linux-kernel - Re:Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area

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Date:   Tue, 12 May 2020 17:19:50 +0800 (GMT+08:00)
From:   Bernard <bernard@...o.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Lukasz Luba <lukasz.luba@....com>, Kukjin Kim <kgene@...nel.org>,
        linux-pm@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        opensource.kernel@...o.com
Subject: Re:Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area


From: Krzysztof Kozlowski <krzk@...nel.org>
Date: 2020-05-12 17:05:28
To:  Lukasz Luba <lukasz.luba@....com>
Cc:  Bernard Zhao <bernard@...o.com>,Kukjin Kim <kgene@...nel.org>,linux-pm@...r.kernel.org,"linux-samsung-soc@...r.kernel.org" <linux-samsung-soc@...r.kernel.org>,linux-arm-kernel@...ts.infradead.org,"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,opensource.kernel@...o.com
Subject: Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area>On Tue, 12 May 2020 at 10:47, Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> Hi Krzysztof,
>>
>> I am sorry, I was a bit busy recently.
>>
>> On 5/12/20 7:50 AM, Krzysztof Kozlowski wrote:
>> > On Fri, May 08, 2020 at 06:13:38AM -0700, Bernard Zhao wrote:
>> >> Maybe dmc->df->lock is unnecessary to protect function
>> >> exynos5_dmc_perf_events_check(dmc). If we have to protect,
>> >> dmc->lock is more better and more effective.
>> >> Also, it seems not needed to protect "if (ret) & dev_warn"
>> >> branch.
>> >>
>> >> Signed-off-by: Bernard Zhao <bernard@...o.com>
>> >> ---
>> >>   drivers/memory/samsung/exynos5422-dmc.c | 6 ++----
>> >>   1 file changed, 2 insertions(+), 4 deletions(-)
>> >
>> > I checked the concurrent accesses and it looks correct.
>> >
>> > Lukasz, any review from your side?
>>
>> The lock from devfreq lock protects from a scenario when
>> concurrent access from devfreq framework uses internal dmc fields 'load'
>> and 'total' (which are set to 'busy_time', 'total_time').
>> The .get_dev_status can be called at any time (even due to thermal
>> devfreq cooling action) and reads above fields.
>> That's why the calculation of the new values inside dmc is protected.
>
>I looked at this path (get_dev_status) and currently in devfreq it
>will be only called from update_devfreq() -> get_target_freq()... at
>least when looking at devfreq core and governors. On the other hand
>you are right that this is public function and this call scenario
>might change. It could be called directly from other paths sooner or
>later.
>
>> This patch should not be taken IMO. Maybe we can release lock before the
>> if statement, just to speed-up.
>
>Yep.
>
>Bernard, you can send just this part of the patch.
>

Sure, I will resubmit a patch in v2.

Best regards,
Bernard

>Best regards,
>Krzysztof