Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200508131338.32956-1-bernard@vivo.com> <20200512065023.GA10741@kozik-lap>
 <e762ce12-eff0-d3a5-f083-2b592921de59@arm.com>
In-Reply-To: <e762ce12-eff0-d3a5-f083-2b592921de59@arm.com>
From:   Krzysztof Kozlowski <krzk@kernel.org>
Date:   Tue, 12 May 2020 11:05:28 +0200
Message-ID: <CAJKOXPekrkyDf2TMCnX7Nvbdaj-JQwuyqrsurFM4moALqVx8Sw@mail.gmail.com>
Subject: Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area
To:     Lukasz Luba <lukasz.luba@arm.com>
Cc:     Bernard Zhao <bernard@vivo.com>, Kukjin Kim <kgene@kernel.org>,
        linux-pm@vger.kernel.org,
        "linux-samsung-soc@vger.kernel.org" 
        <linux-samsung-soc@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        opensource.kernel@vivo.com
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, 12 May 2020 at 10:47, Lukasz Luba <lukasz.luba@arm.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@vivo.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.

Best regards,
Krzysztof