From: Steven Price <steven.price@arm.com>
To: "Clément Péron" <peron.clem@gmail.com>,
"Robin Murphy" <robin.murphy@arm.com>
Cc: Nishanth Menon <nm@ti.com>,
Tomeu Vizoso <tomeu.vizoso@collabora.com>,
Stephen Boyd <sboyd@kernel.org>,
Viresh Kumar <vireshk@kernel.org>,
dri-devel <dri-devel@lists.freedesktop.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Chen-Yu Tsai <wens@csie.org>,
Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Subject: Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic
Date: Fri, 29 May 2020 13:47:16 +0100 [thread overview]
Message-ID: <33b045d6-deb9-2c09-3f74-5ca13f4d2e46@arm.com> (raw)
In-Reply-To: <CAJiuCcemwM-11ZT5+-4JfyTgTRD-_JjWz=HDCme8743M5Epf0g@mail.gmail.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.
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next prev parent reply other threads:[~2020-05-29 12:47 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-10 16:55 [PATCH 00/15][RFC] Add regulator devfreq support to Panfrost Clément Péron
2020-05-10 16:55 ` [PATCH 01/15] drm/panfrost: avoid static declaration Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 02/15] drm/panfrost: clean headers in devfreq Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 03/15] drm/panfrost: don't use pfdevfreq.busy_count to know if hw is idle Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 04/15] drm/panfrost: introduce panfrost_devfreq struct Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 05/15] drm/panfrost: use spinlock instead of atomic Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-29 12:20 ` Robin Murphy
2020-05-29 12:35 ` Clément Péron
2020-05-29 12:47 ` Steven Price [this message]
2020-05-10 16:55 ` [PATCH 06/15] drm/panfrost: properly handle error in probe Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 07/15] drm/panfrost: use device_property_present to check for OPP Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-29 12:45 ` Clément Péron
2020-05-10 16:55 ` [PATCH 08/15] drm/panfrost: move devfreq_init()/fini() in device Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-29 12:38 ` Clément Péron
2020-06-08 11:55 ` Tomeu Vizoso
2020-05-10 16:55 ` [PATCH 09/15] drm/panfrost: dynamically alloc regulators Clément Péron
2020-05-28 13:22 ` Steven Price
2020-05-10 16:55 ` [PATCH 10/15] drm/panfrost: add regulators to devfreq Clément Péron
2020-05-28 13:23 ` Steven Price
2020-05-29 12:37 ` Clément Péron
2020-05-10 16:55 ` [PATCH 11/15] drm/panfrost: set devfreq clock name Clément Péron
2020-05-28 13:23 ` Steven Price
2020-05-29 12:35 ` Clément Péron
2020-05-10 16:55 ` [PATCH 12/15] arm64: defconfig: Enable devfreq cooling device Clément Péron
2020-05-10 16:55 ` [PATCH 13/15] arm64: dts: allwinner: h6: Add cooling map for GPU Clément Péron
2020-05-10 16:55 ` [PATCH 14/15] [DO NOT MERGE] arm64: dts: allwinner: h6: Add GPU OPP table Clément Péron
2020-05-10 16:55 ` [PATCH 15/15] [DO NOT MERGE] arm64: dts: allwinner: force GPU regulator to be always Clément Péron
2020-05-11 5:43 ` [PATCH 00/15][RFC] Add regulator devfreq support to Panfrost Tomeu Vizoso
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