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

[Steven Price]

On 29/05/2020 13:35, ClÃment PÃron wrote:
> Hi Robin,
>
> On Fri, 29 May 2020 at 14:20, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 2020-05-10 17:55, ClÃment PÃron 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: ClÃment PÃron <peron.clem@xxxxxxxxx>
> > > ---
> > [...]
> > > 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,

Lima ('Utgard') has separate units for geometry and pixel processing 
(GP/PP). For Panfrost ('Midgard'/'Bifrost') we don't have that 
separation, however there are multiple job slots. which are implemented 
as multiple DRM schedulers. So the same fix is appropriate, but clearly 
I missed this comment because it's referring to GP/PP which don't exist 
for Midgard/Bifrost.

> > 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.

Technically this is a fix - there's a small race which could cause the 
devfreq information to become corrupted. However it would resolve itself 
on the next devfreq interval when panfrost_devfreq_reset() is called. So 
the impact is very minor (devfreq gets some bogus figures). The 
important variable (busy_count) was already an atomic so won't be affected.

Steve

> Thanks for you review,
>
>
> > Robin.