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

[Steven Price <steven.price@arm.com>]

On 29/05/2020 13:35, Clément Péron wrote:
> Hi Robin,
> 
> On Fri, 29 May 2020 at 14:20, Robin Murphy <robin.murphy@arm.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@gmail.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,

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.