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Date:   Fri, 21 Jul 2023 20:12:48 +0200
From:   Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= <u.kleine-koenig@pengutronix.de>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Nuno Sa <nuno.sa@analog.com>
Subject: Re: [PATCH v2 1/8] iio: core: Add opaque_struct_size() helper and
 use it
Message-ID: <20230721181248.s7dnkcdhn3hb6ihh@pengutronix.de>
References: <20230721170022.3461-1-andriy.shevchenko@linux.intel.com>
 <20230721170022.3461-2-andriy.shevchenko@linux.intel.com>
 <ZLq+2+9q6To3uAkf@smile.fi.intel.com>
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Hello Andy,

On Fri, Jul 21, 2023 at 08:25:12PM +0300, Andy Shevchenko wrote:
> On Fri, Jul 21, 2023 at 08:22:35PM +0300, Andy Shevchenko wrote:
> > On Fri, Jul 21, 2023 at 08:00:15PM +0300, Andy Shevchenko wrote:
> > > Introduce opaque_struct_size() helper, which may be moved
> > > to overflow.h in the future, and use it in the IIO core.
>=20
> ...
>=20
> > > +#define opaque_struct_size(p, a, s)	({		\
> > > +	size_t _psize =3D sizeof(*(p));			\
> > > +	size_t _asize =3D ALIGN(_psize, (a));		\
> > > +	size_t _ssize =3D s;				\
> > > +	_ssize ? size_add(_asize, _ssize) : _psize;	\
> > > +})
> > > +
> > > +#define opaque_struct_data(p, a)			\
> > > +	((void *)(p) + ALIGN(sizeof(*(p)), (a)))
> >=20
> > Hmm... This can potentially evaluate p twice.

I don't think sizeof evaluates its parameter? It should only defer its
type and then calculate the respective size.

> > Perhaps this variant is better:
> >=20
> > #define opaque_struct_data(p, a)	ALIGN((p) + 1, (a)))

Still I like this better.

> > Besides, I don't think we should worry about @s evaluation in the main =
macro
> > which may be simplified to
> >=20
> > #define opaque_struct_size(p, a, s)					\
> > 	((s) ? size_add(ALIGN(sizeof(*(p)), (a)), (s)) : sizeof(*(p)))
> >=20
> > Thoughts?
>=20
> Also we may leave the struct always be aligned which makes the above even
> simpler (but might waste bytes if @s =3D 0).

Depending on the value of a, the bytes "wasted" here might not be saved
anyhow because the memory allocator will round up the allocation size to
a multiple of some chunk size anyhow.

> #define opaque_struct_size(p, a, s)	size_add(ALIGN(sizeof(*(p)), (a)), (s=
))
>=20
> (with the respective documentation update).

That would be my favourite.

Possible users of this helper include:

	__spi_alloc_controller() in drivers/spi/spi.c
	alloc_netdev_mqs in net/core/dev.c

Best regards
Uwe

--=20
Pengutronix e.K.                           | Uwe Kleine-K=F6nig            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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