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

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Message-ID: <6db3a6a4-fe56-d448-14c7-ed43de809acb@arm.com>
Date:   Tue, 11 Aug 2020 10:27:31 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
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 8/9/20 10:12 AM, Krzysztof Kozlowski wrote:
> 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.

There are different paths that framework goes and mainly they are
protected by the df->lock.
But I tend to agree, I will send a patch which adds some locking.

Regards,
Lukasz