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Date:   Wed, 23 Nov 2022 16:14:21 +0100
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Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *
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To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
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        Sean Young <sean@mess.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
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        Hans de Goede <hdegoede@redhat.com>,
        Mark Gross <markgross@kernel.org>,
        Vinod Koul <vkoul@kernel.org>,
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        Ming Lei <ming.lei@redhat.com>,
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        Alan Stern <stern@rowland.harvard.edu>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
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 <711d5275-7e80-c00d-0cdc-0f3d52175361@gmail.com>
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 <97be39ed-3cea-d55a-caa6-c2652baef399@gmail.com>
 <Y34zyzdbRUdyOSkA@casper.infradead.org>
From:   Maximilian Luz <luzmaximilian@gmail.com>
In-Reply-To: <Y34zyzdbRUdyOSkA@casper.infradead.org>
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On 11/23/22 15:52, Matthew Wilcox wrote:
> On Wed, Nov 23, 2022 at 02:59:00PM +0100, Maximilian Luz wrote:
>> On 11/23/22 14:34, Andy Shevchenko wrote:
>>> On Wed, Nov 23, 2022 at 02:14:31PM +0100, Maximilian Luz wrote:
>>>> On 11/23/22 13:25, Greg Kroah-Hartman wrote:
>>>>> The uevent() callback in struct device_type should not be modifying the
>>>>> device that is passed into it, so mark it as a const * and propagate the
>>>>> function signature changes out into all relevant subsystems that use
>>>>> this callback.
>>>
>>> [...]
>>>
>>>>> -static inline struct ssam_device *to_ssam_device(struct device *d)
>>>>> +static inline struct ssam_device *to_ssam_device(const struct device *d)
>>>>>     {
>>>>>     	return container_of(d, struct ssam_device, dev);
>>>>>     }
>>>>
>>>> I am slightly conflicted about this change as that now more or less
>>>> implicitly drops the const. So I'm wondering if it wouldn't be better to
>>>> either create a function specifically for const pointers or to just
>>>> open-code it in the instance above.
>>>>
>>>> I guess we could also convert this to a macro. Then at least there
>>>> wouldn't be an explicit and potentially misleading const-conversion
>>>> indicated in the function signature.
>>>
>>> This is an intermediate step as far as I know since moving container_of to
>>> recognize const is a bit noisy right now. I guess you can find a discussion
>>> on the topic between Greg and Sakari.
>>
>> Thanks! I assume you are referring to the following?
>>
>> 	https://lore.kernel.org/lkml/4218173bd72b4f1899d4c41a8e251f0d@AcuMS.aculab.com/T/
>>
>> As far as I can tell this is only a warning in documentation, not
>> compile time (which would probably be impossible?).
>>
>> As I've said I'd be fine with converting the function to a macro (and
>> preferably adding a similar warning like the one proposed in that
>> thread). The point that irks me up is just that, as proposed, the
>> function signature would now advertise a conversion that should never be
>> happening.
>>
>> Having two separate functions would create a compile-time guarantee, so
>> I'd prefer that, but I can understand if that might be considered too
>> noisy in code. Or if there is a push to make container_of() emit a
>> compile-time warning I'd also be perfectly happy with converting it to a
>> macro now as that'd alleviate the need for functions in the future.
> 
> Can't we do:
> 
> static inline const struct ssam_device *to_ssam_device(const struct device *d)
> {
> 	return container_of(d, const struct ssam_device, dev);
> }

There are at least a couple of places (device/driver-management and
device-removal related, I think) using this function and requiring
non-const access.

A bunch of other instances could be converted to const-access only, but
that would require a couple more function signature changes (I should
probably set up a patch for that regardless of this here as being a bit
more strict about this makes sense).

Regards,
Max