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

From: Fei Shao <fshao@chromium.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: "Jason-JH.Lin" <jason-jh.lin@mediatek.com>,
	Chun-Kuang Hu <chunkuang.hu@kernel.org>,
	 Alexandre Mergnat <amergnat@baylibre.com>,
	Eugen Hristev <eugen.hristev@collabora.com>,
	 Jason-ch Chen <jason-ch.chen@mediatek.com>,
	Johnson Wang <johnson.wang@mediatek.com>,
	 Singo Chang <singo.chang@mediatek.com>,
	Nancy Lin <nancy.lin@mediatek.com>,
	 Shawn Sung <shawn.sung@mediatek.com>,
	dri-devel@lists.freedesktop.org,
	 linux-mediatek@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	 linux-kernel@vger.kernel.org,
	 Project_Global_Chrome_Upstream_Group@mediatek.com
Subject: Re: [PATCH] drm/mediatek: Add spinlock for setting vblank event in atomic_begin
Date: Mon, 18 Sep 2023 18:47:28 +0800	[thread overview]
Message-ID: <CAC=S1ng8033sSGNFYr1vYfYfHbdG0Dk_0xai08=uomGO-EN66A@mail.gmail.com> (raw)
In-Reply-To: <5a4a0d12-d16a-0836-f6c5-6e6c2f3a9097@collabora.com>

Hi Angelo,

On Wed, Sep 13, 2023 at 4:35 PM AngeloGioacchino Del Regno
<angelogioacchino.delregno@collabora.com> 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@mediatek.com>
> > ---
> >   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'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?

Regards,
Fei

>
> Regards,
> Angelo
>

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