and encoder_spinlock in the same thread
is leading to deadlock.

In MDP5, I see the same pattern between vblank_time_lock and list_lock which
is used to track the irq handlers.

I believe that explains why msm_drv is queuing the vblank enable/disable
works to WQ after acquiring vblank_time_lock.

Thanks,
Jeykumar S.

Sean


Signed-off-by: Jeykumar Sankaran <jsanka@xxxxxxxxxxxxxx>
---
 drivers/gpu/drm/msm/msm_drv.c | 67
+++++++++++++------------------------------
 drivers/gpu/drm/msm/msm_drv.h |  7 -----
 2 files changed, 20 insertions(+), 54 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_drv.c
b/drivers/gpu/drm/msm/msm_drv.c
index 6d6c73b..8da5be2 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -203,61 +203,44 @@ u32 msm_readl(const void __iomem *addr)
 	return val;
 }

-struct vblank_event {
-	struct list_head node;
+struct msm_vblank_work {
+	struct work_struct work;
 	int crtc_id;
 	bool enable;
+	struct msm_drm_private *priv;
 };

 static void vblank_ctrl_worker(struct work_struct *work)
 {
-	struct msm_vblank_ctrl *vbl_ctrl = container_of(work,
-						struct msm_vblank_ctrl,
work);
-	struct msm_drm_private *priv = container_of(vbl_ctrl,
-					struct msm_drm_private,
vblank_ctrl);
+	struct msm_vblank_work *vbl_work = container_of(work,
+						struct msm_vblank_work,
work);
+	struct msm_drm_private *priv = vbl_work->priv;
 	struct msm_kms *kms = priv->kms;
-	struct vblank_event *vbl_ev, *tmp;
-	unsigned long flags;
-
-	spin_lock_irqsave(&vbl_ctrl->lock, flags);
-	list_for_each_entry_safe(vbl_ev, tmp, &vbl_ctrl->event_list, node)
{
-		list_del(&vbl_ev->node);
-		spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
-
-		if (vbl_ev->enable)
-			kms->funcs->enable_vblank(kms,
-
priv->crtcs[vbl_ev->crtc_id]);
-		else
-			kms->funcs->disable_vblank(kms,
-
priv->crtcs[vbl_ev->crtc_id]);

-		kfree(vbl_ev);
-
-		spin_lock_irqsave(&vbl_ctrl->lock, flags);
-	}
+	if (vbl_work->enable)
+		kms->funcs->enable_vblank(kms,
priv->crtcs[vbl_work->crtc_id]);
+	else
+		kms->funcs->disable_vblank(kms,
priv->crtcs[vbl_work->crtc_id]);

-	spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
+	kfree(vbl_work);
 }

 static int vblank_ctrl_queue_work(struct msm_drm_private *priv,
 					int crtc_id, bool enable)
 {
-	struct msm_vblank_ctrl *vbl_ctrl = &priv->vblank_ctrl;
-	struct vblank_event *vbl_ev;
-	unsigned long flags;
+	struct msm_vblank_work *vbl_work;

-	vbl_ev = kzalloc(sizeof(*vbl_ev), GFP_ATOMIC);
-	if (!vbl_ev)
+	vbl_work = kzalloc(sizeof(*vbl_work), GFP_ATOMIC);
+	if (!vbl_work)
 		return -ENOMEM;

-	vbl_ev->crtc_id = crtc_id;
-	vbl_ev->enable = enable;
+	INIT_WORK(&vbl_work->work, vblank_ctrl_worker);

-	spin_lock_irqsave(&vbl_ctrl->lock, flags);
-	list_add_tail(&vbl_ev->node, &vbl_ctrl->event_list);
-	spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
+	vbl_work->crtc_id = crtc_id;
+	vbl_work->enable = enable;
+	vbl_work->priv = priv;

-	schedule_work(&vbl_ctrl->work);
+	schedule_work(&vbl_work->work);

 	return 0;
 }
@@ -269,14 +252,13 @@ static int msm_drm_uninit(struct device *dev)
 	struct msm_drm_private *priv = ddev->dev_private;
 	struct msm_kms *kms = priv->kms;
 	struct msm_mdss *mdss = priv->mdss;
-	struct msm_vblank_ctrl *vbl_ctrl = &priv->vblank_ctrl;
-	struct vblank_event *vbl_ev, *tmp;
 	int i;

 	/* We must cancel and cleanup any pending vblank enable/disable
 	 * work before drm_irq_uninstall() to avoid work re-enabling an
 	 * irq after uninstall has disabled it.
 	 */
+
 	msm_gem_shrinker_cleanup(ddev);

 	drm_kms_helper_poll_fini(ddev);
@@ -292,12 +274,6 @@ static int msm_drm_uninit(struct device *dev)
 #endif
 	drm_mode_config_cleanup(ddev);

-	flush_work(&vbl_ctrl->work);
-	list_for_each_entry_safe(vbl_ev, tmp, &vbl_ctrl->event_list, node)
{
-		list_del(&vbl_ev->node);
-		kfree(vbl_ev);
-	}
-
 	/* clean up event worker threads */
 	for (i = 0; i < priv->num_crtcs; i++) {
 		if (priv->event_thread[i].thread) {
@@ -469,9 +445,6 @@ static int msm_drm_init(struct device *dev, struct
drm_driver *drv)
 	priv->wq = alloc_ordered_workqueue("msm", 0);

 	INIT_LIST_HEAD(&priv->inactive_list);
-	INIT_LIST_HEAD(&priv->vblank_ctrl.event_list);
-	INIT_WORK(&priv->vblank_ctrl.work, vblank_ctrl_worker);
-	spin_lock_init(&priv->vblank_ctrl.lock);

 	drm_mode_config_init(ddev);

diff --git a/drivers/gpu/drm/msm/msm_drv.h
b/drivers/gpu/drm/msm/msm_drv.h
index 05d33a7..d4cbde2 100644
--- a/drivers/gpu/drm/msm/msm_drv.h
+++ b/drivers/gpu/drm/msm/msm_drv.h
@@ -77,12 +77,6 @@ enum msm_mdp_plane_property {
 	PLANE_PROP_MAX_NUM
 };

-struct msm_vblank_ctrl {
-	struct work_struct work;
-	struct list_head event_list;
-	spinlock_t lock;
-};
-
 #define MSM_GPU_MAX_RINGS 4
 #define MAX_H_TILES_PER_DISPLAY 2

@@ -225,7 +219,6 @@ struct msm_drm_private {
 	struct notifier_block vmap_notifier;
 	struct shrinker shrinker;

-	struct msm_vblank_ctrl vblank_ctrl;
 	struct drm_atomic_state *pm_state;
 };

--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum,
a Linux Foundation Collaborative Project

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--
Jeykumar S
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