linux-kernel - Re: [PATCH v5 07/12] PM / devfreq: export devfreq

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Date:   Wed, 01 Aug 2018 17:18:40 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Benson Leung <bleung@...omium.org>,
        Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v5 07/12] PM / devfreq: export devfreq_class

Hi Matthias,

On 2018년 08월 01일 04:29, Matthias Kaehlcke wrote:
> On Mon, Jul 16, 2018 at 12:41:14PM -0700, Matthias Kaehlcke wrote:
>> Hi Chanwoo,
>>
>> On Thu, Jul 12, 2018 at 06:08:36PM +0900, Chanwoo Choi wrote:
>>> Hi Matthias,
>>>
>>> On 2018년 07월 07일 03:09, Matthias Kaehlcke wrote:
>>>> Hi,
>>>>
>>>> On Wed, Jul 04, 2018 at 02:30:32PM +0900, Chanwoo Choi wrote:
>>>>
>>>>> I didn't see any framework which exporting the class instance.
>>>>> It is very dangerous. Unknown device drivers is able to reset
>>>>> the 'devfreq_class' instance. I can't agree this approach.
>>>>
>>>> While I agree that it is potential dangerous it is actually a common
>>>> practice to export the class:
>>>>
>>>
>>> I tried to find the real usage of exported class instance
>>> and I add the comment for each class instance. Almost exported class
>>> instance are used in the their own director or some exported class
>>> like rio_mport_class/switchtec_class are created from specific device driver
>>> instead of subsystem.
>>>
>>> Only following two cases are used on outside of subsystem directory.
>>> devtmpfs.c and alarmtimer.c are core feature of linux kernel.
>>>
>>> 	drivers/base/devtmpfs.c uses 'block_class'.
>>> 	kernel/time/alarmtimer.c uses 'rtc_class'.
>>>
>>> I cannot yet agree this approach due to only block_class and rtc_class.
>>
>> I thought your main concern was that the class is exported, which is
>> what several other subsystems do. That the class isn't used outside of
>> the subsystem directory most likely means that there is no need for
>> it, rather than that it shouldn't be done at all (depending on the
>> type of use of course).
>>
>> In any case not exporting the class object provides a limited
>> protection against potential abuse of the class at best. To use the
>> class API all that is needed is a 'struct device' of a devfreq device,
>> which has a pointer to the class object (dev->class).
>>
>> Theoretically I could register a fake devfreq device to obtain access
>> to the class object, though that doesn't seem a very neat approach ;-)
>>
>>> You added the following comment why devfreq_class instance is necessary.
>>> Actullay, I don't know the best solution right now. But, all device drivers
>>> can be added or removed if driver uses the module type. It is not a problem
>>> for only devfreq instance.
>>
>> Certainly it's not a problem limited to devfreq devices. In many other
>> cases bus notifiers can be used, but since devfreq devices areen't
>> tied to a specific bus this is not an option here.
>>
>> If you really don't want to export the class we could add wrappers
>> for (un)registering a class interface:
>>
>> int devfreq_class_interface_register(struct class_interface *)
>> void devfreq_class_interface_unregister(struct class_interface *)

About this approach, I agree because it doesn't export the devfreq_class
instance as you commented.


>>
>> The wrappers would have to assign ci->class since the throttler
>> can't see the class object.
>>
>> Or add notifiers for device addition/removal, though the throttler
>> relies on the behavior of the class_interface which also notifies
>> about devices added before registration. This might not be what other
>> potential users of the notifiers expect.
> 
> Ping
> 
> Could we please try to find a solution/reach a conclusion for this?
> 
> Not that it should affect the outcome of this discussion, but I want
> to mention that from my point of view it is a bit unfortunate that
> this and other fundamental concerns were only raised after I spent
> significant time on repeatedly refactoring the throttler driver to
> address other comments. Since you and MyungJoo Ham previously had only
> minor comments on the other devfreq patches in this series I assumed
> there were no major concerns from your side :(
> 
> 

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics