Re: [PATCH] drm/i915: Flush outstanding unpin tasks before pageflipping

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On Thu,  1 Nov 2012 09:26:26 +0000
Chris Wilson <chris@chris-wilson.co.uk> wrote:

> If we accumulate unpin tasks because we are pageflipping faster than the
> system can schedule its workers, we can effectively create a
> pin-leak. The solution taken here is to limit the number of unpin tasks
> we have per-crtc and to flush those outstanding tasks if we accumulate
> too many. This should prevent any jitter in the normal case, and also
> prevent the hang if we should run too fast.
> 
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=46991
> Reported-and-tested-by: Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_display.c |   22 ++++++++++++++++------
>  drivers/gpu/drm/i915/intel_drv.h     |    4 +++-
>  2 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 69b1739..800b195 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6908,14 +6908,19 @@ static void intel_unpin_work_fn(struct work_struct *__work)
>  {
>  	struct intel_unpin_work *work =
>  		container_of(__work, struct intel_unpin_work, work);
> +	struct drm_device *dev = work->crtc->dev;
>  
> -	mutex_lock(&work->dev->struct_mutex);
> +	mutex_lock(&dev->struct_mutex);
>  	intel_unpin_fb_obj(work->old_fb_obj);
>  	drm_gem_object_unreference(&work->pending_flip_obj->base);
>  	drm_gem_object_unreference(&work->old_fb_obj->base);
>  
> -	intel_update_fbc(work->dev);
> -	mutex_unlock(&work->dev->struct_mutex);
> +	intel_update_fbc(dev);
> +	mutex_unlock(&dev->struct_mutex);
> +
> +	BUG_ON(atomic_read(&to_intel_crtc(work->crtc)->unpin_work_count) == 0);
> +	atomic_dec(&to_intel_crtc(work->crtc)->unpin_work_count);
> +
>  	kfree(work);
>  }
>  
> @@ -6963,9 +6968,9 @@ static void do_intel_finish_page_flip(struct drm_device *dev,
>  
>  	atomic_clear_mask(1 << intel_crtc->plane,
>  			  &obj->pending_flip.counter);
> -
>  	wake_up(&dev_priv->pending_flip_queue);
> -	schedule_work(&work->work);
> +
> +	queue_work(dev_priv->wq, &work->work);
>  
>  	trace_i915_flip_complete(intel_crtc->plane, work->pending_flip_obj);
>  }
> @@ -7266,7 +7271,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
>  		return -ENOMEM;
>  
>  	work->event = event;
> -	work->dev = crtc->dev;
> +	work->crtc = crtc;
>  	intel_fb = to_intel_framebuffer(crtc->fb);
>  	work->old_fb_obj = intel_fb->obj;
>  	INIT_WORK(&work->work, intel_unpin_work_fn);
> @@ -7291,6 +7296,9 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
>  	intel_fb = to_intel_framebuffer(fb);
>  	obj = intel_fb->obj;
>  
> +	if (atomic_read(&intel_crtc->unpin_work_count) >= 2)
> +		flush_workqueue(dev_priv->wq);
> +

Have you by chance tested this with the async flip patch?  I wonder if
in that case whether 2 is too small, and something like 100 might be
better (though really async flips are for cases where we can't keep up
with refresh, so a small number shouldn't hurt too much there either).

-- 
Jesse Barnes, Intel Open Source Technology Center