linux-kernel - Re: [PATCH v5] drm/msm/dp: Always clear mask bits to disable interrupts at dp_ctrl_reset_irq

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Date:   Wed, 11 May 2022 18:01:26 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>, agross@...nel.org,
        airlied@...ux.ie, bjorn.andersson@...aro.org, daniel@...ll.ch,
        dianders@...omium.org, robdclark@...il.com, sean@...rly.run,
        vkoul@...nel.org
Cc:     quic_abhinavk@...cinc.com, quic_aravindh@...cinc.com,
        quic_sbillaka@...cinc.com, freedreno@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] drm/msm/dp: Always clear mask bits to disable
 interrupts at dp_ctrl_reset_irq_ctrl()

Quoting Dmitry Baryshkov (2022-05-11 17:41:50)
> On 12/05/2022 03:02, Kuogee Hsieh wrote:
> > diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > index af7a80c..f3e333e 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > @@ -1389,8 +1389,13 @@ void dp_ctrl_reset_irq_ctrl(struct dp_ctrl *dp_ctrl, bool enable)
> >
> >       dp_catalog_ctrl_reset(ctrl->catalog);
> >
> > -     if (enable)
> > -             dp_catalog_ctrl_enable_irq(ctrl->catalog, enable);
> > +     /*
> > +      * all dp controller programmable registers will not
> > +      * be reset to default value after DP_SW_RESET
> > +      * therefore interrupt mask bits have to be updated
> > +      * to enable/disable interrupts
> > +      */
> > +     dp_catalog_ctrl_enable_irq(ctrl->catalog, enable);
> >   }
> >
> >   void dp_ctrl_phy_init(struct dp_ctrl *dp_ctrl)
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index c388323..79439b8 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -98,6 +98,8 @@ struct dp_display_private {
> >       struct dp_ctrl    *ctrl;
> >       struct dp_debug   *debug;
> >
> > +     atomic_t suspended;
>
> I think it'd be better to protect it with event_lock rather than using
> atomics.

Agreed. I think the concern is that the event queue will have "stuff" in
it. If the event queue was all a threaded irq we could simply call
synchronize_irq() after disabling the irq bit in the DP hardware and
then we would know it is safe to power down the DP logic. Unfortunately
the event queue is a kthread so we can't do that and we have to rewrite
synchronize_irq() by checking that the event queue is empty and waiting
for it to empty out otherwise. It's not safe enough to simply do the
power operations underneath the event_lock because there's a queue in
the kthread that might be waiting to grab the event_lock to process.