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

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jan 23, 2019 at 12:35:02PM +0100, Philipp Zabel wrote:
> On Fri, 2018-10-05 at 17:11 +0200, Philipp Zabel wrote:
> > 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.
> 
> How do we proceed with this? Keep the spin lock?

Yeah, standard practice is to protect these things with a spinlock,
usually the drm->event_lock. Then the flip_done wait should make sure
overall ordering is correct, too.

Might be good to improve the kerneldocs that this is a recommended
pattern.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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