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

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Date:   Tue, 12 May 2020 09:47:19 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Krzysztof Kozlowski <krzk@...nel.org>,
        Bernard Zhao <bernard@...o.com>
Cc:     Kukjin Kim <kgene@...nel.org>, linux-pm@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        opensource.kernel@...o.com
Subject: Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area

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.

This patch should not be taken IMO. Maybe we can release lock before the
if statement, just to speed-up.

Regards,
Lukasz

> 
> Best regards,
> Krzysztof
>