linux-kernel - Re: [RFC] memory: exynos5422-dmc: Document mutex scope

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Message-ID: <CAJKOXPe3OeKFhmtbF4OZup_ii_rxRHTaSK5BT-3T6ijqUukqtg@mail.gmail.com>
Date:   Sun, 9 Aug 2020 11:12:20 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Kukjin Kim <kgene@...nel.org>, linux-pm@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC] memory: exynos5422-dmc: Document mutex scope

On Tue, Aug 04, 2020 at 11:40:07AM +0100, Lukasz Luba wrote:
> Hi Krzysztof,
>
> On 7/24/20 7:08 PM, Krzysztof Kozlowski wrote:
> > Document scope of the mutex used by driver.
> >
> > Signed-off-by: Krzysztof Kozlowski <krzk@...nel.org>
> >
> > ---
> >
> > It seems mutex was introduced to protect:
> > 1. setting actual frequency/voltage,
> > 2. dmc->curr_rate (in exynos5_dmc_get_cur_freq()).
> >
> > However dmc->curr_rate in exynos5_dmc_get_status() is not protected. Is
> > it a bug?
>
> The callback get_dev_status() from devfreq->profile, which here is the
> exynos5_dmc_get_status() should be already called with devfreq->lock
> mutex hold, like e.g from simple_ondemand governor or directly
> using update_devfreq exported function:
> update_devfreq()
>   ->get_target_freq()
>     devfreq_update_stats()
>         df->profile->get_dev_status()
>
> The dmc->curr_rate is also used from sysfs interface from devfreq.
> The local dmc lock serializes also this use case (when the HW freq
> has changed but not set yet into curr_rate.

These are different locks. You cannot protect dmc->curr_rate with
devfreq->lock in one place and dmc-lock in other place. This won't
protect it.

> > ---
> >   drivers/memory/samsung/exynos5422-dmc.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/memory/samsung/exynos5422-dmc.c b/drivers/memory/samsung/exynos5422-dmc.c
> > index 93e9c2429c0d..0388066a7d96 100644
> > --- a/drivers/memory/samsung/exynos5422-dmc.c
> > +++ b/drivers/memory/samsung/exynos5422-dmc.c
> > @@ -114,6 +114,7 @@ struct exynos5_dmc {
> >     void __iomem *base_drexi0;
> >     void __iomem *base_drexi1;
> >     struct regmap *clk_regmap;
> > +   /* Protects curr_rate and frequency/voltage setting section */
> >     struct mutex lock;
> >     unsigned long curr_rate;
> >     unsigned long curr_volt;
> >
>
> I assume this missing comment for the lock was required by some scripts.
> In this case LGTM:
>
> Reviewed-by: Lukasz Luba <lukasz.luba@....com>

Such comments are always useful. It is also pointed by strict
checkpatch:
CHECK: struct mutex definition without comment

Best regards,
Krzysztof