linux-kernel - Re: [PATCH v3 1/9] drm/vc4: Switch to container_of

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Message-ID: <6ckl6zmizxsm52imeau55bicb4hmd3p46btsqih64qaoeptvww@bktsqhuq4lek>
Date:   Tue, 25 Apr 2023 09:38:52 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Emma Anholt <emma@...olt.net>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans Verkuil <hverkuil@...all.nl>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH v3 1/9] drm/vc4: Switch to container_of_const

Hi Javier,

On Sat, Apr 22, 2023 at 07:26:13AM +0200, Javier Martinez Canillas wrote:
> Maxime Ripard <maxime@...no.tech> writes:
> > container_of_const() allows to preserve the pointer constness and is
> > thus more flexible than inline functions.
> >
> > Let's switch all our instances of container_of() to
> > container_of_const().
> >
> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> > ---
> 
> [...]
> 
> > -static inline struct vc4_dpi *
> > -to_vc4_dpi(struct drm_encoder *encoder)
> > -{
> > -	return container_of(encoder, struct vc4_dpi, encoder.base);
> > -}
> > +#define to_vc4_dpi(_encoder)						\
> > +	container_of_const(_encoder, struct vc4_dpi, encoder.base)
> >
> 
> A disadvantage of this approach though is that the type checking is lost.

Not entirely, the argument is still type-checked, but yeah, it's true
for the returned value.

> Since you already had these, I would probably had changed them to return
> a const pointer and just replace container_of() for container_of_const().
> 
> But I see that there are a lot of patches from Greg all over the kernel
> that do exactly this, dropping static inline functions in favor of using
> container_of_const() directly. So it seems the convention is what you do.

More importantly, container_of_const() isn't always returning a const
pointer or always taking a const argument, it's returning the pointer
with the same const-ness than the argument.

This is why it makes sense to remove the inline function entirely,
because it removes the main benefit it brings.

> Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>

Thanks, I've applied this series

Maxime

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