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

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Message-ID: <Y34zyzdbRUdyOSkA@casper.infradead.org>
Date:   Wed, 23 Nov 2022 14:52:59 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Maximilian Luz <luzmaximilian@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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>,
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        Sakari Ailus <sakari.ailus@...ux.intel.com>,
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Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *

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);
}