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

[Jeykumar Sankaran <jsanka-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

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@codeaurora.org>
>> ---
>>  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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