linux-kernel - Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic

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Date:   Fri, 29 May 2020 14:35:08 +0200
From:   Clément Péron <peron.clem@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic

Hi Robin,

On Fri, 29 May 2020 at 14:20, Robin Murphy <robin.murphy@....com> wrote:
>
> On 2020-05-10 17:55, Clément Péron wrote:
> > Convert busy_count to a simple int protected by spinlock.
>
> A little more reasoning might be nice.

I have follow the modification requested for lima devfreq and clearly
don't have any argument to switch to spinlock.

The Lima Maintainer asked to change witht the following reason :
"Better make this count a normal int which is also protected by the spinlock,
because current implementation can't protect atomic ops for state change
and busy idle check and we are using spinlock already"

>
> > Signed-off-by: Clément Péron <peron.clem@...il.com>
> > ---
> [...]
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_devfreq.h b/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> > index 0697f8d5aa34..e6629900a618 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> > +++ b/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> > @@ -4,6 +4,7 @@
> >   #ifndef __PANFROST_DEVFREQ_H__
> >   #define __PANFROST_DEVFREQ_H__
> >
> > +#include <linux/spinlock.h>
> >   #include <linux/ktime.h>
> >
> >   struct devfreq;
> > @@ -14,10 +15,17 @@ struct panfrost_device;
> >   struct panfrost_devfreq {
> >       struct devfreq *devfreq;
> >       struct thermal_cooling_device *cooling;
> > +
> >       ktime_t busy_time;
> >       ktime_t idle_time;
> >       ktime_t time_last_update;
> > -     atomic_t busy_count;
> > +     int busy_count;
> > +     /*
> > +      * Protect busy_time, idle_time, time_last_update and busy_count
> > +      * because these can be updated concurrently, for example by the GP
> > +      * and PP interrupts.
> > +      */
>
> Nit: this comment is clearly wrong, since we only have Job, GPU and MMU
> interrupts here. I guess if there is a race it would be between
> submission/completion/timeout on different job slots.

It's copy/paste from lima I will update it,

>
> Given that, should this actually be considered a fix for 9e62b885f715
> ("drm/panfrost: Simplify devfreq utilisation tracking")?

I can't say if it can be considered as a fix, I didn't see any
improvement on my board before and after this patch.
I'm still facing some issue and didn't have time to fully investigate it.

Thanks for you review,


>
> Robin.