Re: [PATCH 4/4] drm/imx: only send commit done event when all state has been applied

[Philipp Zabel <p.zabel@pengutronix.de>]

On Fri, 2018-09-14 at 18:59 +0200, Lucas Stach wrote:
> Currently there is a small race window where we could manage to arm the
> vblank event from atomic flush, but programming the hardware was too close
> to the frame end, so the hardware will only apply the current state on the
> next vblank. In this case we will send out the commit done event too early
> causing userspace to reuse framebuffes that are still in use.
> 
> Instead of using the event arming mechnism, just remember the pending event
> and send it from the vblank IRQ handler, once we are sure that all state
> has been applied sucessfully.
> 
> Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
> ---
>  drivers/gpu/drm/imx/ipuv3-crtc.c | 32 ++++++++++++++++++++++++++++----
>  1 file changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/imx/ipuv3-crtc.c b/drivers/gpu/drm/imx/ipuv3-crtc.c
> index 7d4b710b837a..b0c95565a28d 100644
> --- a/drivers/gpu/drm/imx/ipuv3-crtc.c
> +++ b/drivers/gpu/drm/imx/ipuv3-crtc.c
> @@ -42,6 +42,7 @@ struct ipu_crtc {
>  	struct ipu_dc		*dc;
>  	struct ipu_di		*di;
>  	int			irq;
> +	struct drm_pending_vblank_event *event;
>  };
>  
>  static inline struct ipu_crtc *to_ipu_crtc(struct drm_crtc *crtc)
> @@ -181,8 +182,31 @@ static const struct drm_crtc_funcs ipu_crtc_funcs = {
>  static irqreturn_t ipu_irq_handler(int irq, void *dev_id)
>  {
>  	struct ipu_crtc *ipu_crtc = dev_id;
> +	struct drm_crtc *crtc = &ipu_crtc->base;
> +	unsigned long flags;
> +	int i;
> +
> +	drm_crtc_handle_vblank(crtc);
> +
> +	if (ipu_crtc->event) {
> +		for (i = 0; i < ARRAY_SIZE(ipu_crtc->plane); i++) {
> +			struct ipu_plane *plane = ipu_crtc->plane[i];
> +
> +			if (!plane)
> +				continue;
> +
> +			if (!ipu_plane_atomic_update_done(&plane->base))

			if (ipu_plane_atomic_update_pending(&plane->base))

> +				break;
> +		}
>  
> -	drm_crtc_handle_vblank(&ipu_crtc->base);
> +		if (i == ARRAY_SIZE(ipu_crtc->plane)) {
> +			spin_lock_irqsave(&crtc->dev->event_lock, flags);
> +			drm_crtc_send_vblank_event(crtc, ipu_crtc->event);
> +			ipu_crtc->event = NULL;

These two happen under the event spinlock, but where event is set in
ipu_crtc_atomic_flush, the locking is removed.

> +			drm_crtc_vblank_put(crtc);
> +			spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
> +		}
> +	}
>  
>  	return IRQ_HANDLED;
>  }
> @@ -229,13 +253,13 @@ static void ipu_crtc_atomic_begin(struct drm_crtc *crtc,
>  static void ipu_crtc_atomic_flush(struct drm_crtc *crtc,
>  				  struct drm_crtc_state *old_crtc_state)
>  {
> -	spin_lock_irq(&crtc->dev->event_lock);
> +	struct ipu_crtc *ipu_crtc = to_ipu_crtc(crtc);
> +
>  	if (crtc->state->event) {
>  		WARN_ON(drm_crtc_vblank_get(crtc));
> -		drm_crtc_arm_vblank_event(crtc, crtc->state->event);
> +		ipu_crtc->event = crtc->state->event;

We assume here that ipu_crtc->event is NULL and the irq handler is never
running at the same time, otherwise we would drop an event. This is non-
obvious to me, and I think it warrants a comment.

My understanding is the following:

- It is virtually impossible for atomic_flush to race against a delayed
  previous ipu_irq_handler because the previous commit's commit_tail
  would still be waiting for the vblank event to release it from
  drm_atomic_helper_wait_for_flip_done.

  However, if the last commit's tail finishes after the irq_handler
  calls drm_crtc_send_vblank_event(), and the new commit is issued, and
  its tail work scheduled, all before the next line in the irq_handler,
  ipu_crtc->event = NULL, then the new commit's tail could call
  drm_atomic_helper_commit_planes and therefore ipu_crtc_atomic_flush
  and ipu_crtc->event would be overwritten.

- It is unproblematic for a delayed atomic_flush to race against the
  next ipu_irq_handler because ipu_crtc->event will just not be set
  when the irq handler checks it, and the vblank event will be deferred
  to the next interrupt.

regards
Philipp
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