linux-kernel - Re: [PATCH 3/5] driver core: make struct device

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Message-ID: <97be39ed-3cea-d55a-caa6-c2652baef399@gmail.com>
Date:   Wed, 23 Nov 2022 14:59:00 +0100
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        Wolfram Sang <wsa@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Sean Young <sean@...s.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        Vinod Koul <vkoul@...nel.org>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Ming Lei <ming.lei@...hat.com>,
        Jilin Yuan <yuanjilin@...rlc.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Won Chung <wonchung@...gle.com>, alsa-devel@...a-project.org,
        devicetree@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-block@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-i3c@...ts.infradead.org, linux-input@...r.kernel.org,
        linux-media@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-usb@...r.kernel.org, linux1394-devel@...ts.sourceforge.net,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *

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