linux-kernel - Re: [Freedreno] [PATCH 3/3] drm/msm/gpu: Remove mutex from wait

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Date:   Thu, 17 Mar 2022 14:07:45 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Akhil P Oommen <quic_akhilpo@...cinc.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Jonathan Marek <jonathan@...ek.ca>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sean Paul <sean@...rly.run>, Daniel Vetter <daniel@...ll.ch>,
        freedreno <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Subject: Re: [Freedreno] [PATCH 3/3] drm/msm/gpu: Remove mutex from wait_event condition

On Thu, Mar 17, 2022 at 1:45 PM Akhil P Oommen <quic_akhilpo@...cinc.com> wrote:
>
> On 3/11/2022 5:16 AM, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > The mutex wasn't really protecting anything before.  Before the previous
> > patch we could still be racing with the scheduler's kthread, as that is
> > not necessarily frozen yet.  Now that we've parked the sched threads,
> > the only race is with jobs retiring, and that is harmless, ie.
> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> >   drivers/gpu/drm/msm/adreno/adreno_device.c | 11 +----------
> >   1 file changed, 1 insertion(+), 10 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > index 0440a98988fc..661dfa7681fb 100644
> > --- a/drivers/gpu/drm/msm/adreno/adreno_device.c
> > +++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > @@ -607,15 +607,6 @@ static int adreno_runtime_resume(struct device *dev)
> >       return gpu->funcs->pm_resume(gpu);
> >   }
> >
> > -static int active_submits(struct msm_gpu *gpu)
> > -{
> > -     int active_submits;
> > -     mutex_lock(&gpu->active_lock);
> > -     active_submits = gpu->active_submits;
> > -     mutex_unlock(&gpu->active_lock);
> I assumed that this lock here was to ensure proper barriers while
> reading active_submits. Is that not required?

There is a spinlock in prepare_to_wait_event() ahead of checking the
condition, which AFAIU is a sufficient barrier

BR,
-R

>
> -Akhil.
> > -     return active_submits;
> > -}
> > -
> >   static int adreno_runtime_suspend(struct device *dev)
> >   {
> >       struct msm_gpu *gpu = dev_to_gpu(dev);
> > @@ -669,7 +660,7 @@ static int adreno_system_suspend(struct device *dev)
> >       suspend_scheduler(gpu);
> >
> >       remaining = wait_event_timeout(gpu->retire_event,
> > -                                    active_submits(gpu) == 0,
> > +                                    gpu->active_submits == 0,
> >                                      msecs_to_jiffies(1000));
> >       if (remaining == 0) {
> >               dev_err(dev, "Timeout waiting for GPU to suspend\n");
>