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Date:   Wed, 23 Nov 2022 14:59:00 +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:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>,
        Stefan Richter <stefanr@s5r6.in-berlin.de>,
        Wolfram Sang <wsa@kernel.org>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Sean Young <sean@mess.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Mark Gross <markgross@kernel.org>,
        Vinod Koul <vkoul@kernel.org>,
        Bard Liao <yung-chuan.liao@linux.intel.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
        Sanyog Kale <sanyog.r.kale@intel.com>,
        Andreas Noever <andreas.noever@gmail.com>,
        Michael Jamet <michael.jamet@intel.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Yehezkel Bernat <YehezkelShB@gmail.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Ming Lei <ming.lei@redhat.com>,
        Jilin Yuan <yuanjilin@cdjrlc.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ira Weiny <ira.weiny@intel.com>,
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        Won Chung <wonchung@google.com>, alsa-devel@alsa-project.org,
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References: <20221123122523.1332370-1-gregkh@linuxfoundation.org>
 <20221123122523.1332370-3-gregkh@linuxfoundation.org>
 <711d5275-7e80-c00d-0cdc-0f3d52175361@gmail.com>
 <Y34hgIW8p1RlQTBB@smile.fi.intel.com>
From:   Maximilian Luz <luzmaximilian@gmail.com>
In-Reply-To: <Y34hgIW8p1RlQTBB@smile.fi.intel.com>
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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.

Regards,
Max