Re: [PATCH] drm/mediatek: Add spinlock for setting vblank event in atomic_begin

[AngeloGioacchino Del Regno]

Il 19/09/23 09:37, Jason-JH Lin (林睿祥) ha scritto:
> Hi Angelo
>
> Thanks for the reviews.
>
> Hi Fei,
>
> Thanks for the testing.
>
> On Mon, 2023-09-18 at 18:47 +0800, Fei Shao wrote:
> >   	
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >   Hi Angelo,
> >
> > On Wed, Sep 13, 2023 at 4:35 PM AngeloGioacchino Del Regno
> > <angelogioacchino.delregno@xxxxxxxxxxxxx> wrote:
> > > Il 22/08/23 15:26, Jason-JH.Lin ha scritto:
> > > > Add spinlock protection to avoid race condition on vblank event
> > > > between mtk_drm_crtc_atomic_begin() and
> > mtk_drm_finish_page_flip().
> > > >
> > >
> > > Hello Jason,
> > >
> > > Can you please provide more information about this race condition?
> > > (check below)
> > >
> > > > Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek
> > SoC MT8173.")
> > > > Signed-off-by: Jason-JH.Lin <jason-jh.lin@xxxxxxxxxxxx>
> > > > ---
> > > >    drivers/gpu/drm/mediatek/mtk_drm_crtc.c | 5 +++++
> > > >    1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> > b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> > > > index d40142842f85..128a672fe3c9 100644
> > > > --- a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> > > > +++ b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> > > > @@ -746,6 +746,9 @@ static void mtk_drm_crtc_atomic_begin(struct
> > drm_crtc *crtc,
> > >                                                                     
> >       crtc);
> > > >        struct mtk_crtc_state *mtk_crtc_state =
> > to_mtk_crtc_state(crtc_state);
> > > >        struct mtk_drm_crtc *mtk_crtc = to_mtk_crtc(crtc);
> > > > +     unsigned long flags;
> > > > +
> > > > +     spin_lock_irqsave(&crtc->dev->event_lock, flags);
> > > >
> > > >        if (mtk_crtc->event && mtk_crtc_state->base.event)
> > > >                DRM_ERROR("new event while there is still a pending
> > event\n");
> > > > @@ -756,6 +759,8 @@ static void mtk_drm_crtc_atomic_begin(struct
> > drm_crtc *crtc,
> > > ...because my suspect is that what creates the race condition in
> > this function is
> > > the unlocked *assignment* to mtk_crtc->event, not the rest.
> > >
> > > If I'm right, you don't need to unconditionally spinlock at the
> > beginning of this
> > > function hence ever-so-slightly improving performance compared to
> > this version.
> > > Can you please try this one and check if this *also* solves the
> > issue?
> > >          if (mtk_crtc_state->base.event) {
> > >                  mtk_crtc_state->base.event->pipe =
> > drm_crtc_index(crtc);
> > >                  WARN_ON(drm_crtc_vblank_get(crtc) != 0);
> > >
> > >                  spin_lock_irqsave(&crtc->dev->event_lock, flags);
> > >                  mtk_crtc->event = mtk_crtc_state->base.event;
> > >                  spin_lock_irqrestore(&crtc->dev->event_lock,
> > flags);
> > >                  mtk_crtc_state->base.event = NULL;
> > >          }
> > >
> > > P.S.: I'd try that myself, but I can't seem to reproduce the issue.
> I can't reproduce the system hang issue reported from customer by
> toggling the night light mode in UI panel either.
> But I can see the error message "new event while there is still a
> pending event" when I was reproducing the issue.
>
> Here is the debug info from "Wei-Shun <weishunc@xxxxxxxxxx>":
>
>  From the kernel tracing:
> 99342.377173: mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.377226:
> drm_crtc_send_vblank_event <-mtk_crtc_ddp_irq
> 99342.393831:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.393887:
> drm_crtc_send_vblank_event <-mtk_crtc_ddp_irq
> 99342.410469:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.410519:
> drm_crtc_send_vblank_event <-mtk_crtc_ddp_irq
> 99342.427094:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.443831:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.460495:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.477157:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.493841:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
> 99342.510453:
> mtk_crtc_ddp_irq <-mtk_disp_ovl_irq_handler
>
> Every mtk_crtc_ddp_irq should come with a drm_crtc_send_vblank_event.
> However, when the issue happens, the mtk_crtc_ddp_irq keeps firing but
> no drm_crtc_send_vblank_event. I suspect this is the main reason
> causing flip_done timeout.
>
> In mtk_drm_crtc.c, mtk_crtc->config_updating and mtk_crtc-
> > pending_needs_vblank are the conditions that may impact the vblank
> event.
>
> static void mtk_drm_finish_page_flip(struct mtk_drm_crtc *mtk_crtc)
> {
>      struct drm_crtc *crtc = &mtk_crtc->base;
>      unsigned long flags;
>
>      drm_crtc_handle_vblank(&mtk_crtc->base);
>
>      spin_lock_irqsave(&crtc->dev->event_lock, flags);
> ==> if (!mtk_crtc->config_updating && mtk_crtc->pending_needs_vblank) {
>           mtk_drm_crtc_finish_page_flip(mtk_crtc);
>           mtk_crtc->pending_needs_vblank = false;
>      }
>      spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
> }
>
>
> There are 3 lines are called in mtk_drm_crtc_finish_page_flip():
> drm_crtc_send_vblank_event(crtc, mtk_crtc->event);
> drm_crtc_vblank_put(crtc);
> mtk_crtc->event = NULL;
>
> So I want to protect these 3 things to avoid them encountering race
> conditions:
> mtk_crtc_state->base.event //will be provided on atomic_commit complete
> drm_crtc_vblank_get(crtc)
> mtk_crtc->event
>
>
> I have tried to protect this line only:
>
> spin_lock_irqsave(&crtc->dev->event_lock, flags);
> mtk_crtc->event = mtk_crtc_state->base.event;
> spin_lock_irqrestore(&crtc->dev->event_lock,flags);
>
> I can still see the error message "new event while there is still a
> pending event" when I toggled the night light mode.
>
> Maybe that is mtk_crtc->event here:
> if (mtk_crtc->event && mtk_crtc_state->base.event)
> haven't set to NULL by mtk_drm_crtc_finish_page_flip().
>
> But that not a problem because we will protect mtk_crtc->event here:
> mtk_crtc->event = mtk_crtc_state->base.event;
>
>
> So should we remove that error message since it doesn't help and may
> cause the confuse after we fix this issue?
>
>
> > I'm still able to reproduce it so I gave it a try, and this approach
> > also seems to fix the issue.  :)
> > FWIW, the way I reproduce that is to toggle the night light mode on
> > and off repeatedly through the UI panel while playing YouTube videos
> > on my device.
> >
> > Jason, can you post a new version with Angelo's suggestion?
>
> OK, I can take Angelo's suggestion. Thanks!
> But I am wondering if we should remove this log?
> DRM_ERROR("new event while there is still a pending event\n");

Thanks for the analysis.

Please don't remove that line, this indicates that there might be another issue
somewhere else, and someone will tackle that later.

Regards,
Angelo

> Regards,
> Jason-JH.Lin
>
> > Regards,
> > Fei
> >
> > > Regards,
> > > Angelo