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@xxxxxxxxxxxxxx>
> > > ---
> > > 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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